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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]

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]

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]

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]

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]

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]

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 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]

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]

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]

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]

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]

Recent studies have shown that the Himalayan region is under the threat of earthquakes of magnitude nine or larger. These estimates are based on comparisons of the geodetically inferred moment deficit rate with the seismicity of the region. However, these studies did not account for the physics of fault slip, specifically the influence of frictional barriers on earthquake rupture dynamics, which controls the extent and therefore the magnitude of large earthquakes. Here we combine an improved probabilistic estimate of moment deficit rate with results from dynamic models of the earthquake cycle to more fully assess the seismogenic potential of the Main Himalayan Thrust (MHT). We propose a straightforward and efficient methodology for incorporating outcomes of physics‐based earthquake cycle models into hazard estimates. We show that, accounting for uncertainties on the moment deficit rate, seismicity and earthquake physics, the MHT is prone to rupturing in Mw 8.7 earthquakes every T > 200 years. Plain Language Summary: Recent studies have shown that the Himalayan region is under the threat of earthquakes of magnitude nine or larger. To draw such estimates, researchers compare how surface deformation translates as potential slip on a fault with the rate at which measured earthquakes occur. In short, these studies only consider the kinematics of the region and compare current seismicity with how much earthquakes are to be expected in the future considering the statistics of earthquakes do not change with time. Therefore, these studies do not take advantage of the recent advances in fault physics. Here, we compare surface deformation rates, earthquake catalogs and numerical simulations of the earthquake cycle to assess the seismogenic potential of the Main Himalayan Thrust (MHT). Our methodology incorporates outcomes of physics‐based earthquake cycle simulations into hazard estimates. We show that, accounting for all available uncertainties, the MHT is prone to rupturing in Mw 8.7 earthquakes every T > 200 years. Key Points: Combine probabilistic estimate of moment deficit rate with results from earthquake cycle dynamic models to assess the seismogenic potentialStraightforward and efficient methodology for incorporating outcomes of physics‐based earthquake cycle models into hazard estimatesMain Himalayan Thrust prone to rupturing in M8.7, accounting for uncertainties on moment deficit rate, seismicity and earthquake physics [ABSTRACT FROM AUTHOR]

Extreme temperature and pressure conditions on the surface of Venus present formidable technological challenges against performing ground‐based seismology. Efficient coupling between the Venusian atmosphere and the solid planet theoretically allows the study of seismically generated acoustic waves using balloons in the upper atmosphere, where conditions are far more clement. However, earthquake detection from a balloon has never been demonstrated. We present the first detection of an earthquake from a balloon‐borne microbarometer near Ridgecrest, CA in July 2019 and include a detailed analysis of the dependence of seismic infrasound, as measured from a balloon on earthquake source parameters, topography, and crustal and atmospheric structure. Our comprehensive analysis of seismo‐acoustic phenomenology demonstrates that seismic activity is detectable from a high‐altitude platform on Earth, and that Rayleigh wave‐induced infrasound can be used to constrain subsurface velocities, paving the way for the detection and characterization of such signals on Venus. Plain Language Summary: The interior structure of Venus remains unknown due to lack of in situ seismic observations. Adverse temperature and pressure conditions on the Venusian surface limit the lifetimes of landers to a few hours, which poses a technological challenge against performing ground‐based seismology to detect venusquakes. Seismic energy on Venus, as on Earth, can be transmitted into the atmosphere through mechanical coupling and propagate as low‐frequency sound (infrasound). Infrasound from earthquakes travels long distances and has been detected from ground‐based stations. This mechanism may allow the detection of seismically generated pressure disturbances on Venus using balloons, enabling remote seismology from its upper atmosphere, where temperature and pressure conditions are far more clement and longer mission lifetimes are likely. However, the feasibility of such a technique has yet to be established through the detection of ground motion following an earthquake using a freely floating balloon. We demonstrate the first detection of an earthquake from a high‐altitude balloon. We explore the dependence of the pressure signal seen by the balloon on parameters such as the magnitude, focal mechanism, location of the earthquake, surface topography, and atmospheric structure. We also show how the signal recorded at the balloon can be used to study the subsurface. Key Points: First detection of a natural earthquake using balloon‐borne infrasound dataRayleigh wave‐induced infrasound dispersion characteristics provide constraints on subsurface velocitiesShallow waveguides, focal mechanism, and subwavelength topographic changes control infrasound amplitude and dispersion by weak earthquakes [ABSTRACT FROM AUTHOR]

Deep focus earthquakes within the Tonga‐Fiji subduction system account for about two‐thirds of the global total and provide significant constraints on slab deformation. The factors controlling the intense deformation remain unclear. Here, we use two‐dimensional, time‐dependent geodynamic models to study the morphology, stress state, and thermal structure of the Tonga‐Fiji subduction zone. The results, consistent with tomographic images and focal mechanisms, demonstrate that collision between a relic slab from the Vanuatu Trench and the Tonga slab may control the steeper dip of the Tonga slab and earthquakes in the mantle transition zone. We suggest that the magnitude 8.2 and 7.9 earthquakes in 2018 mostly ruptured within the warm rim of the Tonga slab and occurred beneath the folding relic slab with high temperatures of at least ∼900°C and ∼1100°C, respectively. The findings support the hypothesis that local slab temperature likely controls rupture of deep earthquakes. Plain Language Summary: Deep earthquakes at the depths between 300 and 700 km are controversial as the increase of temperature and pressure with depth will promote ductile flow and inhibit brittle failure. Approximately two‐thirds of the deep earthquakes globally are located below the North Fiji Basin which provides significant constraints on the deformation of the Tonga slab. However, the factors controlling the intense deformation remain unclear. Here, a time‐evolving mechanical model is used to study the morphology, state‐of‐stress, and thermal structure of the Tonga‐Fiji subduction zone. The models reproduce the high‐resolution Tonga slab morphology and can better fit the focal mechanisms of the deep earthquakes, which demonstrate that collision between a relic slab detached from the Vanuatu Trench and the Tonga slab leads to the nearly vertical Tonga slab and controls the occurrences of the deep earthquakes in the mantle transition zone. We suggest that two large deep earthquakes with moment magnitude (Mw) 8.2 and 7.9 in 2018 occurred within the warm rim of the Tonga slab and beneath the high‐temperature folded relic slab, respectively, supporting the hypothesis that local slab temperature probably controls the rupture details of deep earthquakes. Key Points: Geodynamic models reproduce the high‐resolution tomographic images of the Tonga slabCollision between a relic and Tonga slabs controls the intense deformation within the two slabsSimulated thermal structures predict deep earthquake doublet (Mw 8.2 and 7.9) occurs in the warm slab areas [ABSTRACT FROM AUTHOR]

Earthquake early warning (EEW) systems are required to report earthquake locations and magnitudes as quickly as possible before the damaging S wave arrival to mitigate seismic hazards. Deep learning techniques provide potential for extracting earthquake source information from full seismic waveforms instead of seismic phase picks. We developed a novel deep learning EEW system that utilizes fully convolutional networks to simultaneously detect earthquakes and estimate their source parameters from continuous seismic waveform streams. The system determines earthquake location and magnitude as soon as very few stations receive earthquake signals and evolutionarily improves the solutions by receiving continuous data. We apply the system to the 2016 M 6.0 Central Apennines, Italy Earthquake and its first‐week aftershocks. Earthquake locations and magnitudes can be reliably determined as early as 4 s after the earliest P phase, with mean error ranges of 8.5–4.7 km and 0.33–0.27, respectively. Plain Language Summary: Earthquake early warning (EEW) systems detect hazardous earthquakes, estimate their source parameters, and transmit warnings to the public. Conventional EEW algorithms depend on picking and analyzing the first seismic compressional wave (i.e., P wave). Seismic waveforms contain more information and can potentially be used to estimate earthquake source parameters with the fewest possible number of stations and to promptly transmit warning information. Deep learning techniques provide opportunities for extracting and exploiting the features behind seismic waveforms. In this study, we develop a fully automatic real‐time EEW system by directly mapping seismic waveform data to earthquake source parameters using deep learning techniques. We apply this system to the 2016 M 6.0 Central Apennines, Italy Earthquake and its first‐week aftershocks. Our results show EEW can be reliably issued as early as 4 s after the earliest P arrival. Key Points: A fully convolutional network is designed for real‐time earthquake detection, location, and magnitude estimationEarthquake locations and magnitudes can be determined as early as a few seconds of earthquake signals received at very few stationsThe system evolutionarily improves and updates earthquake source parameters by receiving continuous data [ABSTRACT FROM AUTHOR]

The 2019 M6.4 Searles Valley, Southern California, earthquake nucleated on a buried right‐lateral fault segment and propagated around a perpendicular fault intersection to a surface‐outcropping left‐lateral segment, but it did not propagate coseismically to the intersecting fault of the subsequent M7.1 Ridgecrest mainshock. We use the three dimensional finite element method to explore the physical reasons for this curious rupture path. Rather than model the details of the measured and inferred slip distribution, we use simple constant‐traction assumptions to explore how initial stress, hypocenter location, and the depth of burial of the initial right‐lateral segment may have influenced rupture propagation. The results suggest that only a narrow range of fault stresses, fault burial depths, and hypocenter locations would result in the observed rupture path in this earthquake. The results may have implications for interactions between faults in Southern California and beyond. Plain Language Summary: The magnitude 6.4 Searles Valley earthquake in the desert of Southern California happened only 36 h before a larger magnitude 7.1 Ridgecrest earthquake in almost the same location, with the two fault systems intersecting each other. The earlier earthquake ruptured two perpendicular fault segments, but somehow did not continue onto the also perpendicular magnitude 7.1 fault system. This work investigates the physical reasons why the earlier magnitude 6.4 earthquake took such a curious rupture path. We use three‐dimensional dynamic models to construct a number of different earthquake scenarios for the magnitude 6.4 earthquake under different assumptions about the overall stress level of the fault system, the earthquake nucleation location, and the depth of burial of the initial fault segment. We find that only a narrow range of these parameters results in an earthquake rupture path like the observed one. Our models also help shed light on the complex physical interactions that led to the observed earthquake behavior. The results may have broader implications for earthquakes on geometrically complex fault systems. Key Points: We model the rupture path of the 2019 M6.4 Searles Valley earthquake with its lack of immediate triggering of the M7.1 Ridgecrest earthquakeThe observed rupture path requires the right‐lateral fault to have a shallow burial depth, and a narrow range of stress on all segmentsThe results imply that it would have been difficult to predict the rupture path of this earthquake prior to its occurrence [ABSTRACT FROM AUTHOR]

Until the Mw 6.8 Elazığ earthquake ruptured the central portion of the East Anatolian Fault (EAF, Turkey) on January 24, 2020, the region had only experienced moderate magnitude (Mw < 6.2) earthquakes over the last century. We use geodetic data to constrain a model of subsurface fault slip. We adopt an unregularized Bayesian sampling approach relying solely on physically justifiable prior information and account for uncertainties in both the assumed elastic structure and fault geometry. The rupture of the Elazığ earthquake was mostly unilateral, with two primary disconnected regions of slip. This rupture pattern may be controlled by structural complexity. Both the Elazığ and 2010 Mw 6.1 Kovancılar events ruptured portions of the central EAF that are believed to be coupled during interseismic periods, and the Palu segment is the last portion of the EAF showing a large fault slip deficit which has not yet ruptured in the last 145 years. Plain Language Summary: The Elazığ earthquake ruptured the central portion of the East Anatolian Fault (EAF), a major strike‐slip fault in eastern Turkey, on January 24, 2020. Before this event, the region had only experienced moderate magnitude earthquakes over the last century. We aim at understanding the rupture of this earthquake, and how it relates to the historical ruptures of the EAF. To do so, we use measurements of displacement at the surface to image the subsurface slip on the fault that occurred during the earthquake. As the characteristics of the crust are poorly known, we make realistic assumptions on the fault geometry and Earth structure, and build on novel approaches to account for the possible biases of our assumptions and to characterize the uncertainties of the imaged slip. We suggest that the Elazığ earthquake rupture may be controlled by structural complexity of the fault, and that two main regions of slip surround a fault bend acting as a barrier to rupture propagation. We also suggest that the fault segment located between Lake Hazar and the city of Palu is the last portion of the central EAF, showing a large deficit of the fault slip, which has not yet ruptured in the last 145 years. Key Points: We infer a stochastic model for the distribution of subsurface fault slip associated with the 2020 Elazığ earthquakeWe account for uncertainties in both the depth‐dependence of the assumed elastic structure and the location and geometry of the faultOur models are characterized by two primary patches of fault slip where distribution appears to be controlled by geometrical complexities [ABSTRACT FROM AUTHOR]

Aseismic deformation has been suggested as a mechanism to release the accumulated strain in rifts. However, the fraction and the spatial distribution of the aseismic strain are poorly constrained during amagmatic episodes. Using Sentinel‐1 interferograms, we identify the surface deformation associated with the 2014 Mw5.2 Karonga earthquake, Malawi, and perform inversions for fault geometry. We also analyze aftershocks and find a variety of source mechanisms within short timescales. A significant discrepancy in the earthquake depth determined by geodesy (3–6 km) and seismology (11–13 km) exists, although both methods indicate Mw5.2. We propose that the surface deformation is caused by aseismic slip from a shallow depth. This vertical partitioning from seismic to aseismic strain is accommodated by intersecting dilatational faults in the shallow upper crust and sedimentary basin, highlighting the importance of considering aseismic deformation in active tectonics and time‐averaged strain patterns, even in rifts with little volcanism. Plain Language Summary: During the early stages of continental rifting, some accumulated tectonic energy is released with little or no earthquake activity (i.e., aseismic strain). This is most often observed in places where magma intrusion occurs. However, in rift basins lacking evidence of magma intrusion, how much and where the aseismic strain is released remains largely unknown. We investigate a magnitude 5.2 earthquake that occurred in 2014 near Karonga, Malawi, using both local seismic data and satellite ground deformation data. Our analyses show that although the main earthquake and other aftershocks ruptured at ∼12 km deep, the surface deformation is caused by an aseismic source from a shallower region (3–6 km deep). The aftershocks occur seconds to minutes apart and have different rupture sources, suggesting a highly damaged zone. The depth discrepancy between earthquake locations and the aseismic energy source might be caused by a weak, damaged layer where many faults intersect. Since intersecting faults are common in rift systems, our study suggests that this depth discrepancy might be common as well, indicating a large portion of energy released as aseismic strain in a continental rifting system. Key Points: InSAR implies deformation from a shallow depth (3–6 km), while seismicity suggests a deeper origin (11–13 km)Aftershock source mechanisms suggest fault intersection along steep east and west dipping fault planesFractured zone might separate shallow aseismic and deeper seismic strain beneath sedimentary basin [ABSTRACT FROM AUTHOR]

The eastern portion of the Shumagin gap along the Alaska Peninsula ruptured in an MW 7.8 thrust earthquake on 22 July 2020. The megathrust fault space‐time slip history is determined by joint inversion of regional and teleseismic waveform data along with co‐seismic static Global Navigation Satellite System (GNSS) displacements. The rupture expanded westward and along‐dip from the hypocenter, located adjacent to the 1938 MW 8.2 Alaska earthquake, with slip and aftershocks extending into the gap about 180 to 205 km, respectively, at depths from 15 to 40 km. The deeper half of ~75% of the Shumagin gap experienced faulting. However, the patchy slip is significantly less than possible accumulated slip since the region's last major rupture in 1917, compatible with geodetic seismic‐coupling estimates of 10‐40% beneath the Shumagin Islands. The rupture terminated in the western region of very low seismic coupling. There was a regional decade‐scale decrease in b‐value prior to the 2020 event. Plain Language Summary: A large MW 7.8 underthrusting earthquake ruptured on the plate boundary between the Pacific and North American Plates along the Alaska Peninsula on 22 July 2020. The fault slipped in the region between large plate boundary earthquakes in 1938 and 1946 that has been called the Shumagin seismic gap. The last major earthquake on this part of the plate boundary occurred in 1917, with both that event and the 2020 earthquake rupturing the eastern half of the Shumagin gap. The slip was concentrated in several patches, with the largest slip patch located directly below the Shumagin Islands. Global Positioning Satellite (GPS) stations on the islands recorded the ground displacement clearly. Using GPS displacements along with seismic wave ground motions recorded by regional strong motion and global broadband stations, the slip space‐time history during the faulting is determined. The coseismic slip is significantly less than the amount that could have accumulated from relative plate motions since 1917, which is consistent with geodetic estimates of low seismic coupling on this portion of the plate boundary. There was a regional decrease in the slope (b‐value) of the earthquake magnitude distribution over the decade before the event. Key Points: The 22 July 2020 MW 7.8 thrust faulting earthquake is the largest in the partially coupled eastern Shumagin gap since 1917Inversion of seismic and GNSS data for the slip history indicates about approximately five patches of 1–3.2 m slip distributed from 15 to 40 km deepA gradual decrease in b‐value in the source region occurred over the decade prior to the 2020 Shumagin earthquake [ABSTRACT FROM AUTHOR]

We address geometric and kinematic properties of the Mw 6.3 26 November 2019 Durrës earthquake, the strongest earthquake in Albania in the past 40 years. Using coseismic surface displacements from Sentinel‐1 Differential Interferometric Synthetic Aperture Radar (DInSAR) and nearby Global Navigational Satellite System (GNSS) stations, we invert for the geometry and slip of the causative fault. We find that both a steep SW‐dipping fault (dip 71°) and a shallow NE‐dipping fault (dip 15°) can fit the data equally well. However, the slip on the SW‐dipping fault occurs at depths 11–23 km, similar to the depths of the mainshock and aftershock seismicity, and thus, we prefer that model. The location of our preferred fault plane correlates with the mapped SW‐dipping backthrust, the Vore fault. The fault rupture did not reach the surface, which implies that an updip stress propagation onto the unruptured shallow portion of the Vore fault and its secondary structures pose an increased seismic hazard for cities in Albania, including the capital, Tirana. Plain Language Summary: The magnitude 6.3 earthquake near Durrës, Albania, on 26 November 2019 was the largest earthquake in the country for over 40 years. It caused 51 deaths and damaged over 2,000 buildings in Durrës and the capital city Tirana. The earthquake occurred below the surface, and it was not immediately clear in the aftermath which fault it occurred on. We investigated that question using a combination of satellite observation techniques: DInSAR (a radar method that maps movements of the ground in one dimension over the large area) and GNSS (observations of three‐dimensional movements of the ground at specific locations). Out of two possibilities, we prefer a model in which the earthquake occurred on a fault that steeply dips (tilts) to the southwest, between 11‐ and 23‐km depth, agreeing with the depths of the mainshock and aftershocks from seismology. This fault, the Vore fault, is partly mapped at the surface and runs close to Tirana. The upper 11 km of the Vore fault and its hanging wall structures did not move in this earthquake, and therefore, they could still sustain a damaging earthquake in the future, threatening Tirana and other cities in northwestern Albania. Key Points: Geodetic source modeling of the Mw 6.3 2019 Durrës earthquake is performed based on Sentinel‐1 DInSAR and GNSS observationsA SW‐dipping fault‐plane agrees better with the seismic source parameters, depths, and locations of mainshock and aftershocksRupture did not reach the surface, and unruptured part of fault potentially poses elevated seismic hazard for the Albanian capital Tirana [ABSTRACT FROM AUTHOR]

We attempt to clarify processes associated with the 2019 Ridgecrest earthquake sequence by analyzing space‐time variations of seismicity, potency values, and focal mechanisms of earthquakes leading to and during the sequence. Over the 20 years before the Mw7.1 mainshock, the percentage of normal faulting events decreased gradually from 25% to below 10%, indicating a long‐term increase of shear stress. The Mw6.4 and Mw7.1 ruptures terminated at areas with strong changes of seismic velocity or intersections with other faults producing arresting barriers. The aftershocks are characterized by highly diverse focal mechanisms and produced volumetric brittle deformation concentrated in a 5–10 km wide zone around the main ruptures. Early aftershocks of the Mw7.1 event extended over a wide area below typical seismogenic depth, consistent with a transient deepening of the brittle‐ductile transition. The Ridgecrest earthquake sequence produced considerable rock damage in the surrounding crust including below the nominal seismogenic zone. Plain Language Summary: The Eastern California Shear Zone is one of the seismically most active regions in Southern California and hosted in the last few decades several large earthquakes. The most recent of these is the 5 July 2019, Ridgecrest earthquake with magnitude 7.1, which was followed by a vigorous aftershock sequence. To clarify processes associated with the Ridgecrest earthquake sequence, we analyze properties of earthquakes before, during and after the 2019 Ridgecrest mainshock. The fraction of normal faulting events was reduced gradually in the 20 years before the mainshock, indicating a long‐term increase of shear stress. The Ridgecrest earthquake and a moderate event with magnitude 6.4 about 34 hr earlier terminated in areas with high seismic velocity or intersections with other faults, which might act as barriers that arrested the ruptures. The aftershocks following the Ridgecrest mainshock have highly diverse mechanisms and are widely distributed within 5–10 km wide zone around the main rupture. Many of the early aftershocks are deeper than the regular seismogenic zone. The results highlight the strongly heterogeneous volumetric nature of crustal deformation during large earthquakes off main plate‐boundary faults. Key Points: The fraction of normal faulting events in the area dropped in the past 20 years from >25% to <10%, implying increasing shear stressThe Mw6.4 and Mw7.1 events terminated at areas with strong changes of seismic velocity or junctions with other faults acting as barriersAftershocks with diverse mechanisms produced significant potency in a 5–10 km wide zone, including deeper‐than‐usual early events [ABSTRACT FROM AUTHOR]

Asperities are patches where the fault surfaces stick until they break in earthquakes. Locating asperities and understanding their causes in subduction zones is challenging because they are generally located offshore. We use seismicity, interseismic and coseismic slip, and the residual gravity field to map the asperity responsible for the 2014 M8.1 Iquique, Chile, earthquake. For several years prior to the mainshock, seismicity occurred exclusively downdip of the asperity. Two weeks before the mainshock, a series of foreshocks first broke the upper plate then the updip rim of the asperity. This seismicity formed a ring around the slip patch (asperity) that later ruptured in the mainshock. The asperity correlated both with high interseismic locking and a circular gravity low, suggesting that it is controlled by geologic structure. Most features of the spatiotemporal seismicity pattern can be explained by a mechanical model in which a single asperity is stressed by relative plate motion. Plain Language Summary: At the northern Chile subduction zone, where the oceanic Nazca plate glides several centimeters per year under the South American continent, the two plates got stuck at an irregularity. This irregularity got stressed for more than 100 years until it broke in 2014 in a magnitude 8.1 earthquake. We mapped small earthquakes in the years preceding the large earthquake. We found that small earthquakes formed a ring around the irregularity. First, a half circle formed along the lower side of the irregularity, probably, because the oceanic Nazca plate is pulled down by its own weight and is thus mainly stressing along the lower side. Then, 2 weeks before the largest earthquake, a series of smaller earthquakes closed the circle around the upper side of the irregularity. Finding such irregularities between tectonic plates is important to assess earthquake risk. We also found that the irregularity can be seen in data derived from Earth's gravity field, hinting that it is due to geologic structure. Key Points: Seismicity in the years prior to and immediately before the mainshock form two halves of a "Mogi Doughnut" surrounding the main slip patchMechanical asperity model predicts stress increase where earthquakes are observedSlip region correlates with high interseismic locking and a circular gravity low, suggesting that the asperity is controlled by geologic structure [ABSTRACT FROM AUTHOR]

Paleoseismic rupture histories provide spatiotemporal models of earthquake moment release needed to test numerical models and lengthen the instrumental catalog. We develop a model of the fewest and thus largest magnitude earthquakes permitted by paleoseismic data for the last 1,500 years on the southern San Andreas and San Jacinto Faults, California, USA. The largest geometric complexity appears to regulate the system: Only two ruptures break the San Gorgonio Pass region, followed by episodes of ruptures that could bridge the northern San Jacinto Fault and the San Andreas Fault. When tested against independent data on slip per event, the model produces comparable values indicating the end‐member model does not underpredict rupture rates. Rupture of >85% of the fault length in the historic period between 1800 and 1857 and the subsequent quiescence is similar to epochs of activity in the prehistoric model, suggesting that regional clustering of seismicity could be a trait of the system. Plain Language Summary: Large, damaging earthquakes are rare, so geologic data are essential to supplement the record of earthquakes from seismographs. We use the geologic record to determine the magnitude and timing of past earthquakes on the San Andreas and San Jacinto Faults. A pattern emerges in which the biggest bends and splays control the length of most ruptures. Because the model estimates the largest‐possible earthquakes on this system, it is useful as an upper limit on earthquake size used in engineering and earthquake forecasting. Key Points: End‐member model examines the fewest, but longest, ruptures required by ages of ground‐rupturing earthquakes from 31 paleoseismic sitesRegional clustering emerges, with largest moment release ruptures similar to the historic record with ruptures breaking much of the systemRuptures through San Gorgonio Pass are infrequent, and anticorrelated with episodes when fault sections near Cajon Pass are active [ABSTRACT FROM AUTHOR]

Seismicity in the Raton Basin over the past two decades suggests reactivation of basement faults due to waste‐water injection. In the summer of 2018, 96 short period three‐component nodal instruments were installed in a highly active region of the basin for a month. A machine‐learning based phase picker (PhaseNet) was adopted and identified millions of picks, which were associated into events using an automated algorithm—REAL (Rapid Earthquake Association and Location). After hypocenter relocation with hypoDD, the earthquake catalog contains 9,259 ML −2.2 to 3 earthquakes focused at depths of 4–6 km. Magnitude of completeness (Mc) varies from −1 at nighttime to −0.5 in daytime, likely reflecting noise variation modulated by wind. The clustered hypocenters with variable depths and focal mechanisms suggest a complex network of basement faults. Frequency‐magnitude statistics and the spatiotemporal evolution of seismicity are comparable to tectonic systems. Plain Language Summary: Earthquakes induced by waste‐water injection are widely observed worldwide and have been occurring in the Raton Basin (located at the border of New Mexico and Colorado) for two decades. We deployed 96 short period seismic stations in the summer of 2018 to investigate the faults in the southern section of Raton Basin. Earthquake detection was performed with state‐of‐the‐art machine‐learning techniques, which led to a catalog with ~10,000 earthquakes with magnitude ranging from −2.2 to 3. Clusters of earthquakes were investigated in detail. We found that the orientation of the faults varies within the study region and that induced earthquakes may exhibit spatial‐temporal‐magnitude clustering just like tectonic ones. Successful application of the automated catalog‐building workflow also sheds light on the power of "hands‐free" processing of large‐volume seismic data. Key Points: A machine‐learning phase picker and dense nodal array enabled location of ~10,000 earthquakes in a monthHypocenter patterns and moment tensors vary among clusters, unveiling reactivation of complex basement faultsRaton Basin seismicity exhibits frequency‐magnitude distribution and spatiotemporal evolution comparable to tectonic events [ABSTRACT FROM AUTHOR]

Earthquake early warning would be improved if the final size of an ongoing earthquake could be predicted early in the rupture process. Previous research relies largely on parameters derived from seismic waveforms, resulting in widely varied estimates of how much of the rupture must be complete before final sizes can be predicted. We demonstrate here that incorporating prior information on along‐strike variation in source fault structural maturity helps improve constraints on the earthquake's final size. Using surface slip profiles from 26 large continental earthquakes worldwide, we derive a generic empirical relation between earthquake slip, rupture length, and along‐strike variability in source fault maturity. Using this equation, we fit successive slip profiles developing during each earthquake and find that, on average, we predict the final length and magnitude of an earthquake once it has reached ~20% of its total length. Our findings thus demonstrate a clear determinism in the rupture process. Plain Language Summary: When an earthquake initiates, an important question is whether and when we can anticipate what its final size will be. Previous studies have searched the answer to this question in seismic data from the earthquake alone. Here we suggest that we should incorporate additional information on the tectonic source fault that is rupturing. Based on earthquake slip‐length functions measured at the ground surface, we develop a generic empirical equation between earthquake slip, rupture length, and along‐strike changes in source fault structural maturity, for continental earthquakes. We show that applying this relationship to evolving earthquake slip profiles improves how quickly earthquake length and magnitude can be determined. These findings demonstrate that part of the rupture process is controlled by the physical properties of the source faults and may have important implications for earthquake early warning. Key Points: We parameterize a generic relationship between along‐strike fault maturity, coseismic slip, and final rupture lengthUsing fault maturity as a prior improves earthquake magnitude and rupture length predictions as needed for earthquake early warningWe predict final length and magnitude once earthquake rupture has reached ~20% of its total length, what shows a clear determinism in the rupture process [ABSTRACT FROM AUTHOR]

An outstanding question for induced seismicity is whether the volume of injected fluid and/or the spatial extent of the resulting pore pressure and stress perturbations limit rupture size. We simulate ruptures with and without injection‐induced pore pressure perturbations, using 2‐D dynamic rupture simulations on rough faults. Ruptures are not necessarily limited by pressure perturbations when (1) background shear stress is above a critical value, or (2) pore pressure is high. Both conditions depend on fault roughness. Stress heterogeneity from fault roughness primarily determines where ruptures stop; pore pressure has a secondary effect. Ruptures may be limited by fluid volume or pressure perturbation extent when background stress and fault roughness are low, and the maximum pore pressure perturbation is less than 10% of the background effective normal stress. Future work should combine our methodology with simulation of the loading, injection, and nucleation phases to improve understanding of injection‐induced ruptures. Plain Language Summary: Earthquakes can be induced or triggered by fluid injected deep underground, if the fluid encounters faults. Previous studies of induced seismicity at different injection sites around the world have empirically found that in many cases the maximum magnitude earthquake is found to scale with total volume of injected fluid. However, this is not always the case, and the level and heterogeneity of preexisting stress on faults likely plays an important role in determining the final earthquake size. In this paper, we use numerical simulations of earthquakes to quantify one source of stress heterogeneity—that arising from geometric roughness—and study how changes in pore pressure and stress from fluid injection interact with preexisting stress to influence earthquake size. We find that earthquakes are not limited by the injected volume, except under specific conditions. Instead, earthquakes stop where preexisting conditions are unfavorable for continued rupture; in our case because of bends in the fault geometry. Earthquakes can well exceed the predicted maximum magnitude, depending on the preexisting stress on the fault, how rough it is, and the magnitude and extent of the perturbation from injection. Key Points: Rupture size is not necessarily limited by the volume of injected fluid in earthquake simulations with imposed pore pressure perturbationsStress heterogeneity arising from geometric roughness may be the primary cause for rupture termination on rough faultsHigh initial shear stress or pore pressure can trigger a rupture larger than the volume‐based magnitude limit on faults with low roughness [ABSTRACT FROM AUTHOR]

Earthquakes caused by human activities are a growing societal concern. A pioneering study of induced seismicity in the 1970s involved cooperation between the U.S. Geological Survey and Chevron, Inc., to adjust operations in the Rangely oil field, Colorado, to initiate and control earthquakes. Here we revisit this experiment using coupled flow and geomechanical modeling to determine the physical mechanisms that caused these earthquakes and investigate the strength and state of stress acting on the fault that sourced them. The controlled nature of the Rangely experiment, combined with data from decades of field operations, presents a unique opportunity to understand the phenomenon of induced seismicity in ways that can be applied to manage these risks. Plain Language Summary: We revisit the classic field experiment conducted in the Rangely oil field, Colorado, to initiate and control the occurrence of earthquakes. Our study applies state‐of‐the‐art numerical modeling to investigate the physical mechanisms that caused these events. In addition, knowledge about the structure and state of stress at Rangely from decades of field operations enables us to determine the strength of the fault and the relative contributions of tectonic versus anthropogenic stress that led to these earthquakes. These insights suggest that modest perturbations to stress fields caused by field operations can trigger seismicity on long dormant faults that are far from active tectonic plate boundaries, which has important implications for assessing and managing risks associated with induced and triggered seismicity. Key Points: Our work revisits the classic experiment in earthquake control at Rangely, Colorado, to determine the physical mechanism that caused these earthquakesThe coefficient of friction for the earthquake‐sourcing fault at Rangely experiment is inferred to be ~0.70, consistent with Byerlee valuesWe define the tectonic and anthropogenic stress contributions that caused these events, thus distinguishing "Induced" versus "Triggered" seismicity [ABSTRACT FROM AUTHOR]

We analyze source characteristics of global, deep‐focus (>350 km) earthquakes with moment magnitudes (Mw) larger than 6.0–8.2 using teleseismic P‐wave and S‐wave spectra and an empirical Green's functions approach. We estimate the corner frequency assuming Brune's source model and calculate stress drops assuming a circular crack model. Based on P‐wave and S‐wave spectra, the one standard deviation ranges are 3.5–369.8 and 8.2–328.9 MPa, respectively. Based on the P‐wave analysis, the median of our stress drop estimates is about a factor of 10 higher than the median stress drop of shallow earthquakes with the same magnitude estimated by Allmann and Shearer (2009, https://doi.org/10.1029/2008JB005821). This suggests that, on average, the shear stress of deep faults in the mantle transition zone is an order of magnitude higher than the shear stress of faults in the crust. The wide range of stress drops implies coexistence of multiple physical mechanisms. Plain Language Summary: The change of shear stress (i.e., stress drop) during an earthquake is thought to be larger for deeper earthquakes than shallow earthquakes because of higher overburden pressure. However, the observational evidence for stress drop dependence on depth is still inconclusive. We estimate stress drops of earthquakes deeper than 400 km from recorded ground motion spectra. We find that the median stress drop of deep earthquakes is about one order of magnitude higher than the stress drop of shallow (<50 km) earthquakes. This implies that the shear stress of deep faults is moderately higher than of faults in the crust. The wide range of our stress drop estimates suggests that various mechanisms producing deep earthquakes coexist. Key Points: EmpiricalGreen's functions are applied to analyze stress drops of deep‐focus earthquakesOne standard deviation ranges are 3.5–369.8 MPa for P waves and 8.2–328.9 MPa for S wavesThe median stress drops suggest that fault shear stress is an order of magnitude higher in the mantle than in the crust [ABSTRACT FROM AUTHOR]