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We compare Fourier Amplitude Spectra of Fault Normal (FN) and Fault Parallel (FP) seismograms at near‐fault sites for seven strike‐slip earthquakes with moment magnitudes Mw ≥ 6. For all events we find large FN/FP ratios at low frequencies consistent with near‐fault S‐wave radiation patterns for strike‐slip earthquakes. However, the difference diminishes with increasing frequency and FN/FP is about 1 above a transition frequency. The results may reflect small tensile/isotropic components in the earthquake rupture zones that homogenize the high‐frequency radiation in different directions at near‐fault sites. The FN/FP ratios at low frequencies and transition frequencies above which FN ∼ FP vary among the analyzed earthquakes and have no clear correlation with the magnitudes. The lack of correlation may signify a characteristic scale (e.g., process zone size, duration of source time function) controlling the isotropic radiation, and/or wave propagation and other effects that mask the source effects. Plain Language Summary: Earthquake source processes have significant impacts on many topics including generation of frictional heat on the fault and seismic ground motion away from the fault. The classical model with pure shear motion predicts that strike‐slip earthquakes should produce considerably larger fault‐normal motion close to the fault than fault‐parallel motion. Analyzing near‐fault seismic waveforms generated by seven moderate to large earthquakes, we find that larger fault‐normal motion is observed only for relatively low frequencies, and that above a transition frequency the fault‐normal and fault‐parallel motions are very similar. The observations may reflect local dilatational processes in the earthquake rupture zones that modify the directional dependence of the high‐frequency seismic radiation. The transition frequencies do not show clear scaling with the size of earthquakes, which suggests a process with a characteristic length and/or time scale, or a mixture of multiple effects. The results highlight the need to further improve our understanding of earthquake source processes with detailed near‐fault data. Key Points: Near‐fault seismograms of Mw ≥ 6 strike‐slip events show significantly larger fault‐normal than fault‐parallel motion at low frequenciesThe near‐fault seismic motions have similar amplitudes in different directions above certain transition frequenciesThe isotropic high‐frequency radiation suggests that local dilatational processes accompany the overall shear motion in the rupture zones [ABSTRACT FROM AUTHOR]

The 2024 Mw 7.5 Noto Peninsula, Japan, earthquake was initiated within the source region of intense swarm activity. To reveal the mainshock early process, we relocated the earthquake hypocenters and found that many key phenomena, including the mainshock initiation, foreshocks, swarm earthquakes, and deep aseismic slip, occurred at parts of a previously unrecognized fault in intricate fault network. This fault is subparallel (several kilometers deeper) to a known active fault, and the mainshock initiation and foreshocks occurred at the front of a 2‐year westward swarm migration. The initiation location coincides with the destination of the upward migration of a deeper earthquake cluster via several smaller faults. Fluid supply, small earthquakes, and aseismic slip on the fault likely triggered the mainshock, leading to the first major rupture at the western region, propagating further to the west and east sides, resulting in an Mw7.5 event, exceeding 100 km in length. Plain Language Summary: In 2024, an earthquake of magnitude 7.5 happened on the Noto Peninsula, Japan. This earthquake started in an area where many small earthquakes occurred. To reveal how this large earthquake occurred, we precisely determined the hypocenters of the mainshock, foreshocks, aftershocks, and swarm earthquakes. We observed that the mainshock initiation, foreshocks, and some aftershocks occurred on the largest fault in the complex network. Aseismic slip also occurred near the deeper extension. This fault is not a previously known active fault, but small earthquakes migrated westward on it for over 2 years, and the mainshock rupture was initiated at the front. Directly below the mainshock initiation area, small earthquakes moved upward through several small faults. Aseismic slip propagation, fluid supply, and small earthquakes likely triggered the mainshock. The initiated mainshock rupture caused the first major slip in the western region, causing considerable uplift there and further propagating to the east and west. Despite the existence of many minor faults, many key phenomena have occurred on this previously unrecognized fault, which plays a vital role in stress release and tectonic processes in this crust. Key Points: The mainshock initiation, foreshocks, and deep aseismic slip occurred on a previously unknown fault deeper than a known active faultMany swarm earthquakes that appeared to have occurred on different faults actually occurred on different parts of the same faultMainshock rupture was initiated in a westward earthquake migration front where fluid was supplied from depth through several small faults [ABSTRACT FROM AUTHOR]

The interplay between seismic activity and fluid flow is essential during the evolution of hydrothermal systems. Although earthquakes can trigger transient fluid flow and phase changes in dilational jogs, the temporal scale and the geologic conditions that enhance such process are poorly quantified. Here, we use numerical simulations of deformation and fluid flow to constrain the conditions that maximize adiabatic boiling—referred to as flashing—and estimate the extent and duration of such process. We show that there is an optimal geometry for a dilational jog that maximizes co‐seismic flashing within the jog. Fluid flow simulations indicate that the duration, intensity, and propagation of the flashing front are limited and highly dependent on the magnitude of the co‐seismic slip and the initial pressure‐enthalpy conditions. Our results are valuable to better understand the implications of pressure fluctuations during the seismogenic cycle, as well the mineralization processes in the Earth's crust. Plain Language Summary: Earthquakes can strongly affect circulating fluids within the Earth's crust, mainly where faults bend or split into different fault segments and produce dilatant areas. In these areas, earthquakes play an important role in forming ore deposits, because the co‐seismic volume change can produce a pressure drop that drives boiling with gas exsolution and subsequent mineralization. This process, in which boiling is triggered by a pressure drop rather than a temperature rise, is called flash vapourization or flashing. Here, we used a computer code to unravel scenarios where optimal geometry and pressure‐temperature conditions maximize flash vapourization. Furthermore, we found that the duration and extension of the flashing event are limited and highly dependent on the magnitude of the triggering earthquake and the physico‐chemical conditions of the system. Such results are valuable for assessing the implications of pressure fluctuations during the seismogenic cycle and for better understanding mineralization processes in the Earth's crust. Key Points: We use numerical simulations to constrain the intensity and temporal scale of co‐seismic flash vapourization or flashing triggered by earthquakesWe found an optimal geometry of a dilational jog that maximizes co‐seismic flashing within the jogsUnder optimal conditions, the flashing front propagates up to 1 m away from the dilational jog and persists for almost 3 hr [ABSTRACT FROM AUTHOR]

A M6.8 earthquake struck the High Atlas Mountains in Morocco on 8 September 2023, ending a 63‐year seismic silence. We herein attempt to clarify the seismogenic fault and explore the underlying mechanism for this seismic event based on multiple data sets. Utilizing probabilistic Bayesian inversion on interferometric radar data, we determine a seismogenic fault plane centered at a depth of 26 km, striking 251° and dipping 72°, closely aligned with the Tizi n'Test fault system. Given a hypocenter at the Moho depth, the joint inversion of radar and teleseismic data reveals that the rupture concentrates between depths of 12 and 36 km, offsetting the Mohorovičić discontinuity (Moho) at ∼32 km. Considering a strong link between magma activity and failure in lower crust, we propose that the triggering of the earthquake possibly was mantle upwelling that also supports the high topography. Plain Language Summary: On 8 September 2023, a devastating earthquake with a magnitude of 6.8, struck the High Atlas Mountains in Morocco, breaking a 63‐year seismic silence. In this work we utilized geodetic, seismic, and seismicity data to investigate the earthquake, pinpointing its origin within the Tizi n'Test fault system at 12–36 km depths and causing displacement of the Mohorovičić discontinuity (Moho), where Earth's crust meets the mantle. Highlighting the impact of the mantle upwelling, this event underscores its significance in shaping regional topography and driving crustal deformation along established faults, leading to unusual, significant seismic activity in areas of minimal plate convergence. This earthquake highlights the importance of incorporating deep crustal dynamics into seismic hazard evaluations, challenging traditional risk models with its implications on earthquake genesis far from tectonic boundaries. Key Points: Interferometric radar data, Bayesian inversion techniques and teleseismic data reveal the seismogenic fault and underlying mechanismsThe seismogenic fault centered at a depth of 26 km, striking 251° and dipping 72°, closely aligned with the Tizi n'Test fault systemWe propose that the 2023 M6.8 earthquake in Morocco was possibly triggered by mantle upwelling on a steep fault that offsets the Moho [ABSTRACT FROM AUTHOR]

A devastating earthquake with moment magnitude 7.5 occurred in the Noto Peninsula in central Japan on 1 January 2024. We estimate the rupture evolution of this earthquake from teleseismic P‐wave data using the potency‐density tensor inversion method, which provides information on the spatiotemporal slip distribution including fault orientations. The results show a long and quiet initial rupture phase that overlaps with regions of preceding earthquake swarms and associated aseismic deformation. The following three major rupture episodes evolve on segmented, differently oriented faults bounded by the initial rupture region. The irregular initial rupture process followed by the multi‐scale rupture growth is considered to be controlled by the preceding seismic and aseismic processes and the geometric complexity of the fault system. Such a discrete rupture scenario, including the triggering of an isolated fault rupture, adds critical inputs on the assessment of strong ground motion and associated damages for future earthquakes. Plain Language Summary: On 1 January 2024, a moment magnitude 7.5 earthquake occurred in the northern Noto Peninsula, Japan. The strong ground motion and tsunami associated with the earthquake caused severe damage to buildings and infrastructure, resulting in at least 245 causalities in the affected areas. The Noto Peninsula is affected by northwest‐southeast compression, and active reverse faults are known along the northern coast of the peninsula and its offshore region. Before the 2024 earthquake, the source region experienced long‐lasting earthquake swarm activity, which is a set of seismic events without an obvious mainshock‐aftershock pattern. Our seismological analysis found that there was a 10‐s‐long initial rupture episode around the hypocenter that overlapped with the earthquake swarm region. The initial rupture was followed by a series of three different rupture episodes on differently oriented fault segments. This earthquake highlights a multi‐scale rupture growth across a segmented fault network after a very quiet initial rupture process that was controlled by the preceding earthquake swarms and associated aseismic deformation related to fluid injection from depth. The rupture process advances our understanding of earthquake source physics and can lead to a better assessment of future earthquake hazards. Key Points: The 2024 Mw 7.5 Noto Peninsula earthquake involves a multi‐segmented rupture sequence on differently oriented faultsThe long and quiet initial rupture domain coincides with the preceding earthquake swarm regionFluid‐induced earthquake swarms and a segmented fault network control the complex earthquake rupture growth [ABSTRACT FROM AUTHOR]

Very low‐frequency earthquakes (VLFs) are characterized by longer source duration and smaller stress drop than regular earthquakes of similar magnitude. Recent studies have shown their frequent correlation with low‐frequency earthquakes (LFEs) on shared faults. The underlying source processes governing the occurrence of VLFs and their interaction with LFEs remain elusive. Here, we employ a slip‐weakening model for slow earthquakes. By comparing the source parameters of simulations and observations, it is suggested that VLFs are slow self‐arresting earthquakes that self‐terminate within the nucleation patch. Additionally, we adopt a composite model to reproduce the records of the simultaneous occurrences of a VLF and an LFE in the Nankai area. Our results present the possibility that VLFs, LFEs, and regular earthquakes can be distinguished using a unified dynamic framework. Plain Language Summary: Earthquakes can be divided into two groups: regular earthquakes and slow earthquakes. Regular earthquakes are the results of quick shear slip on faults, while slow earthquakes are generated from slow transient movements along the faults and radiate anomalous small energy. The mechanism that drives their different source characteristics remains elusive. Here, we employ an independent and composite dynamic source model to elucidate the unique attributes of very low‐frequency earthquakes (LFEs), a type of slow earthquakes. By comparing the source parameters and waveforms of observations with simulations. The outcomes indicate that very LFEs potentially represent a seismic type that self‐terminate within the region where the rupture is initially triggered. Key Points: Very low‐frequency earthquakes (LFEs) are characterized by slow self‐arresting processesOur model can simultaneously reproduce both very low‐frequency and LFEThe slip‐weakening frictional model generates both fast and slow earthquakes [ABSTRACT FROM AUTHOR]

Current earthquake forecasting approaches are mainly based on probabilistic assumptions, as earthquakes seem to occur randomly. Such apparent randomness can however be caused by deterministic chaos, rendering deterministic short‐term forecasts possible. Due to the short historical and instrumental record of earthquakes, chaos detection has proven challenging, but more frequently occurring slow slip events (SSE) are promising candidates to probe for determinism. Here, we characterize the SSE signatures obtained from GNSS position time series in the Hikurangi Subduction Zone (New Zealand) to investigate whether the seemingly random SSE occurrence is governed by chaotic determinism. We find evidence for deterministic chaos for stations recording shallow SSEs, suggesting that short‐term deterministic forecasting of SSEs, similar to weather forecasts, might indeed be possible over timescales of a few weeks. We anticipate that our findings could open the door for next‐generation SSE forecasting, adding new tools to existing probabilistic approaches. Plain Language Summary: Since earthquakes appear to occur randomly, the currently available probabilistic predictions are based on past earthquake records. These predictions estimate the likelihood of an earthquake of a given magnitude occurring within a defined time period. In contrast to such probabilistic approaches, deterministic systems are fully predictable, albeit often confined to short time scales due to their potential chaotic behavior. Probing for deterministic predictability in the earthquake cycle is intractable due to the limited historical instrumental record. However, frequently occurring slow slip events ‐ captured by transient GNSS displacements that can last several weeks ‐ provide a unique opportunity to explore deterministic predictability in these types of slow earthquakes. By studying GNSS time series from various stations on New Zealand's North Island, we have discovered evidence suggesting that these irregularly occurring slow slip events might be governed by chaotic determinism. This implies the potential to forecast both timing and magnitude of slow slip events a few weeks in advance using deterministic methods, much like we predict weather patterns. Consequently, our theoretical findings could therefore pave the way for innovative approaches to short‐term slow slip forecasting. Key Points: Nonlinear analysis of GNSS displacement time series unveils evidence for deterministic chaos in slow slip events in New ZealandOur theoretical findings imply that irregularly occurring slow slip events could potentially be forecasted a few weeks in advance [ABSTRACT FROM AUTHOR]

Earthquake swarms are most commonly composed of small‐magnitude earthquakes. However, a recent study by Yoshida, Uchida, et al. (2023, https://doi.org/10.1029/2023GL106023) analyzed a swarm beneath the Noto Peninsula in Japan that, after more than two years of moderate‐magnitude seismicity, triggered the moment magnitude (Mw) 6.2 Suzu mainshock in May 2023. Based on high‐precision earthquake locations and a slip inversion of the mainshock, these authors found that the Mw 6.2 Suzu earthquake occurred on the updip extension of a fault that was active during the swarm, likely driven by increased fluid pressure. After publication of that paper, a much larger and more destructive Mw 7.5 event occurred nearby on 1 January 2024. These events underscore the potential for swarms to be precursors to large, damaging earthquakes. Forecasting the eventual evolution of swarms is currently very challenging but could be aided in the future by new observations and models. Plain Language Summary: Although earthquake swarms often only contain small earthquakes, occasionally they can trigger large, damaging earthquakes. Yoshida, Uchida, et al. (2023, https://doi.org/10.1029/2023GL106023) examined the triggering of a magnitude 6.2 earthquake in Japan, which caused extensive damage, following two years of smaller earthquakes. This sequence, like many swarms, may have been driven by elevated fluid pressure migrating upwards within the crust. After publication of that paper, a much larger and more destructive Mw 7.5 event occurred nearby. Forecasting the eventual evolution of ongoing swarms is currently very challenging but could be aided in the future by new observations and models. Key Points: Earthquake swarms can be precursors to damaging earthquakes, including the 1 January 2024, Mw 7.5 Noto Peninsula, Japan eventFluid pressure changes often drive swarm activity, and their degree of confinement may influence swarm evolutionDirect observations of fluids in fault zones combined with numerical modeling may aid swarm forecasting in the future [ABSTRACT FROM AUTHOR]

On 8 September 2023 an MW 6.8 earthquake struck the High Atlas Mountains of western Morocco, about 70 km southwest from Marrakesh, causing significant devastation and casualties. In this study, we investigate a comprehensive geodetic data set, employing interferometric synthetic aperture radar measurements to assess the fault segment responsible for the seismic event. Our findings suggest two potential fault scenarios: either a transpressive NNW‐dipping high‐angle (70°) fault related to the Tizi n'Test alignment or a transpressive SSW‐dipping low‐angle (22°) fault associated with the North Atlas Fault, with slip (up to 2.2 m) only occurring on deeper parts of the fault. While seismic catalogs couldn't definitively determine the dip direction of the fault, evidence from mainshock locations, gravity and heat flow data and modeling, and active shortening direction suggest the activation of a low‐angle south‐westerly dipping oblique thrust of the North Atlas fault during the 2023 Moroccan earthquake. Plain Language Summary: On 8 September 2023, a powerful earthquake with a magnitude of 6.8 hit the High Atlas Mountains in western Morocco, causing significant damage and loss of life. In this research, a thorough geodetic analysis was conducted using a technique called interferometric synthetic aperture radar to identify the causative fault. The results suggest two potential fault scenarios: one involves a steeply dipping fault related to the Tizi n'Test crustal structure, while the other suggests a gently dipping fault associated with the North Atlas Fault. As far as existing aftershock records do not allow determination of the angle of the fault, clues from the locations of the mainshock, gravity and heat flow data and modeling, and the direction of active compression suggest that a gently dipping fault was likely activated during the 2023 earthquake in Morocco. Key Points: Coseismic ground deformation of the 2023 Morocco earthquake measured with interferometric synthetic aperture radar (InSAR) dataInSAR data allow for two possibilities: the rupture of an intramontane high‐angle fault or of a border low‐angle thrustBased on aftershock distribution, crustal thickness, and regional tectonic stress orientation, the low‐angle solution is more likely [ABSTRACT FROM AUTHOR]

The reliable estimation of earthquake magnitude and stress drop are key in seismology. The novel technology of distributed acoustic sensing (DAS) holds great promise for source parameter inversion owing to the measurements' high spatial density. In this study, I demonstrate the robustness of DAS for magnitude and stress drop estimation using the empirical Green's function deconvolution method. This method is applied to nine co‐located earthquakes recorded in Israel following the 2023 Turkey earthquakes. Spectral ratios were stacked along the fiber, and fitted with a relative Boatwright source spectral model. Excellent fits were obtained even for similar sized earthquakes. Stable seismic moments and stress drops were calculated assuming that the moment of one earthquake is known. DAS derived estimates were found to be more stable and reliable than those obtained using a dense accelerometer network. The results demonstrate the great potential of DAS for source studies. Plain Language Summary: Estimating earthquake magnitude and stress drop, the two most fundamental source parameters, is key for various seismological investigations, including earthquake self‐similarity and hazard mitigation. The extraction of source parameters requires that path and site effects are reliably deconvolved from source contributions. One approach is to deconvolve a small earthquake from a co‐located larger one in the frequency domain such that common path and site functions cancel and relative source function, between both earthquakes, remain. In this study, I apply the deconvolution approach to nine earthquakes that followed the 2023 Turkey earthquakes, recorded using distributed acoustic sensing (DAS) in Israel. DAS converts standard optical fibers into dense arrays, with seismic measurements every few meters along tens‐of‐kilometers long fibers. The deconvolution was calculated for different earthquake pairs and excellent fits were obtained with a relative source spectral model. Stable seismic moments and stress drops were calculated. These parameters were found to be more reliable than those obtained using a dense standard‐accelerometer network. The results demonstrate the great potential of DAS for source studies. Key Points: The spectral ratios approach was applied to nine earthquakes in Israel recorded by distributed acoustic sensing (DAS) and accelerometersAccurate relative magnitudes and stress drops were calculatedDAS can provide reliable source parameter estimates that may outperform accelerometers [ABSTRACT FROM AUTHOR]

The evolution of a fault's frictional strength during the interseismic period is a critical component of the earthquake cycle, yet there have been relatively few studies that examine the time‐dependent evolution of strength at conditions representative of seismogenic depths. Using a simulated fault in Westerly granite, we examined how frictional strength evolves under hydrothermal conditions up to 250°C during slide‐hold‐slide experiments. At temperatures ≤100°C, frictional strength generally increases with hold duration but, at 200 and 250°C, an initial increase in strength transitions to rapid time‐dependent weakening for holds longer than 14 hr. Forward modeling of long hold periods at 250°C using the rate and state friction constitutive equations requires a second, strongly negative, state variable with a long evolution distance. This implies that significant hydrothermal alteration is occurring at 250°C, consistent with microstructural observations of dissolution and secondary mineral precipitation. Plain Language Summary: A fault can regain strength following an earthquake. The rates, conditions, and mechanisms by which this recovery of strength occurs are not well understood but they are critical to understanding earthquake recurrence and induced seismicity. By conducting a series of experiments in which a simulated fault alternates between shear displacement and quasi‐stationary hold periods, we examine how fault strength evolves with time at temperatures up to 250°C. At elevated temperatures the fault initially strengthens but then weakens over long timescales. This weakening appears to be due to hydrothermal alteration, potentially including the precipitation of secondary minerals in the fault. Relating this observed weakening behavior and associated mechanisms to constitutive equations will facilitate modeling earthquake behavior in different materials and at variety of environmental conditions. Key Points: Time‐dependent weakening of simulated fault surfaces at hydrothermal conditions is observedWeakening mechanism is water‐assisted, temperature‐dependent, and depends on host rock compositionCapturing frictional behavior across all time scales requires a rate and state model with two state variables and a time delay parameter [ABSTRACT FROM AUTHOR]

Frictional healing is fundamental to the seismic cycle and plays a role in the energy balance, dynamics, and recurrence interval of earthquakes and slow slip events. Although the healing behavior of quartz has been studied extensively, the role of clay content is less understood. We tested synthetic mixtures of quartz and smectite (10%–100% smectite) in a double‐direct shear configuration to measure frictional healing. We show that the magnitude and rate of healing decreases systematically with higher clay content (from 0.008/decade at 10% smectite to 0.002/decade at 100% smectite). Healing scales with both the magnitude of stress relaxation during holds and layer‐normal compaction of the gouge. We suggest this reflects the alignment of clay minerals, leading to saturation of the real area of contact that limits restrengthening during holds. The low healing rates of clay‐rich faults—together with rate‐neutral to rate‐strengthening friction—should promote frequent, small failures or stable sliding. Plain Language Summary: Healing is a time‐dependent strengthening process that makes it possible for faults to regain strength after an earthquake and fail again in future events. Faults are commonly hosted in clay‐rich, fine‐grained gouges. While there is a wealth of data on the frictional behavior of these gouges, there is a relative lack of data regarding their healing behavior. Here, we measure the change in frictional healing with increasing clay content using experimental deformation of synthetic fault gouges. We find that the amount and rates of healing decrease with clay content. This decrease is likely caused by the tendency for clay grains, which are elongate and plate‐shaped, to align and limit the ability for the fault gouge to increase the area of contact between grains, thus limiting healing. This small healing, together with their low strength and tendency toward stable behavior, may promote small frequent earthquakes—or inhibit earthquakes—on faults hosted in clay‐rich gouges. Key Points: We conducted experiments on a suite of quartz‐smectite mixtures to investigate the role of clay in frictional healingIncreasing smectite content reduces both healing and frictional strength, interpreted to result from saturation of real area of contactLow healing rates in clay‐rich gouges may favor more frequent and smaller failures than in quartz‐dominated gouges [ABSTRACT FROM AUTHOR]

Submarine landslides shape continental margins, transfer massive amounts of sediment downslope, and can generate deadly and destructive tsunamis. Submarine landslides are common globally, yet constraining hazard potential of future events is limited by a short historical record and a wide range of possible slide dynamics. We test a novel approach to investigate slide dynamics using properties of the deformation zone induced by a large submarine landslide along the Cascadia margin, offshore Oregon. We use a simple model of a line load on a poroelastic half space to show the deformation zone size required rapid transport and deceleration. We argue that the slide moved at high speeds, aided by low dynamic frictional resistance, suggesting this event could have generated a tsunami. This method is applicable where slide‐induced impact zones are observed. Plain Language Summary: The Cascadia margin is susceptible to underwater landslides and tsunami hazards due to the active subduction zone that can produce large magnitude earthquakes. However, determining whether future submarine landslides will be tsunamigenic is challenging. While past landslide deposits can easily be identified and in many cases age‐dated, the dynamic process including the initial acceleration and velocity that created the deposit is almost never known. We present a novel approach to back‐analyze slide velocity of past landslides by using the characteristics of the deformation zone that occurred when the landslide deposit came to rest on the seafloor. Our models suggest that the slide hydroplaned while moving at high‐speeds (up to 60 m/s) with a run‐out distance of 10‐km from the source, which match the observations. While our method does not model tsunamis explicitly, the high speeds are consistent with known tsunamigenic slides. This approach provides critically important constraints on the slide velocity and therefore the hazards of submarine landslides that can occur at Cascadia. Key Points: A broad zone of impact‐induced deformation is observed immediately adjacent to the massive 44‐N Slide offshore OregonThe deformation of seafloor sediments observed at the slide terminus requires slide deceleration, implying catastrophic emplacementModeled impact forces require a slide velocity of up to 60 m/s, which may have been sufficient to generate a tsunami [ABSTRACT FROM AUTHOR]

We considered various non‐uniformities such as branch faults, rotation of stress field directions, and changes in tectonic environments to simulate the dynamic rupture process of the 6 February 2023 Mw 7.8 Kahramanmaraş earthquake in SE Türkiye. We utilized near‐fault waveform data, GNSS static displacements, and surface rupture to constrain the dynamic model. The results indicate that the high initial stress accumulated in the Kahramanmaraş‐Çelikhan seismic gap leads to the successful triggering of the East Anatolian Fault (EAF) and the supershear rupture in the northeast segment. Due to the complexity of fault geometry, the rupture speed along the southeastern segment of the EAF varied repeatedly between supershear and subshear, which contributed to the unexpectedly strong ground motion. Furthermore, the triggering of the EAF reminds us to be aware of the risk of seismic gaps on major faults being triggered by secondary faults, which is crucial to prevent significant disasters. Plain Language Summary: On 6 February 2023, the south‐central Türkiye was hit by two major earthquakes with magnitudes of Mw 7.8 and Mw 7.6 respectively. Among them, the complex rupture process and unexpected ground motion of the Mw 7.8 event attracted the attention of seismologists. In this paper, the 3D dynamic rupture process of this mainshock is simulated based on complex multi‐fault system and heterogeneous initial stress. And the simulation results are in good agreement with the observations. Our results show that high initial stress is required for the EAF to be triggered. The supershear rupture occurred only in certain fault segments and is unable to sustain itself in a significant area on the fault due to the along‐strike variations in fault geometry and strength. More importantly, the dynamic model suggests that we must be alert to the risk of major fault being triggered by earthquakes on nearby small faults, especially when there are seismic gaps on the major fault. Key Points: The high initial stress accumulated in the seismic gap leads to the successful triggering of the East Anatolian FaultThe change of fault geometry in the southwest segment prevented the sustained supershear ruptureThe risk of earthquake nucleation on the secondary fault triggering the major fault rupture and the related disaster was highlighted [ABSTRACT FROM AUTHOR]

Earth's crust is continuously subjected to oscillatory stress perturbations due to the solid Earth and ocean tides. The seismic response to such stress modulations carries information on earthquake physics and crustal properties. Experimental and observational studies suggested but could not demonstrate that the strength of tidal modulation of seismicity increases before large earthquakes. We tested this hypothesis by (a) developing a new, comprehensive 10‐year long earthquake catalog preceding the 6 July 2019 magnitude 7.1 Ridgecrest, CA earthquake, and (b) applied our novel method for extracting a statistical signal of tidal modulation. Our results show enhanced tidal sensitivity of seismicity along the fault starting about 1.5 years before the mainshock, corroborating the hypothesis. This observation suggests that small magnitude earthquakes may be used to gain insight into subtle changes in fault conditions, bringing new promise for studying the earthquake preparation process. Plain Language Summary: The solid Earth experiences tides, like the ocean, it deforms under the gravitational attraction of the Sun and Moon. This deformation induces an oscillatory stress change in the crust with small peak‐to‐peak amplitudes of the order of 1 kPa (about 1% of the atmospheric pressure). Tidal stresses weakly influence the rate of earthquake occurrence and the characteristics of this modulation carry information on earthquake physics and crustal properties. On the basis of experimental and field observation studies, it has been proposed that the modulation of seismicity by the tides increases before a large earthquake. We tested this hypothesis by analyzing 10 years of seismicity before the M7.1 2019 Ridgecrest, CA earthquake. We first built a new, comprehensive earthquake catalog with our automated method and used our novel method to extract the signal of tidal modulation throughout the study period. We found that seismicity became strongly modulated by the tides about 2 years before the mainshock, thus corroborating the hypothesis. Key Points: Seismicity in the Ridgecrest area is modulated by the solid Earth tidesPeak seismicity occurs when tidal Coulomb stress or stress rate favors ruptureStrength of tidal triggering is gradually increasing in the rupture area about 2 years before the Mw 7.1 2019 mainshock [ABSTRACT FROM AUTHOR]

A devastating doublet of earthquakes with moment magnitude MW 7.9 and MW 7.6 earthquakes contiguously occurred in SE Türkiye near the NW border of Syria. Here we perform a potency‐density tensor inversion to simultaneously estimate rupture evolution and fault geometry for the doublet. We find the initial MW 7.9 earthquake involved discrete episodes of supershear rupture and back‐rupture propagation, and was triggered by initial rupture along a bifurcated splay of the East Anatolian Fault. The second MW 7.6 event was triggered by the earlier MW 7.9 event, and it involved more extensive supershear rupture along a favorably curved fault, and was likely stopped by geometric barriers at the fault ends. Our results highlight the multi‐scale cascading rupture growth across the complex fault network that affects the diverse rupture geometries of the 2023 Türkiye earthquake doublet, contributing to the strong ground shaking and associated devastation. Plain Language Summary: On 6 February 2023, devastating dual earthquakes; moment magnitude 7.9 and 7.6 events struck southern Türkiye near the northern border of Syria. The two earthquakes were only separated ∼90 km and ∼9 hr apart. The strong shaking from the two earthquakes caused significant damage to the buildings and people, having caused over 50,000 fatalities in Türkiye and Syria. The source region is where the Anatolian, Arabian and African plates meet, developing the network of faults that hosted the large devastating earthquakes. Seismological analyses using observed seismic waveforms are effective for rapidly estimating how the rupture of the two earthquakes evolves over such distinctively oriented and possibly segmented faults. We use the globally observed seismic records to simultaneously estimate rupture evolution and fault geometry of the earthquake doublet. We find the sequence of both earthquakes involves curved and segmented fault ruptures, including the back‐propagating rupture for the initial earthquake, which is facilitated by the complex active fault network. The 2023 earthquake doublet displays the irregular rupture evolution and diverse triggering behaviors both in a single event and across the earthquake sequence, which provide critical inputs in both our understanding of earthquake‐rupture dynamics and better assessment of future damaging earthquakes. Key Points: An earthquake doublet of MW 7.9 and MW 7.6 ruptured multiple segments and curved faultsInitial splay fault rupture triggered a large MW 7.9 rupture involving pulses of back‐propagating supershear ruptureMulti‐scale rupture growth in a complex fault network may facilitate diverse rupture behaviors and triggering interactions in the doublet [ABSTRACT FROM AUTHOR]

The 2016 Mw 5.9 Menyuan earthquake occurred near the Haiyuan fault system in the northeastern Tibetan plateau. Although some kinematic properties of this event have been investigated, more dynamic properties still need to be clarified. First, we use the empirical Green's function method to obtain this event's apparent source time functions (ASTFs), which present a bell shape with a duration of 3s and corresponding smooth spectra. We then conduct dynamic rupture simulations, which are well constrained by the seismic and geodetic observations, to explore more in‐depth source properties. The results show that the characteristics of ASTFs can be explained by the self‐arresting rupture model, which implies that the rupture propagation of this event may spontaneously terminate before reaching the barriers. This result suggests that the initial conditions when this earthquake nucleated may have controlled the rupture process. Our work contributes to understanding the stopping mechanism of some moderate earthquake ruptures. Plain Language Summary: A quantitative understanding of what controls earthquake rupture propagation is important, helping to estimate the eventual magnitude of a future earthquake and thus has significant implications for earthquake hazard assessment. Previous studies indicate that the complex fault geometry and the heterogeneous material properties of the fault zone can influence earthquake rupture propagation. In this study, we first use the empirical Green's function approach to obtain the single‐peak apparent source time functions for the 2016 Mw 5.9 Menyuan earthquake, and the corresponding spectra have no spectral troughs. We then conduct the spontaneous dynamic rupture simulations, which are well constrained by the geodetic and seismic observations, to reveal more source properties. The features of apparent source time functions can be well explained by the self‐arresting rupture process, which implies that the rupture propagation might spontaneously stop without external interference. This result indicates that this earthquake's rupture propagation might have been deterministic by the initial conditions in which it prepared for rupture. Key Points: The apparent source time functions (ASTFs) of the 2016 Mw 5.9 Menyuan earthquake display a bell shape and corresponding smooth spectraThe features of ASTFs can be explained by the self‐arresting rupture model, implying that the rupture propagation might spontaneously stopThe dynamic rupture simulations are constrained by the interferometric synthetic aperture radar data and strong ground motion observations [ABSTRACT FROM AUTHOR]

Distributed Acoustic Sensing (DAS) is a promising technique to improve the rapid detection and characterization of earthquakes. Previous DAS studies mainly focus on the phase information but less on the amplitude information. In this study, we compile earthquake data from two DAS arrays in California, USA, and one submarine array in Sanriku, Japan. We develop a data‐driven method to obtain the first scaling relation between DAS amplitude and earthquake magnitude. Our results reveal that the earthquake amplitudes recorded by DAS in different regions follow a similar scaling relation. The scaling relation can provide a rapid earthquake magnitude estimation and effectively avoid uncertainties caused by the conversion to ground motions. Our results show that the scaling relation appears transferable to new regions with calibrations. The scaling relation highlights the great potential of DAS in earthquake source characterization and early warning. Plain Language Summary: Distributed Acoustic Sensing (DAS) is an emerging technique that can convert an optical fiber cable into a dense array to record seismic waves from earthquakes. The recorded seismic signals contain essential information about earthquakes. For example, DAS can record high‐amplitude signals from earthquakes with large magnitudes. However, the exact setting of the optical cables (i.e., installation conditions and coupling with the surrounding medium) is often unknown, thus preventing quantitative estimations of earthquake magnitudes with DAS. In this study, we analyze earthquake data recorded by different DAS arrays and develop a data‐driven method to obtain an empirical relation between the earthquake magnitude and the amplitude of DAS signals. We show that this empirical relation can accurately estimate the earthquake magnitude directly from the DAS data. Furthermore, the empirical relation we obtain from one area can also be applied to new regions with slight calibrations. Our empirical relation can significantly expand the applications of DAS in earthquake research, such as seismic hazard assessment and earthquake early warning. Key Points: We present the first data‐based scaling relation between Distributed Acoustic Sensing (DAS) amplitude and earthquake magnitudeEarthquake magnitude can be reliably estimated from DAS amplitude with the scaling relationThe DAS scaling relation can be transferred from one region to another with minor calibrations [ABSTRACT FROM AUTHOR]

Subducting seamounts are recognized as one of the key features influencing megathrust earthquakes. However, whether they trigger or arrest ruptures remains debated. Here, we use analog models to study the influence of a single seamount on megathrust earthquakes, separating the effect of topography from that of friction. Four different model configurations have been developed (i.e., flat interface, high and low friction seamount, low friction patch). In our models, the seamount reduces recurrence time, interseismic coupling, and fault strength, suggesting that it acts as a barrier: 80% of the ruptures concentrate in flat regions that surround the seamount and only smaller magnitude earthquakes nucleate above it. The low‐friction zone, which mimics the fluid accumulation or the establishment of fracture systems in natural cases, seems to be the most efficient in arresting rupture propagation in our experimental setting. Plain Language Summary: Seamounts ‐ extinct volcanoes ‐ are ubiquitous features of the seafloor of subducting plates. During their descent toward the mantle, seamounts are squeezed between the overriding‐ and subducting plates, creating a geometrical and mechanical discontinuity that is thought to control the largest earthquakes that occur on Earth. It is not known, however, whether they trigger or arrest earthquakes, as a variety of observations have been used to support one or the other hypothesis. Here, we tackle this subject using scaled laboratory models that allow reproduction of large earthquakes. Our models support a scenario where subducting seamounts primarily arrest rupture propagation, limiting therefore expected maximum magnitudes. Our models also suggest that subducting seamounts might influence the recurrence time of large earthquakes by decreasing their frequency. Key Points: We investigated the effect of seamount subduction on megathrust earthquakes by isolating the role of geometry from that of frictionSeamounts act primarily as barriers to earthquake propagation, promoting small ruptures and decreasing seismic couplingReduced friction displays maximum barrier efficiency, a configuration likely associated with fluids or fracture systems in nature [ABSTRACT FROM AUTHOR]

Two submarine earthquakes (Mw 5.8) occurred near volcanic islands, Curtis and Cheeseman, in the Kermadec Arc in 2009 and 2017. Following both earthquakes, similar tsunamis with wave heights of about a meter, larger than expected from their moderate seismic magnitudes, were observed by coastal tide gauges. We investigate the source mechanism for both earthquakes by analyzing tsunami and seismic data of the 2017 event. Tsunami waveform analysis indicates that the earthquake uplifted a submerged caldera around the islands. Combined analysis of tsunami and seismic data suggests that trapdoor faulting, or sudden intra‐caldera fault slip interacted with magma reservoir deformation, occurred due to magma overpressure, possibly in association with caldera resurgence. The earthquake scaling relationship for trapdoor faulting events at three calderas deviates from that for regular earthquakes, possibly due to the fault‐reservoir interaction in calderas. Plain Language Summary: Most tsunamis are generated by large earthquakes with seismic magnitudes M > ∼7, but two moderate‐sized earthquakes with only M 5.8 near volcanic islands, north of New Zealand, caused unusual tsunamis with a maximum wave height of about a meter. In this study, we examine the unusual source mechanism of the volcanic earthquakes that excited unexpected tsunamis. By analyzing the records of tsunamis and seismic waves from the earthquakes, we suggest that the inside of a curved fault system beneath a submerged volcano with a caldera structure suddenly moved upward, together with a large intra‐caldera fault slip and a volume increase of a shallow magma reservoir. Overpressure created by magma accumulation beneath the submarine caldera recurrently induces meter‐scale tsunamis without significant ground motions, calling for attention to tsunami hazards from submarine calderas. Key Points: Unusual tsunamis were caused by two Mw 5.8 volcanic earthquakes in 2009 and 2017 near Curtis and Cheeseman Islands in the Kermadec ArcBy using tsunami and seismic data of the 2017 event, we suggest that a trapdoor faulting event occurred at a submerged resurgent calderaTrapdoor faulting events at calderas had abnormally large slips, possibly due to the fault‐reservoir interaction in calderas [ABSTRACT FROM AUTHOR]

The September 2022 Ms 6.8 Luding earthquake broke more than 230 years of seismic calm on the Moxi fault, providing a unique opportunity to understand its seismogenic environment, rupture dynamics, and seismic hazard. Using teleseismic body waves, regional strong‐motion observations, GNSS, and InSAR data, we decipher the spatiotemporal rupture evolution of the mainshock. Combining the elastic dislocation model with surface creep, we find that the coseismic slip correlates closely with a locked patch with a loading rate of 9.7 mm/yr, but the creeping rate is insufficient to make up the shallow slip deficit. Notably, the Luding earthquake ruptured only ∼1/4 of the Moxi seismic gap, and it further increased the stress in the unruptured northern part. We thus argue that the Moxi fault has the potential for magnitude 7+ earthquakes in the near future, although geodetic prediction may overestimate the actual seismic moment released by the coseismic rupture. Plain Language Summary: The Xianshuihe fault is an area of intense tectonic activity on the Tibetan Plateau and plays an important role in accommodating the postcollisional convergence of the Eurasian and Indian plates. However, the Moxi fault, at the southeastern end of the Xianshuihe fault, has remained seismically quiescent since the last Ms 7.75 event in 1786. In September 2022, a Ms 6.8 Luding earthquake struck the Moxi seismic gap, resulting in destructive landslide damage, with 93 people dead and 25 missing. Using seismic and geodetic recordings, we recover its space‐time rupture process in detail. Our model reveals an asymmetric bilateral rupture with dominant propagation and maximum slip to the SSE. It is important to note that the 2022 Luding earthquake ruptured only the southern Moxi seismic gap, leaving the northern part unruptured. Based on the departure time of the last major earthquake and the slip rate obtained from the elastic dislocation model, we estimate the unreleased seismic energy of the Moxi fault, indicating that it still has the potential for strong earthquakes. Besides, the Luding earthquake further increased its seismic hazard through stress transfer. Therefore, we should pay great attention to the kinematic state of the Moxi fault at this time. Key Points: The kinematic evolution of the Luding earthquake is revealed by seismic and geodetic observationsThe coseismic slip asperity from 3 to 20 km depth correlates closely with the preseismic surface creep and locked patchThe unruptured Moxi seismic gap still has the potential for magnitude 7+ earthquakes and deserves further attention [ABSTRACT FROM AUTHOR]

In August of 2020, an earthquake sequence initiated within the Maacama fault zone in northern California, raising questions about its relationship with the larger‐scale fault. To investigate the faulting geometry and its implications for physical processes driving seismicity, we applied an integrated, multi‐faceted seismic analysis including waveform‐correlation‐based event detection, relative relocation, and high‐precision focal mechanisms. To determine mechanisms for a large population of very small earthquakes (predominantly M < 1), we combined correlation‐derived polarity analysis with a recently developed technique to determine S/P amplitude ratios from single seismic components. Finally, we applied an iterative stress inversion to distinguish between the likely fault and auxiliary planes. This analysis reveals that although the sequence initiated on a right‐lateral fault, plausibly a strand of the Maacama Fault, it also activated numerous en echelon left‐lateral conjugate faults. Together, these interlocking faults form a fracture mesh, consistent with structures associated elsewhere with fluid‐induced faulting and earthquake swarms. Plain Language Summary: In August of 2020, an earthquake sequence initiated within the Maacama fault zone in northern California, raising questions about its relationship with the larger‐scale fault. To investigate the faulting geometry and its implications for physical processes driving seismicity, we applied multiple high‐resolution seismic analyses to determine precise locations and slip orientations. This analysis reveals complex fracture mesh faulting geometry consisting of interlocking faults with two primary orientations, consistent with structures associated elsewhere with fluids and earthquake swarms. Key Points: A swarm‐like earthquake sequence occurred in 2020 near the Maacama Fault in northern California, with magnitudes up to Mw 4.2We determined precise locations for 3,520 events and focal mechanisms for 3,253 events, expanding the routine catalog of 388 earthquakesAn iterative stress inversion distinguishes likely fault planes, revealing en echelon and conjugate faults forming a fracture mesh [ABSTRACT FROM AUTHOR]

The Mw 6.4 right‐lateral Petrinja earthquake (2020, Croatia) is one of the largest continental earthquakes of central Europe for decades. The slip pattern of such events is usually difficult to retrieve with terrestrial geodesy due to limited monitoring means. This study takes advantage of a unique data set of dense measurements of coseismic displacement in the epicentral area, obtained by repeated measurements of benchmark networks designed for civilian purposes, and supplemented by far‐field continuous GNSS measurements. Elastic modeling of these data shows two coseismic slip patches that extend over a 15 × 10 km rupture plane, locally reaching the surface, and that a right‐lateral sub‐parallel secondary fault in the central part of the rupture likely accommodated part of the coseismic deformation. This study demonstrates that rapid re‐measurement of pre‐existing civilian networks offers unique coseismic constrains in the near‐field where InSAR or optical image correlation may decorrelate. Plain Language Summary: The magnitude 6.4 Petrinja earthquake that stroke Croatia on 29 December 2020, is one of the largest earthquakes registered for decades in continental Europe. Large damage and surface ruptures were observed, suggesting that the earthquake occurred at a very shallow depth. The slip pattern for such moderate magnitude earthquakes is usually difficult to retrieve with terrestrial geodesy because of limited monitoring means and small deformations. In this study, we use a unique data set made of dense coseismic displacement estimates in the epicentral area obtained by repeated positioning measurements of benchmark networks designed for civilian purposes. The inversion of this displacement field completed by far‐field continuous GNSS measurements shows that the coseismic slip on the Petrinja‐Pokupsko fault is limited to depths no greater than 10 km and has reached the surface locally. A single‐fault model explains well the data, but the fit is largely improved with a sub‐parallel secondary fault in the central part of the rupture. This study demonstrates that quick remeasurement of pre‐existing civilian networks can offer unique constrains on the coseismic deformation and the associated fault geometry when spatial techniques such as InSAR or optical correlation may decorrelate. Key Points: Real Time Kinematics measurements of civilian networks capture near‐field coseismic displacements of the 2020 strike‐slip Petrinja earthquakeTime‐series from regional continuous GNSS stations constrain coseismic deformation in the far‐fieldElastic modeling argues for two slip patches on the main fault and for significant slip on a parallel secondary fault [ABSTRACT FROM AUTHOR]

The Sulaiman Fold Thrust (SFT) in Central Pakistan formed during the India‐Eurasia collision in the late Cenozoic. However, the mechanics of shortening of the brittle crust at time scales of seismic cycles is still poorly understood. Here, we use radar interferometry to analyze the deformation associated with the 2015 magnitude (Mw) 5.7 Dajal blind earthquake at the eastern boundary of the SFT. We use kinematic inversions to determine the distribution of slip on the frontal ramp and of flexural slip along active axial surfaces for the forward‐ and backward‐verging two end‐member models: a double fault‐bend‐fold system and a fault‐propagation‐fold. In both models, a décollement branches into a shallow ramp at approximately 7.5 km depth with coseismic folding in the hanging wall. The Dajal earthquake ruptured the base of the Boundary Thrust buried under the sediment from the Indus‐River floodplain, representing fault‐bend or fault‐propagation folding some 30 km off its nearest surface exposure. Plain Language Summary: The relative motion between tectonic plates is accommodated across wide fault zones that concentrate deformation. Yet, how these deformation zones grow over time is poorly known. In this study, we investigate the Sulaiman Fold‐Thrust (SFT) belt, Pakistan at the plate boundary between the Indian and Eurasian continents. We use radar interferometry to estimate the crustal deformation induced by the 2015 Mw 5.7 Dajal earthquake at the eastern margin of the SFT. The rupture did not reach the surface at the eastern extension of the SFT, buried under the younger sediments of the Indus River floodplain. The rupture and associated folding in the hanging wall documents the eastward growth of the SFT. These observations document the seismic potential of hidden ramps in fold‐and‐thrust belts and the control on final rupture size by fault‐bends and surrounding folds. Key Points: 2015 Mw 5.7 Dajal, Pakistan earthquake ruptured the blind front of the Sulaiman Fold Thrust beltCoseismic deformation includes slip on a blind thrust and flexural slip in the hanging wallThe buried section of the Boundary Thrust extends 30 km farther south of the fault trace [ABSTRACT FROM AUTHOR]

Intraplate tectonic stress fields are complex due to the imprint of a long geological history. Here we use a new data set of earthquake focal mechanism solutions and relocated events to investigate the relationship between regional stress, crustal strength, and seismicity in the Charlevoix Seismic Zone (CSZ), the most active seismic zone in eastern Canada. Our stress inversion shows that SHmax gradually rotates clockwise from approximately St. Lawrence River‐parallel (∼54°) near the surface to river‐perpendicular (∼120°) in the lower crust, as postglacial rebound stress becomes increasingly dominant at greater depth. The stress rotation occurs primarily between ∼13 and ∼26 km depth, where glacial rebound induced stress perturbation is further amplified by a "weaker" middle crust. Finally, depth‐dependent b‐values confirm the rheological difference between upper and middle crust in the CSZ. Plain Language Summary: The occurrence of earthquakes in continental interiors is controlled by several geological and geophysical conditions which are often poorly known. High‐resolution earthquake catalogs and information about faulting mechanisms are therefore key for recognizing and quantifying the crustal stress and strength conditions in these challenging regions. This study focuses on the Charlevoix Seismic Zone (CSZ), one of the most seismically active regions in eastern North America. By analyzing a new set of earthquake distribution and faulting data, we find that the combination between a "weak" fault zone and crustal stresses generated by postglacial rebound possibly controls the distribution of earthquakes in the CSZ. In particular, seismicity rate and faulting style in the CSZ strongly relate with the depth‐dependent frictional and rheological properties of the crust. Key Points: Preexisting weaknesses in the Charlevoix Seismic Zone amplify postglacial horizontal stresses and cause large clockwise stress rotationsStress rotation is further amplified in mid‐crust due to low friction and semi‐brittle behaviorThe frequency‐magnitude distribution of earthquakes suggests an inverse relationship of b‐value with differential stress in the upper crust [ABSTRACT FROM AUTHOR]

The ability to estimate the likelihood of given earthquake magnitudes is critical for seismic hazard assessment. Earthquake magnitude‐recurrence statistics are empirically linked to stress, yet which fault‐zone processes explain this link remains debated. We use numerical models to reproduce the interplay between viscous creep and frictional sliding of a fault‐zone, for which inter‐seismic locking becomes linked to stress. The models reproduce the empirical stress‐dependent earthquake magnitude distribution observed in nature. Stress is related to the likelihood a fault section is near frictional failure, influencing likely rupture lengths. An analytical model is derived of a fault consisting of identical patches, each with a probability of inter‐seismic locking. It reproduces a similar magnitude‐recurrence relationship, which may therefore be caused by probabilistic clustering of locked fault patches. Contrasts in earthquake statistics between regions could therefore be explained by stress variation, which has future potential to further constrain statistical models of regional seismicity. Plain Language Summary: The frequency of earthquakes with a given magnitude is empirically described by the Gutenberg‐Richter law, where large earthquakes occur less frequently than small ones. Variations in magnitude distribution between regions have been correlated with the tectonic force acting on a fault. However, it is unclear which mechanism is responsible for this relationship, restricting its predictive capability. Here, we create computational models in which some portions of a fault can generate earthquakes and others can move slowly (not generating earthquakes). This slow movement acts to relax and limit the elastic forces that build up around the fault. This approach is used to reproduce realistic earthquake statistics, indicating that processes that limit the stress build‐up between earthquakes may be responsible for the varying likelihoods of earthquake magnitudes observed in nature. Key Points: Combining viscous creep and rate‐and‐state friction in fault slip models limits the peak loading stress over the earthquake cycleNumerical and analytical models reproduce Gutenberg‐Richter earthquake size‐recurrence statistics, with a b‐value linked to fault stressThe interplay between loading stress and probabilistic fault locking provides an explanation for regional contrasts in b‐value [ABSTRACT FROM AUTHOR]

The 1989, Mw = 6.9 Loma Prieta earthquake resulted in tens of lives lost and cost California almost 3% of its gross domestic product. Despite widespread damage, the earthquake did not clearly rupture the surface, challenging the identification and characterization of these hidden hazards. Here, we show that they can be illuminated by inverting fluvial topography for slip‐and moment accrual‐rates—fundamental components in earthquake hazard assessments—along relief‐generating geologic faults. We applied this technique to thrust faults bounding the mountains along the western side of Silicon Valley in the San Francisco Bay Area, and discovered that these structures may be capable of generating a Mw = 6.9 earthquake every 250–300 years based on moment accrual rates. This method may be deployed broadly to evaluate seismic hazard in developing regions with limited geological and geophysical information. Plain Language Summary: Large, shallow earthquakes nucleated along geologic faults promoting vertical motion of rocks produce instantaneous uplift of the Earth's surface on the order of meters. This process and the intervening deformations, repeated over thousands of earthquake cycles, build up mountain ranges, which are subsequently carved by rivers thanks to the action of climate forces counteracting tectonic uplift. Consequently, the incision pattern along mountain rivers, resulting from rock resistance to erosion and long‐lived distribution of fault‐induced rock uplift, contains information about the past activity of underlying relief‐generating faults. Our study tested this fundamental idea, integrating the topography with simple, yet standard mechanical and erosional modeling, to estimate the accrual of earthquake magnitude potential over time. We applied this new methodology, particularly useful to illuminate hazards posed by difficult‐to‐characterize‐faults not well exposed to direct observation, to the fault‐bounded mountains along the western side of the San Francisco Bay Area. We discovered that a quake of similar size to the 1989 Loma Prieta event, the last devastating earthquake affecting this populous economic hub, could occur every 250–300 years. More importantly, given the wide availability of topographic datasets, this method may be deployed broadly to evaluate seismic hazards in poorly instrumented and studied areas. Key Points: The topographic structure of mountain rivers can be inverted for long‐term slip‐and moment accrual‐rates along relief‐generating faultsThrust faults along the western side of Silicon Valley have the potential for generating a Mw = 6.9 earthquake every 250–300 yearsThis approach may be deployed more broadly to evaluate seismic hazard in regions with limited geological and geophysical information [ABSTRACT FROM AUTHOR]

We investigate postseismic deformation following the 15 May 2020, Mw 6.5 Monte Cristo Range, Nevada earthquake using geodetic and aftershock data. Seven months of Sentinel 1‐A/B SAR images were used to model deformation as afterslip on two subparallel fault planes outlined by aftershocks. Postseismic deformation fits exponential and logarithmic decay models equally well. For exponential decay, the average decay time is ∼38 days. On the western plane, where most coseismic slip was observed, afterslip was minor, occurred at shallower depths than coseismic slip, and is anticorrelated with aftershock distribution. On the eastern plane, afterslip is significant, exceeds coseismic moment release, occurred at and below coseismic slip, and is correlated with aftershock distribution. On both segments, geodetic moment exceeds seismic moment, suggesting most afterslip occurred aseismically. Aseismic creep does not make up for observed coseismic shallow slip deficit, perhaps related to fault immaturity. Plain Language Summary: Monitoring surface deformation and slip at depth after an earthquake can be used to investigate changes in the local stress field caused by earthquakes and infer frictional and other conditions on earthquake‐causing faults. We used InSAR satellite data and ground‐based seismic data to study afterslip associated with a moderate magnitude earthquake in Nevada that occurred on 15 May 2020. Most energy after the mainshock was released aseismically rather than as aftershocks. The large (∼1.4 m) coseismic slip that occurred at a depth of about 8–10 km never propagated to the surface, either during or after the earthquake. Geologic estimates of motion on this fault would therefore underestimate the motion at depth, perhaps leading to underestimation of the fault's long‐term slip rate. Key Points: InSAR time series reveal that the average characteristic relaxation time for postseismic motion is ∼38 days for the Monte Cristo Range earthquakeWhile aseismic afterslip exceeded seismic afterslip, combined afterslip did not compensate for the shallow coseismic slip deficitFault immaturity might explain the shallow slip deficit and the relatively low amount of afterslip compared to coseismic slip [ABSTRACT FROM AUTHOR]

We present a two‐dimensional analytical solution for the static stress field around a rough interlocked interface under background stresses and its applications to faulting. The solution is derived using perturbation theory for the defined interface topography and background stresses. The failure‐ratio, which scales the likelihood to failure of the bulk material around the interface, is calculated adopting the Coulomb failure criterion. We compare model predictions with results of experiments performed on rough rock interfaces and find good agreement between the locations of off‐fault deformation zones and calculated high failure‐ratio values. We further study the effect of fault topography on stress distribution and failure around the San Andreas Fault and find a moderate correlation between Failure‐Ratio values and off‐fault seismic events. We conclude that our model is able to delineate off‐fault regions that are relatively prone to bulk failure due to local effects of fault topography. Plain Language Summary: An earthquake is triggered when the stress build‐up by tectonic motion exceeds the strength of the fault material. The factors that control the earthquake magnitude, location, and time are still largely unknown. The significance of fault roughness to earthquake physics has been recognized for many years; geometrical irregularities across fault zones drive stress heterogeneities that strongly affect the onset of seismic rapture. Different research groups are now attempting to quantify the effect of roughness on faulting, either experientially or theoretically. Particularly, it has long been attempted to develop an analytical solution for the local stress field and yield potential across interlocked interfaces as a proxy to natural faulting, albeit without much success. Here, we present an analytical model that solves the stress field around an interlocked fault segment for a given interface roughness and background stress field, and show how surface geometry affects rock failure around the interface. We argue that any dynamic sliding between surfaces is initiated by fracture of the intact rock material at immediate proximity to the interface. We use the stresses obtained by our analytical solution to assess the failure potential around seismically active faults, a novel contribution with specific implications to seismic risk assessment. Key Points: Analytical solution demonstrates how surface geometry and background stress affect the local stress field around rough interlocked faultsThe stress undulations induced by the rough interface and the background stress field, decay exponentially away from the faultThe likelihood for off‐fault failure is assessed using the Coulomb failure criterion and is demonstrated on laboratory and natural faults [ABSTRACT FROM AUTHOR]

The ground movements induced by seismic waves create acoustic waves propagating upward in the atmosphere, thus providing a practical solution to perform remote sensing of planetary interiors. However, a terrestrial demonstration of a seismic network based on balloon‐carried pressure sensors has not been provided. Here we present the first detection of seismic infrasound from a large magnitude quake on a balloon network. We demonstrate that quake's properties and planet's internal structure can be probed from balloon‐borne pressure records alone because these are generated by the ground movements at the planet surface below the balloon. Various seismic waves are identified, thus allowing us to infer the quake magnitude and location, as well as planetary internal structure. The mechanical resonances of balloon system are also observed. This study demonstrates the interest of planetary geophysical mission concepts based on seismic remote sensing with balloon platforms, and their interest to complement terrestrial seismic networks. Plain Language Summary: After a quake, the surface of our planet vibrates like the surface of a drum. These vibrations generate sound waves at low frequencies that propagate upward in the atmosphere. These signals from two earthquakes have been recorded by barometers on board a network of long duration high altitude balloons deployed by the Strateole‐2 experiment. The analysis of these records demonstrates that the amplitude and arrival time of the vibrations are properly predicted by our modeling tools. The oscillations of the balloon/gondola system forced by the acoustic waves are also observed, but the quake distance and magnitude can be estimated only from the data recorded on board the balloon gondolas. Moreover, the shape of the pressure perturbations recorded by the balloons contains the seismic surface waves that are sounding the first hundred kilometers of the Earth's internal structure. These observations clearly demonstrate the interest of a similar experiment in the atmosphere of Venus to sound its poorly known internal structure. Key Points: An earthquake is detected by a network of barometers on board balloons in the stratosphere for the first timeQuake magnitude and distance are estimated from the infrasound created by seismic body waves and surface wavesBalloons can act as mobile seismometer network in the atmosphere of Earth and Venus [ABSTRACT FROM AUTHOR]

The massive worldwide deglaciation leads to more frequent slope instabilities in mountainous terrains. The physical processes leading to such destabilizations are poorly constrained due to little monitoring of dynamic parameters at the local scale. Here we study a very large slow‐moving landslide (∼0.8 km2), on the flank of Tungnakvíslarjökull glacier in Iceland. Based on a combination of remote sensing images, we monitor the landslide and glacier kinematics over 75 years, with a focus over the period 1999–2019 when rapid glacier wastage has been observed. The landslide accelerates from 2 to 45 m/yr in the 6 years following a sudden increase in glacier mass loss. This acceleration coincides with intense quake activity (Mℓ < 2.8), recorded by a regional seismic network. We show that this seismicity is caused by the landslide sliding on a rough surface. The evolution of the quake magnitudes suggests a progressive segmentation of the landslide mass during its acceleration. Plain Language Summary: Glacier‐retreat may lead to a destabilization of adjacent slopes that can evolve into large landslides. The time‐scales, as well as the controlling factors of the landslide response to this glacier retreat are poorly known, due to few observations of landslide dynamics in the vicinity of glaciers over the long term. To fill this gap, we document the dynamics of a large landslide in Iceland, developing on the flank of Tungnakvíslarjökull glacier, that underwent strong retreat after 1994. Based on aerial images since 1945 and satellite images since 1987, we calculate the motion of the landslide through time. We show that the landslide accelerated from 2 to 45 m/yr in the 6 years following the onset of glacier retreat. This acceleration coincides with earthquakes of small magnitudes (Mℓ < 2.8), produced by the landslide motion. Their analysis suggests that the landslide mass is progressively fragmented during the acceleration. This study opens interesting perspectives for landslide studies by combining seismic records from regional networks and 3D motions from satellites. Key Points: We monitor the dynamics of a very large slow‐moving landslide over 75 years, developing on the flank of a glaciated valleyThe landslide kinematics is strongly controlled by the glacier retreat, and produce seismicity correlated with the landslide velocityThe acceleration is linked with progressive segmentation of the landslide mass [ABSTRACT FROM AUTHOR]

Ruptures of the largest earthquakes can last between a few seconds and several minutes. An early assessment of the final earthquake size is essential for early warning systems. However, it is still unclear when in the rupture history this final size can be predicted. Here we introduce a probabilistic view of rupture evolution ‐ how likely is the event to become large ‐ allowing for a clear and well‐founded answer with implications for earthquake physics and early warning. We apply our approach to real time magnitude estimation based on either moment rate functions or broadband teleseismic P arrivals. In both cases, we find strong and principled evidence against early rupture predictability because differentiation between differently sized ruptures only occurs once half of the rupture has been observed. Even then, it is impossible to foresee future asperities. Our results hint toward a universal initiation behavior for small and large ruptures. Plain Language Summary: Earthquakes are among the most destructive natural hazards known to humankind. While earthquakes can not be predicted, it is possible to record them in real‐time and provide warnings to locations that the shaking has not reached yet. Warning times usually range from few seconds to tens of seconds. For very large earthquakes, the rupture itself, which is the process sending out the seismic waves, can have a similar duration. Whether the final size of the earthquake, its magnitude, can be determined while the rupture is still ongoing is an open question. Here we show that this question is inherently probabilistic ‐ how likely is an event to become large? We develop a formulation of rupture predictability in terms of conditional probabilities and a framework for estimating these from data. We apply our approach to two observables: moment rate functions, describing the energy release over time during a rupture, and seismic waveforms at distances of several thousand kilometers. The final earthquake magnitude can only be predicted after the moment rate peak, at approximately half the event duration. Even then, it is impossible to foresee future subevents. Our results suggests that ruptures exhibit a universal initiation behavior, independent of their size. Key Points: We develop a probabilistic formulation of earthquake rupture predictability and estimate magnitude distributions given early observationsThe final magnitude of an event can not be determined before the peak moment release from moment rate functions or teleseismic waveformsIn multi‐asperity ruptures, the occurrence of future asperities can not be foreseen [ABSTRACT FROM AUTHOR]

We present a novel and robust method for estimating moment magnitudes (Mw) of large earthquakes with long‐period and long‐lasting coda energy. Fitting the energy with a simple decay model, we derive a straightforward relationship between the coda energy and the Mw. Tests with both real and synthetic data of 10 globally distributed large earthquakes (Mw > 8) verify the method and the results are stable and reliable even with a fast calculation of synthetics for a 1D model. Tests also show that the method is applicable for earthquakes with Mw above 7.5–8.0 with energy sufficiently greater than the ambient noise. The method removes or reduces the effects of geometric spreading, focal mechanism, source rupture process, and the actual Earth structure, making it advantageous for estimating the magnitudes of large earthquakes. The new long‐period coda moment magnitude (Mwo) estimations are similar to the conventional solutions but are slightly larger (by 0.04 on average). Plain Language Summary: A method is proposed to measure the size of a large earthquake using the very low‐frequency energy that lasts for a very long time (more than 10 hr) after the earthquake. Using only one parameter from a simple formula describing the decay of the energy, we connect the coda energy and the size of an earthquake. The method is reliable and stable because the coda energy is less affected by the details of the source properties and Earth structure. It is applicable to earthquakes with magnitude greater than 7.5–8.0. Key Points: A new method is proposed to measure moment magnitudes of large earthquakes with long‐period and long‐lasting coda energyGlobally recorded data and synthetic waveforms demonstrate the reliability and robustness of the methodThe method is advantageous for estimating large‐earthquake magnitudes with minimum influence from Earth structure and source complexities [ABSTRACT FROM AUTHOR]

Fractures within ice shelves are zones of weakness, which can deform on timescales from seconds to decades. Icequakes produced during the fracturing process show a higher b‐value in the Gutenberg‐Richter scaling relationship than continental earthquakes. We investigate icequakes on the east side of rift WR4 in the Ross Ice Shelf, Antarctica. Our model suggests a maximum icequake slip depth that is ∼7.8 m below the rift mélange, where the slip area can only grow laterally along the fracture planes. We propose ductile deformation below this depth, potentially due to the saturation of unfrozen water. We use remote sensing and geodetic tools to quantify surface movement on different timescales and find that the majority of icequakes occur during falling tides. The total seismic moment is <1% of the estimated geodetic moment during a tidal cycle. This study demonstrates the feasibility of using seismology and geodesy to investigate ice rift zone rheology. Plain Language Summary: Fractures located on ice shelves are weak compared to the rest of the ice shelf. They deform over seconds to decades, and vibrations that we refer to as "icequakes" can be accompanied by their deformation. We find that tides, particularly falling tides, influence the frequency of icequake occurrences the most. We also find that small magnitude icequakes are a larger proportion of total icequakes when compared to the proportion of small magnitude continental earthquakes in relation to total global earthquakes. We test whether this proportion is due to the maximum depth estimated at 7.8 m below the surface of the interior of the rift zone by using satellite imagery, Global Navigation Satellite Systems measurements, and a seismometer located near a fracture on the Ross Ice Shelf. We propose that the rift zone below 7.8 m depth behaves as a ductile deformation possibly due to saturation with unfrozen water, whereas the region above this depth is more prone to brittle fracture that can generate icequakes. Key Points: Along the rift WR4 in the Ross Ice Shelf, evidence suggests most icequakes are driven by falling tides than long‐term rift openingThe b‐value of icequakes in the Gutenberg‐Richter relationship is generally greater than that for continental earthquakesRift mélange is likely water saturated below ∼7.8 m depth, which limits the occurrence of higher magnitude icequakes [ABSTRACT FROM AUTHOR]

Rupture processes of global large earthquakes have been observed to exhibit great variability, whereas recent studies suggest that the average rupture behavior could be unexpectedly simple. To what extent do large earthquakes share common rupture characteristics? Here, we use a machine learning algorithm to derive a generic model of global earthquake source time functions. The model indicates that simple and homogeneous ruptures are pervasive whereas complex and irregular ruptures are relatively rare. Despite the standard long‐tail and near‐symmetric moment release processes, the model reveals two special rupture types: runaway earthquakes with weak growing phases and relatively abrupt termination, and complex earthquakes with all faulting mechanisms but mostly shallow origins (<40 km). The diversity of temporal moment release patterns imposes a limit on magnitude predictability in earthquake early warning. Our results present a panoptic view on the collective similarity and diversity in the rupture processes of global large earthquakes. Plain Language Summary: Over the past decades, seismologists have observed great variability in the rupture processes of many large earthquakes. However, some recent studies suggest that the average rupture behavior could be unexpectedly simple. Can the average behavior be representative of most earthquakes? To what extent do large earthquakes share common rupture characteristics? Here, we use machine learning to derive a panoptic picture, that is, a generic model of source time functions, for global earthquakes. The model shows that simple and homogeneous ruptures are pervasive whereas complex and irregular ruptures are relatively rare. Besides, it reveals two special rupture types: runaway earthquakes with weak initial phases, and complex earthquakes with all faulting mechanisms but mostly shallow origins (<40 km). Our results present a panoptic view on the collective similarity and diversity in the rupture processes of global large earthquakes, which affects how well we can predict earthquake magnitude in earthquake early warning. Key Points: A generic model of characteristic source time functions is derived from global earthquake observations using machine learningThe model presents a panoptic view of the similarity and the diversity in the rupture processes of large earthquakesThe diversity of moment release patterns, together with absolute duration variability, limits magnitude predictability in early warning [ABSTRACT FROM AUTHOR]

We investigate the preparatory phase of the 2009, Mw 6.1 L'Aquila earthquake in central Italy by analyzing the temporal evolution and spatial distribution of the seismic moment, M0, to radiated energy, ES. Our approach focuses on monitoring the deviations of the scaling between M0 and ES with respect to a model calibrated for the background seismicity. The temporal evolution of these deviations, defined as Energy Index (EI), identifies the onset of the activation phase 1 week before the mainshock. We show that foreshocks are characterized by a progressive increase in slip per unit stress, in agreement with the diffusion of highly pressurized fluids before the L'Aquila earthquake proposed by previous studies. Our results suggest that the largest events occur where energy index (EI) is highest, in agreement with the existing link between energy index (EI) and the mean loading stress. Plain Language Summary: Understanding how the machine generating large earthquakes works is one of the fundamental challenging scientific questions. Pioneering studies have shown that large magnitude earthquakes are sometimes anticipated by foreshocks and slip instabilities. Until now, retrospective analyses of foreshocks during the preparatory phase of large earthquakes have focused mainly on their distribution in space, time and magnitude. In this study, we show that also the dynamic properties of the rupture process associated to foreshocks evolve both in time and space. Our results indicate that the temporal evolution of radiated energy and size of small magnitude earthquakes provide useful information to identify the begin of the final activation phase of the 2009 Mw 6.1 L'Aquila, Italy, earthquake, in agreement with seismicity rate information. Our study suggests that, for the investigated case, foreshocks had dynamic characteristics distinct from those of normal rate (background) seismicity. Thus, it can contribute with new evidence to the open debate whether foreshocks are different phenomena from background seismicity. Key Points: We observe foreshocks dynamic characteristics before the 2009, Mw 6.1 L'Aquila earthquake in central ItalyDeviations of radiated energy over seismic moment from a reference model highlight the preparatory phase of the main eventForeshocks are characterized by a progressive increase in slip per unit stress, in agreement with diffusion of highly pressurized fluids [ABSTRACT FROM AUTHOR]

The southern Ryukyu subduction zone is one of the potential sources for tsunamigenic earthquakes. Despite a great seismic risk, the deformation pattern remains poorly known, primarily due to the absence of seafloor constraints. With GNSS‐acoustic measurements over years, we characterize the convergence rate across this margin growing from 92 mm/yr offshore eastern Taiwan to 123 mm/yr near the Gagua Ridge. The new data suggest the subduction interface is capable of hosting Mw 7.5–8.4 earthquakes. The orientations of seafloor movement and P‐axes in the Nanao Basin are both subnormal to the trench, notably deviate from the direction of plate convergence. By considering the combined effect of plate convergence and backarc rifting, different trends between the forearc convergence, P‐axes, and seafloor movement may indicate some degree of slip‐partitioning. The trench‐parallel component is likely accommodated in part by earthquakes near Taiwan, lower plate deformation, and strike‐slip faults within the accretionary wedge. Plain Language Summary: The southern Ryukyu margin near Taiwan accommodates the relative plate motion and backarc rifting. This region has suffered from tsunamis and large earthquakes with magnitudes of about eight in the history, yet insufficient offshore data still hamper our ability to fully characterize fault slip behaviors. We demonstrate how a seafloor geodetic technique, with the combination of the Global Navigation Satellite System and acoustic ranging, can help us constraining fault slip behaviors and assessing seismic risks. We infer a fraction of oblique plate motion is accommodated by the subduction interface capable of hosting earthquakes with moment magnitudes ranging from 7.5 to 8.4. Incorporating seismological observations, we find different trends of the compressive axes of earthquakes, seafloor movement, and the forearc convergence, which may indicate some degree of strain partitioning. The trench‐parallel motion is partially taken up by seismic slip near Taiwan, subducting plate deformation, and strike‐slip faults within the forearc. More near‐trench observations are crucial to assessing seismic hazard and evaluating strain partitioning along the southern Ryukyu Trench. Key Points: Rapid convergence across the southern Ryukyu margin increases eastward from 92 mm/yr offshore Taiwan to 123 mm/yr near the Gagua RidgeOblique convergence is accommodated in part by the plate interface fault capable of hosting earthquakes with Mw 7.5–8.4Different trends between the forearc convergence, P‐axes, and seafloor movement may indicate some degree of slip partitioning [ABSTRACT FROM AUTHOR]

The seismic moments observed for low‐frequency earthquakes (LFEs) vary over multiple orders of magnitude, even where the LFEs occur within families of similar events. Although this variability is typically interpreted to record a scale‐limited process at the LFE source, neither the slip per LFE nor the rupture area can be determined from seismological constraints. Here, we examine incrementally developed slickenfibers that have been proposed to record LFEs in exhumed subduction zones. These structures form through repeated, micron‐scale slip events across dilational irregularities in the fault plane, which are punctuated by cementation and sealing in the interstitial space. By statistically analyzing the geometry of inclusion trails delineating slip‐parallel mineral‐growth increments, we constrain the variability in slip per inferred LFE and test end‐member hypotheses regarding the controls on LFE moments. We find that that the slickenfibers exhibit characteristic slip increments, favoring a "slip‐limited" model that requires large variability in LFE rupture areas. Plain Language Summary: Low‐frequency earthquakes (LFEs) are a category of seismic phenomena only perceptible by geophysical instruments. Understanding what causes these events, however, may help us understand larger magnitude earthquakes occurring along plate boundaries. Unfortunately, the physical processes that generate LFEs at their source depth are poorly understood. Seismological studies have shown that energy release varies substantially from one LFE to another, for example, but the causes of this variability are unclear. Given that energy release is proportional to the area of LFE rupture and the slip during the LFE, a common conceptual model argues that the variability in LFE energy release results from a wide variation in slip amount occurring over a relatively constant area between events. We utilize a unique record of slip events extracted from geological structures (inferred to record ancient LFEs) to test competing models governing energy release at the LFE source. Using a statistical test that quantifies the variability of slip increment recorded in rock samples, we find that the amount of slip during individual inferred LFEs has a strong preferred value with little variation between events. This result indicates variations in LFE energy release are determined primarily by variations in the area over which the slip occurs. Key Points: We examine slip increments associated with LFE structures in geological exposuresStatistical analysis shows that the slicken fibers record characteristic slip incrementsResults indicate slip‐limited seismic moments during low frequency earthquakes [ABSTRACT FROM AUTHOR]

A devastating magnitude 7.2 earthquake struck Southern Haiti on 14 August 2021. The earthquake caused severe damage and over 2000 casualties. Resolving the earthquake rupture process can provide critical insights into hazard mitigation. Here we use integrated seismological analyses to obtain the rupture history of the 2021 earthquake. We find the earthquake first broke a blind thrust fault and then jumped to a disconnected strike‐slip fault. Neither of the fault configurations aligns with the left‐lateral tectonic boundary between the Caribbean and North American plates. The complex multi‐fault rupture may result from the oblique plate convergence in the region, so that the initial thrust rupture is due to the boundary‐normal compression and the following strike‐slip faulting originates from the Gonâve microplate block movement, orienting SW‐NE direction. The complex rupture development of the earthquake suggests that the regional deformation is accommodated by a network of segmented faults with diverse faulting conditions. Plain Language Summary: On 14 August 2021, a devastating magnitude 7.2 earthquake struck Southern Haiti, causing over 2,000 casualties and severe infrastructure damage. Southern Haiti situates in between the Caribbean and North American plates, where they converge obliquely at the boundary. The relative motion displaces the plates horizontally and accumulates stress along a major left‐lateral fault network. The oblique plate motion also causes an uplift of the region due to the boundary‐normal compression. Therefore, earthquakes in the region rupture in complex ways. However, the physical relations between the tectonic regime and the earthquake rupture development are poorly understood, posing challenges to local risk management. Here we use global seismic records to resolve the rupture history of the 2021 Haiti earthquake. We find the earthquake composed of two distinct rupture episodes: a reverse faulting subevent near the epicenter and a strike‐slip faulting subevent further west. Both subevents ruptured faults that deviate away from the left‐lateral geometry of the Enriquillo‐Plantain Garden fault zone. Our results show that the complex tectonic setting of the convergence boundary is imprinted in a segmented fault network with various distinct faulting styles, which may have been influenced by the local small‐scale plate fragmentation. Key Points: The M 7.2 2021 Haiti earthquake sequentially ruptured two disconnected thrust and strike‐slip faultsNeither the thrust nor strike‐slip fault aligns with the Enriquillo‐Plantain Garden fault configurationFaulting variability of the earthquake likely reflects the complex deformation partition at the tectonic boundary [ABSTRACT FROM AUTHOR]

Subduction zone architecture and properties are thought to control megathrust slip behavior, but few constraints on crustal structure and megathrust properties are available at sufficient resolution and depth, hindering understanding of linkages between structure and behavior. Here we present a P‐wave seismic velocity model based on wide‐angle seismic data integrated with collocated reflection imaging in the weakly coupled Shumagin Gap in the Alaska subduction zone, where a M7.8 occurred in July 2020. We show that this earthquake occurred near and below the Moho of the overriding plate where the megathrust is characterized by a 3‐ to 5‐km‐thick reflection band interpreted to represent tectonic mixing. The rheological heterogeneity of the plate boundary near and below the Moho could account for abundant interplate seismicity, repeated M7.x events, and patchiness of the 2020 rupture. Velocity variations in the overriding continental crust imply changes in rigidity that could further influence megathrust slip. Plain Language Summary: Slip on subduction zone plate boundary faults (megathrusts) produces large and destructive earthquakes, but many questions remain about what controls the size and character of these events. To address this question, we used seismic imaging data to determine subduction zone configuration and properties offshore Alaska in the area of a magnitude 7.8 earthquake that occurred in July 2020. We constrain the thickness of the crust overlying the megathrust, variations in strength within the crust, and changes in the total width and complexity of the fault zone. The M7.8 earthquake occurred on a part of the fault overlain by both continental crust and upper mantle, where imaging shows the fault zone may be particularly heterogeneous, which could explain the patchiness of slip during this event. Variations in strength of the overriding plate may also influence both shallow and deep fault slip in this subduction zone. Key Points: The megathrust intersects the continental Moho in the Shumagin Gap at ∼33 kmMegathrust heterogeneity near and below continental Moho could explain patchy slip in 2020 M7.8 event and abundant megathrust seismicityChanges in seismic velocity in the overriding crust imply changes in rigidity that could influence megathrust slip behavior [ABSTRACT FROM AUTHOR]

The state‐of‐stress within subducting oceanic plates controls rupture processes of deep intraslab earthquakes. However, little is known about how the large‐scale plate geometry and the stress regime relate to the physical nature of the deep intraslab earthquakes. Here we find, by using globally and locally observed seismic records, that the moment magnitude 7.3 2021 East Cape, New Zealand earthquake was driven by a combination of shallow trench‐normal extension and unexpectedly, deep trench‐parallel compression. We find multiple rupture episodes comprising a mixture of reverse, strike‐slip, and normal faulting. Reverse faulting due to the trench‐parallel compression is unexpected given the apparent subduction direction, so we require a differential buoyancy‐driven stress rotation, which contorts the slab near the edge of the Hikurangi plateau. Our finding highlights that buoyant features in subducting plates may cause diverse rupture behavior of intraslab earthquakes due to the resulting heterogeneous stress state within slabs. Plain Language Summary: A key type of tectonic boundary is where two plates collide with one sinking into the mantle beneath. These subduction zones generate the world's largest earthquakes. Quantifying stress in the subducting plate ("slab") is important because slabs drive the global plate‐tectonic system, and large earthquakes can occur within them. These earthquakes can cause strong shaking, and when occurring near cities, can lead to damage. However, mapping stress is challenging as we cannot directly "see" inside deep slabs. Our best indications of slab stress come from earthquakes themselves. A magnitude 7.3 earthquake north of New Zealand in 2021 generated a distinct pattern of seismic waveforms at seismometers installed worldwide. We used these seismic records to probe the earthquake, providing a new view of stress in subduction zones. We found the earthquake generated both vertical and horizontal motions along faults, driven by compressional and extensional stresses deep within the slab. The compressional part is oriented 90 degrees from the subduction direction, which is opposite to the usual compression in subduction zones. This unusual direction of compression can be explained by subduction of a thickened and buoyant part of the Pacific plate, known as the Hikurangi plateau. Key Points: A moment magnitude 7.3 2021 East Cape, New Zealand intraslab earthquake comprised multiple rupture episodes with different faulting stylesThe complex rupture comprises components of shallow trench‐normal extension and unexpectedly, deep trench‐parallel compression in slabThe trench‐parallel compression likely reflects stress rotation at a buoyancy contrast that drives slab contortion [ABSTRACT FROM AUTHOR]

Holocene marine terraces occur globally and record information about the timing and magnitude of past coseismic events. Staircased terraces develop through repetitive coseismic uplift of shore platforms, but are also subject to destruction from subsequent wave erosion and rock weathering. In this study we calibrate a rock coast evolution model using terrace field data from Mahia Peninsula, New Zealand, and use it to investigate how relative sea level (RSL) change influences Holocene terrace development. Analyses of 10,002 simulations reveal time periods of extremely rapid terrace creation and destruction as a result of shore platform development processes that are modulated both by episodic and gradual RSL change scenarios. Subtle differences in these scenarios give rise to completely different terrace sequences, even if coseismic event timing is held constant. Improved interpretation of Holocene terrace sequences require higher resolution paleo RSL data and chronological data on shore platform development. Plain Language Summary: Holocene marine terraces develop when tectonically active rock coasts are uplifted during earthquakes, and thus provide the information about the timing and magnitude of past seismic events that is useful to estimate future seismic hazards. However, the record is often partial because once terraces are uplifted, subsequent marine erosion processes (wave erosion and intertidal rock weathering) start destroying terraces and can completely remove them from the landscape. This can confound paleo‐seismic interpretations of Holocene terrace sequences. In this study we used a numerical model to explore how earthquake‐driven episodic and climate‐driven gradual relative sea level (RSL) changes can influence marine erosion processes, and therefore Holocene terrace preservation. The simulation results reveal that subtle differences in episodic and gradual RSL change histories can lead to periods in which marine erosion are greatly accelerated, which can completely alter terrace sequences that are preserved in the landscape. Until this study, we had little understanding of the way in which RSL modulates the erosion processes that control terrace creation and destruction. We conclude that better interpretation of past seismic event history from Holocene terraces require a more detailed understanding of RSL history and higher resolution data on rates of shore platform formation through time. Key Points: The effects of episodic and gradual relative sea level (RSL) change on Holocene marine terrace development are numerically exploredEven subtle differences in RSL have large impacts on shore platform development rates, thereby changing preserved terrace sequencesBetter understanding of past RSL and shore platform development history will improve paleo‐seismic interpretation of Holocene terraces [ABSTRACT FROM AUTHOR]

How earthquakes initiate is still a largely debated question in earthquake science. On the lab scale, rupture initiation is well studied, and detailed models of how rupture nucleation evolves have been developed. Contrarily, for real earthquakes, only a few high‐resolution observations of this process are available today, mostly limited to mainshock magnitudes > M5. Consequently, there is still no consensus on whether and how laboratory results can be transferred to real earthquakes. Here we show that rupture nucleation phenomena observed on the lab scale can also be imaged on the microearthquake‐scale with little instrumental effort. Our results highlight the potential of the applied analysis workflow to significantly improve the observation quality of seismicity patterns and immediate foreshock phenomena in microearthquake sequences. Our approach can help to narrow the existing observational gap to the lab scale and may contribute to a better understanding of the mechanisms of earthquake initiation in the future. Plain Language Summary: One of the key questions in seismology is how earthquakes start. Until now, many models of earthquake initiation have been established through laboratory experiments. But for real earthquakes, only a few detailed observations of rupture initiation exist because they are mainly restricted to large earthquakes (M > 5). Consequently, it is not known today if and how the results obtained in the laboratory can be transferred to real earthquakes. In this study, we show that rupture initiation can be studied with close‐to‐lab‐scale detail in foreshock sequences to small earthquakes (M > 2.5). Small earthquakes occur much more frequently than large ones ‐ generally 10 times more per unit decrease in earthquake size (i.e., magnitude). Therefore, our results indicate that the database of detailed rupture initiations in real earthquakes can be extended substantially. Our analysis method that generates decade‐long, high‐resolution, and consistent catalogs of sequences of very small earthquakes, helps to narrow the existing observational gap to the lab scale. In this way, it can contribute to a better understanding of earthquake initiation mechanisms in the future. Key Points: Observe precursory phenomena to ML ≤ 3.2 mainevents in a microseismic sequenceImage high‐resolution spatiotemporal evolution of the immediate foreshock zone of small earthquakesObserve lab‐scale‐like rupture initiation phenomena on the field scale with little instrumental effort similar to lab scale experiments [ABSTRACT FROM AUTHOR]

The possibility of controlling induced earthquake magnitude through managed metering of water injection has yet to be rationalized. Mechanisms of reducing magnitudes of induced events through cycled fluid injection remain unclear. To explore such mechanisms and this possibility, we report experiments with water injection into laboratory faults. Water injection results in early triggering for both single and cycled injection. However, the maximum moment magnitude and total energy of the repeating induced earthquakes during cycled water injection are both lower than those of induced earthquakes induced with continuous injection and for natural earthquakes without water injection. Higher permeability of the host reduces the number of injection‐induced earthquakes but increases their moment. With an increasing number of water injection cycles, both maximum moment and total energy decrease, particularly as permeability decreases, while the number of induced events increases. The moment magnitude of induced events can thereby be controlled through cycled fluid injection. Plain Language Summary: Fluid injection‐triggered earthquakes have been documented worldwide. Although it is clear that fluid pressure triggers fault slip and results in induced earthquakes, the possibility of controlling induced earthquakes through managed metering of water injection has not been rigorously examined. We performed experiments with single and cycled injection into laboratory faults. Our results show that cycled water injection into a fault could successfully reduce both the maximum magnitude of the individual induced seismic events and the total radiated energy—by setting appropriate water injection schedules. The magnitude and number of induced events are dependent on the number of water injection cycles and rock permeability. These results may provide a better understanding of managing and preventing large induced earthquakes. Key Points: Water injection results in early occurrence of the fault seismic slip for both single and cycled injectionCycled water injection could reduce both the maximum magnitude of the individual induced seismic events and the total released energyBoth maximum moment and total energy decrease with an increasing number of water injection cycles [ABSTRACT FROM AUTHOR]

Tsunamis from earthquakes of various magnitudes have affected Cascadia in the past. Simulations of Mw 7.5–9.2 earthquake constrained by earthquake rupture physics and geodetic locking models show that Mw ≥ 8.5 events initiating in the middle segments of the subduction zone can create coastal tsunami amplitudes comparable to those from the largest expected event. Our rupture and tsunami simulations reveal that the concave coastline geometry of the Pacific Northwest coastline focuses tsunami energy between latitudes 44° and 45° in Oregon. The possible coastal tsunami amplitudes are largely insensitive to the choice of slip model for a given magnitude. These results are useful for identifying the most hazardous segments of the subduction zone and demonstrate that a worst‐case rupture scenario does not uniquely yield the worst‐case tsunami scenario at a given location. Plain Language Summary: Offshore earthquakes along the Pacific Northwest coast of the U.S. and Canada (Cascadia region) can have magnitudes as high as 9.2, as was probably the case for an earthquake in the year 1700 CE that resulted in a large tsunami in Cascadia and across the Pacific Ocean. To learn more about the future tsunami hazard in the region, we design computer models of tsunamis from a wide range of earthquake scenarios. We find that almost regardless of the earthquake source details, events larger than magnitude 8.5 near the coast of Oregon can create large and widespread tsunamis along the US west coast. These are consequences of the geometry of offshore earthquake faulting and the concave shape of coastline in the region. Key Points: A Mw = 8.5 event in central Cascadia (Oregon) can create coastal tsunami amplitudes comparable to those from the largest possible eventThe concave coastline contributes to larger coastal tsunami amplitudes in central CascadiaThe choice of slip model does not significantly affect the distribution of coastal tsunami amplitudes in Cascadia [ABSTRACT FROM AUTHOR]

Strong strain and pore pressure changes are observed after three Mw 4.5+ local and one Mw 7.2 regional earthquake during 2010–2017 in borehole strainmeters near Anza, California. The strain change emerges immediately after the earthquakes and lasts 40–100 days with amplitudes up to 10−7, larger than the coseismic strain offsets. The pore pressure exhibits change immediately after the earthquakes at some boreholes and with a delay of 4–10 days at the others. A joint analysis of the observed postseismic strain and pore pressure change suggests that the postseismic strains could be explained by combined effects of poroelastic deformation due to earthquake‐induced pore pressure change and elastic deformation due to an earthquake‐triggered aseismic slip on a local fault. Our study indicates that, in addition to possible aseismic fault slips triggered by an earthquake, pore pressure changes after the earthquake could be even more important in producing postseismic deformation. Plain Language Summary: Understanding the physical mechanisms producing postseismic underground deformation is important for assessing fault slip budget and seismic hazards. In this study, we seek to clarify the possible roles of aseismic fault slip and underground water in producing postseismic deformation, through a joint analysis of underground deformation and pressure change in the pores of underground water reservoirs observed in the boreholes in southern California following four middle/large‐magnitude earthquakes. We find that both the underground deformation and pore pressure in the underground water reservoir exhibit changes lasting 1–3 months after the earthquakes, with changing amplitudes larger than the coseismic changes. These observations are well explained by a mechanism in which the mainshock earthquakes instantly trigger aseismic slips on the local faults and alter the hydrological conditions in the region; the change of hydrological condition results in a postseismic change of pore pressure in the underground water reservoir and produces poroelastic deformation in the region, while the aseismic fault slips produce elastic deformation. This study indicates that, in addition to possible aseismic fault slips triggered by an earthquake, changes of pore pressure after the earthquake in the underground water reservoir could play an even more important role in producing postseismic deformation. Key Points: We observe the strong months‐long change of strain and pore pressure after four Mw 4.5+ earthquakes in borehole strainmeters at Anza, CaliforniaThe postseismic strains last 40–100 days, and exhibit different trends and larger amplitudes up to 10−7 compared to coseismic strainsPostseismic strain = poroelastic strain by earthquake‐induced pore pressure change + elastic strain by an earthquake‐triggered aseismic slip [ABSTRACT FROM AUTHOR]

Three devastating earthquakes of MW ≥ 5.9 activated a complex system of high‐angle normal, antithetic, and sub‐horizontal detachment faults during the 2016–2017 central Italy seismic sequence. Waveform cross‐correlation based double‐difference location of nearly 400,000 aftershocks illuminate complex, fine‐scale structures of interacting fault zones. The Mt. Vettore–Mt. Bove (VB) normal fault exhibits wide and complex damage zones, including a system of bookshelf faults that intersects the detachment zone. In the Laga domain, a comparatively narrow, shallow dipping segment of the deep Mt. Gorzano fault progressively ruptures through the detachment zone in four subsequent MW ∼ 5.4 events. Reconstructed fault planes show that the detachment zone is fragmented in four sub‐horizontal, partly overlaying shear planes that correlated with the extent of the mainshock ruptures. We find a new, deep reaching seismic barrier that coincides with a bend in the VB fault and may play a role in controlling rupture evolution. Plain Language Summary: In 2016–2017, a sequence of three devastating earthquakes with magnitudes ∼6 near the towns of Amatrice, Visso, and Norcia in central Italy triggered hundreds of thousands of aftershocks. We compute high‐precision locations for these aftershocks that have the power to illuminate the fine‐scale structure of the complex system of intersecting and interacting faults that were activate during the sequence. The normal faults that ruptured during the three mainshocks are capped at ∼9 km depth by a sub‐horizontal detachment zone. The normal faults intersect the detachment at an acute angle, causing complex fault zone structures and fragmentation of the detachment where they intersect. We observe a deep reaching seismic barrier that we conclude may play a role in controlling rupture extent and evolution of large earthquakes along this section of the Apennines. In general, fault complexity here is higher than in similar sequences in 2009 near L'Aquila to the south and in 1997 near Colfiorito to the north. Key Points: Precise locations of 390,334 aftershocks of the 2016–2017 Central Italy sequence illuminate the complex normal fault systemA system of high‐angle bookshelf faults accommodates slip at the intersection between normal and detachment faultsA deep‐reaching seismic barrier is identified north of the Sibillini Thrust that may control the ruptures of large earthquakes [ABSTRACT FROM AUTHOR]

Aftershock cascades play an important role in forecasting seismicity in natural and human‐made situations. While their behavior including the spatial aftershock zone has been the focus of many studies in tectonic settings, this is not the case when fluid flows are involved. Using high‐quality seismic catalogs, we probe aftershocks dynamics in five settings influenced by fluids: (a) induced seismicity in Oklahoma and Kansas, (b) natural swarms in California and Nevada, and (c) suspected swarms in the Yuha Desert (California). All settings exhibit significant aftershock behavior highlighting the importance of event‐event triggering processes. The spatial aftershock zones scale with mainshock magnitude as expected based on the rupture length. While (a) and (b) show a rapid decay beyond their rupture length, (c) exhibits long‐range behavior suggesting that fluid migration might not be the dominant mechanism. We also find that the scaling of aftershock productivity with mainshock magnitude together with the Gutenberg‐Richter b‐value might allow to distinguish between natural swarms and induced seismicity. Plain Language Summary: While it is known that fluid injection operations can induce seismic activity, it has remained unclear how this activity compares to their natural counterpart, seismic swarms driven by natural fluid flows. The latter are typically characterized by the absence of a dominant event within the seismic sequence, while exhibiting other characteristics consistent with tectonic sequences including aftershock triggering. Our analysis of high‐quality seismic catalogs for both types of fluid‐driven seismicity shows that both exhibit a significant amount of aftershocks arising from "secondary" processes (i.e. stress‐based event‐event triggering as an indirect consequence of fluid injections) leading to spatially localized aftershocks zones. Yet, the trade‐off between the seismic productivity relation, which refers to the average increase in the number of aftershocks with the magnitude of their trigger, and the distribution of earthquake magnitudes controls the relative role of small compared to large triggers and we find that aftershock triggering is much more dominated by smaller events in the induced setting. Both findings are of direct importance for earthquake forecasting and seismic hazard assessment. Key Points: Significant event‐event triggering is present in both natural swarms and induced seismicityBoth fluid‐driven settings are characterized by narrow aftershock zonesAftershock triggering is dominated by smaller triggers in induced seismicity but much less so for natural swarms [ABSTRACT FROM AUTHOR]

Eight Ml ≥ 7.0 earthquakes have occurred around the Bayan Har block, NW Tibet, China, since 2000, resulting in a large number of casualties. Near the western boundary of the Bayan Har block, four Ml ≥ 6.0 Yutian earthquakes have occurred from 2008 to 2020. Stress interactions among them are comprehensively investigated by applying the ETAS (Epidemic‐Type Aftershock Sequence) model and calculating ΔCFS (Coulomb failure stress change) from a 3D linear viscoelastic mode. The combined (coseismic plus postseismic) ΔCFS induced by proceeding Yutian earthquakes on hypocenters of the 2012, 2014, and 2020 Yutian earthquakes are −1.5 × 10−4, 3.6 × 10−3, and 1.5 × 10−1 MPa, respectively. The background probabilities of the 2008, 2012, 2014, and 2020 Yutian earthquakes are 0.87, 0.97, 1.5 × 10−3, and 8.7 × 10−5, respectively. Combining those two independent approaches, we conclude that the 2008 and 2012 Yutian earthquakes are more like background earthquakes and that the 2014 and 2020 Yutian earthquakes were triggered by the proceeding Yutian earthquakes. Plain Language Summary: Along the boundaries of the Bayan Har block, NW Tibet, China, there have been eight large earthquakes since 2000, resulting in a large number of casualties and countless economic losses. At its western boundary (Yutian region), four earthquakes with local magnitudes larger than 6.0 have occurred since 2008. Whether these four large earthquakes were triggered is an important question to assess regional seismic hazards. We combine two methods to address this issue. One is the direct calculation of stress transfer and the other is the estimation of the probabilities of them being background events (an event is driven by tectonic loading). We find that the stress transfer from proceeding major earthquakes to the 2012 Yutian earthquake is smaller than the earthquake triggering threshold (0.01 MPa), while those of the 2014 and 2020 Yutian earthquakes were comparable with the triggering threshold. The background probabilities of the 2008 and 2012 Yutian earthquakes are high (close to 1.0), while the background probabilities of the 2014 and 2020 Yutian earthquakes are very low (close to 0.0). Thus, we conclude that the 2008 and 2012 Yutian earthquakes are more like background earthquakes and that the 2014 and 2020 Yutian earthquakes were triggered by the proceeding Yutian earthquakes. Key Points: Four Ml ≥ 6.0 have occurred on the western boundary of the Bayan Har block, NW Tibet, China, from 2008 to 2020The 2014 Yutian earthquake is the principal contribution to the occurrence of the 2020 Yutian event due to stress triggeringStatistical insights provide a good cross‐reference for the triggering mechanism due to small uncertainties [ABSTRACT FROM AUTHOR]

Two adjacent segments of the Chile margin exhibit significant differences in earthquake magnitude and rupture extents during the 1960 Valdivia and 2010 Maule earthquakes. We use the discrete element method (DEM) to simulate the upper plate as having an inner and outer wedge defined by different frictional domains along the décollement. We find that outer wedge width strongly influences coseismic slip distributions. We use the published peak slip magnitudes to pick best fit slip distributions and compare our models to geophysical constraints on outer wedge widths for the margins. We obtain reasonable fits to published slip distributions for the 2010 Maule rupture. Our best‐fit slip distribution for the 1960 Valdivia earthquake suggests that peak slip occurred close to the trench, differing from published models but being supported by new seismic interpretations along this margin. Finally, we also demonstrate that frictional conditions beneath the outer wedge can affect the coseismic slip distributions. Plain Language Summary: The south‐central Chilean margin is host to the Mw 9.5 1960 Valdivia earthquake and the Mw 8.8 2010 Maule earthquake. Although their earthquake rupture segments are adjacent and partially overlap, the resulting earthquakes are significantly different. We use the DEM to explore the controls on these differences. We use assemblages of discrete particles to simulate wedges that define the upper plate in two dimensions. Each wedge is partitioned into a strong inner wedge, capable of supporting large elastic strains that can be released during earthquakes, and a lower strength outer wedge that resists earthquake rupture. We simulate earthquake unloading by instantaneously reducing the basal friction beneath the inner wedge for a range of relative widths of the outer wedge. The results yield a suite of slip distributions that can be compared with natural earthquake ruptures, including the Chilean events as well as the 2011 Tohoku earthquake. The best‐fit slip distributions also provide constraints on the internal structure of the upper plate, which compares well to the interpreted structures in all three settings. Our simulations lead us to conclude that the 1960 Valdivia earthquake ruptured to the toe of the wedge, which differs from some previous interpretations. Key Points: Upper plate structure and associated variations in megathrust frictional properties influence earthquake size and fault slip distributionsNumerical simulations, validated by geophysical constraints, reproduce slip distributions for the 2010 Maule and 2011 Tohoku earthquakesThe 1960 Valdivia earthquake likely experienced its highest slip close to the trench, in contrast to published models [ABSTRACT FROM AUTHOR]