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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]

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]

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]

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]

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]

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]

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]

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]

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]

Foreshocks have been documented as preceding less than half of all mainshock earthquakes. These observations are difficult to reconcile with laboratory earthquake experiments and theoretical models of earthquake nucleation, which both suggest that foreshock activity should be nearly ubiquitous. Here we use a state‐of‐the‐art, high‐resolution earthquake catalog to study foreshock sequences of magnitude M4 and greater mainshocks in southern California from 2008–2017. This highly complete catalog provides a new opportunity to examine smaller magnitude precursory seismicity. Seventy‐two percent of mainshocks within this catalog are preceded by foreshock activity that is significantly elevated compared to the local background seismicity rate. Foreshock sequences vary in duration from several days to weeks, with a median of 16.6 days. The results suggest that foreshock occurrence in nature is more prevalent than previously thought and that our understanding of earthquake nucleation may improve in tandem with advances in our ability to detect small earthquakes. Plain Language Summary: Earthquakes often occur without warning or detectable precursors. Here we use a new, highly complete earthquake catalog to show that most mainshock earthquakes in southern California are preceded by elevated seismicity rates—foreshocks—in the days and weeks leading up to the event. Many of these foreshock earthquakes are small in magnitude and hence were previously undetected by the seismic network. These observations help bridge the gap between observations of real earth fault systems and laboratory earthquake experiments, where foreshock occurrence is commonly observed. Key Points: We analyze foreshock activity in a catalog of more than 1.8 million earthquakes in southern CaliforniaForeshock occurrence significantly exceeds the background seismicity rate for 72% of candidate mainshocksDurations of elevated foreshock activity range from days to weeks for these sequences [ABSTRACT FROM AUTHOR]