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In the Nazca‐South American subduction zone, the subducted slab is flattened beneath northern Andes, called the Peruvian flat slab. The 2019 Mw 8.0 northern Peru intermediate‐depth normal‐faulting earthquake occurred at the leading edge of the Peruvian flat slab, where the slab rebends and sinks into greater depths. Here we investigate this earthquake by back projection analysis and finite fault inversion using seismic waveforms at teleseismic distances. The rupture process indicates that this earthquake ruptured mainly along strike (353°) ~150 km north‐northwestward within ~55 s (average rupture velocity ~2.7 km/s), resulting in two major slip areas with three high slip rate areas, which are consistent with three high‐frequency energy radiation subevents. Our study suggests that such a heterogeneous rupture may be caused by slab bending forces and dehydration embrittlement associated with morphology of the slab. Plain Language Summary: In the subduction zone, the oceanic plate and the continental plate converge, and then the oceanic plate subducts beneath the continental plate and sinks into great depths. In South America, the subducted oceanic plate is flattened beneath northern Peru instead of steeply sinking, called the Peruvian flat slab. After flattened, the oceanic plate starts to sink again. In the region where the oceanic plate starts to sink again, an earthquake with magnitude of 8.0 occurred at a depth of ~120 km on 26 May 2019. How and why this earthquake occurred at such a depth? Does the occurrence of this earthquake is related to the subducted oceanic plate? We try to answer these questions in this paper. By using two techniques, we obtained a scenario about this earthquake: In the region where the oceanic plate starts to sink again, the water in the oceanic plate releases, along with the effect of sinking to cause this earthquake. After this earthquake initiated, it ruptured mainly unilaterally north‐northwestward ~150 km with a speed of ~2.7 km/s, resulting in two large slip areas with three high slip rate areas. Such an uneven rupture may be produced by the heterogeneous structure of the slab. Key Points: Rupture process of the 26 May 2019 northern Peru earthquake is obtained from back projection analysis and finite fault inversionThe effects of slab bending forces and dehydration embrittlement together may cause this earthquake at an intermediate depthThis intermediate‐depth earthquake is probably associated with morphology of the Peruvian flat slab [ABSTRACT FROM AUTHOR]

Constraining the mechanism of earthquakes in subduction zones requires adequate estimates of source location and near‐source elastic properties. In this study, we propose a P wave autocorrelation‐based method to extract depth phase energy from teleseismic earthquakes with moment magnitude down to 4.0. We apply the method to improve location estimates of intermediate‐depth earthquakes in the Japan and northern Chile subduction zones, which represent so‐called cold and warm slabs, respectively, and which are both marked by double seismic zones. A positive correlation of slab age and double‐seismic‐zone width validates a thermally controlled model of slab morphology. The negative to normal differential times (and, thus, low or normal Vp/Vs) of the deep parts of the double seismic zones suggest that the intermediate‐depth earthquakes considered here are not due to dehydration. Plain Language Summary: Earthquake depth, which can be obtained by using depth phases, is critical for hazard mitigation and a better understanding of their mechanisms. Most techniques that use depth phases with teleseismic data require large events, even when array‐based techniques are used, which limits comprehensive studies of subduction zone seismicity. Here we propose a new autocorrelation‐based method to extract depth phase energy from teleseismic data generated by earthquakes with magnitudes as small as Mw4.0. The application to Japan and northern Chile validates the robustness of the method, and the more precise delineation of the double seismic zones reveals a positive correlation between their widths and the age of the slab when it approaches the trench. Moreover, we find the differential times of sP and pP energy, which are sensitive to the average Vp/Vs between the events and the surface, are different between the upper and the lower layers in the double seismic zone, with the lower layer mostly showing negative to normal values and thus low or normal Vp/Vs. This indicates a lack of water in the lower layer and suggests that other mechanisms are needed to interpret intermediate earthquakes rather than dehydration. Key Points: Autocorrelating teleseismic P wave improves focal depth estimates for intermediate‐depth earthquakes, with moment magnitude down to 4.0Slab age correlates with the width of the double seismic zone observed in relocated catalogs for the Japan and Chile subduction zonesThe dry lower layers in the double seismic zones call for other mechanisms to interpret the intermediate‐depth earthquakes [ABSTRACT FROM AUTHOR]

The largest earthquakes recorded in northern Alaska (Mw 6.4 and Mw 6.0) occurred ∼6 hr apart on 12 August 2018, in the northeastern Brooks Range. The earthquakes were captured by Sentinel‐1 interferometric synthetic aperture radar (InSAR) satellites and Earthscope Transportable Array seismic data, giving insight into the little‐known active tectonic processes of Arctic Alaska, obscured until recently by sparse data availability. In this study, InSAR modeling, teleseismic back projections, calibrated hypocentral relocations, and regional moment tensor solutions resolve two previously unknown, SSW dipping right‐lateral fault segments. These are the first active faults identified as conjugate to the NE trending sinistral Canning displacement zone directly to the west, which is therefore a more complex zone of diffuse faulting than previously thought. The northeastern Brooks Range has been characterized as an area of low to moderate seismic hazard, but these earthquakes illustrate the potential for larger, possibly destructive events in a region earmarked for rapid resource development. Plain Language Summary: The largest earthquakes recorded in northern Alaska (magnitude 6.4 and magnitude 6.0) occurred ∼6 hr apart on 12 August 2018. Few active faults are mapped in this region despite widespread seismicity, and the current tectonic setting remains unclear due to limited available data and the remote location. We use satellite radar images and seismic data to resolve two previously unknown fault segments, along which the magnitude 6.4 earthquake ruptured unilaterally eastward. This fault geometry demonstrates that the Canning displacement zone, the main tectonic feature in the area, is a more complex zone of diffuse faulting than previously thought. These results are also important for reassessing seismic hazard by illustrating the potential for damaging earthquakes on seemingly aseismic faults. Key Points: The largest earthquakes recorded in northern Alaska (Mw 6.4 and Mw 6.0) occurred on previously unknown active faultsInSAR and calibrated hypocenter relocations indicate that the faults are conjugate to the Canning displacement zoneThe Canning displacement zone involves a complex fault network and vertical‐axis block rotations [ABSTRACT FROM AUTHOR]

Seismicity in the Eagle Ford play grew to 33 times the background rate in 2018. We identified how hydraulic fracturing (HF) contributed to seismicity since 2014 by comparing times and locations of HF with a catalog of seismicity extended with template matching. We found 94 ML ≥ 2.0 earthquakes spatiotemporally correlated to 211 HF well laterals. Injected volume and number of laterals on a pad influence the probability of seismicity, but effective injection rate has the strongest effect. Simultaneous stimulation of multiple laterals tripled the probability of seismicity relative to a single, isolated lateral. The 1 May 2018 MW 4.0 earthquake may have been the largest HF‐induced earthquake in the United States. It occurred ~10 km from a MW 4.8 earthquake in 2011 and was thought to be induced by fluid extraction. Thus, faults in this area are capable of producing felt and potentially damaging earthquakes due to operational activities. Plain Language Summary: We investigated the recent increase in seismicity rate in the Eagle Ford oil and gas field of south Texas in 2018 that grew to 33 times higher than previous years and how hydraulic fracturing (HF) contributed. We compared times and locations of HF wells with a catalog of seismicity we enhanced through seismogram similarity detection. Over 200 HF wells had seismicity nearby during operation with ~90 earthquakes having magnitudes ≥2.0, indicating seismicity from HF is more common in this area than previously thought. We found that HF strategy affects the probability of earthquakes. Seismicity was twice as likely when operators inject into multiple nearby wells simultaneously compared to when they inject into multiple wells one at a time. The simultaneous strategy was three times more likely to produce seismicity compared to a single well strategy. Of the ~2,400 HF‐induced earthquakes we identified, a magnitude 4.0 is one of the largest reported in the United States, and it occurred ~10 km from the largest (magnitude 4.8) earthquake in south Texas, thought to be due to fluid extraction in 2011. This study demonstrates that faults in this area are capable of producing felt and potentially damaging earthquakes due to ongoing HF. Key Points: Hydraulic fracturing induced ~90 earthquakes larger than magnitude 2.0 in the Eagle Ford shale play of south Texas from 2014–2018One of the largest events potentially induced by hydraulic fracturing in the US occurred near the largest recorded event in south TexasWe propose hydraulic fracturing of multiple laterals simultaneously increases the probability of earthquakes relative to wells in isolation [ABSTRACT FROM AUTHOR]

A very low‐frequency earthquake is a type of seismic event that is rich in low frequencies and depleted in higher frequencies compared to regular fast local earthquakes of similar magnitude. The source process behind very low‐frequency earthquakes is still poorly understood. Here we present a dynamic rupture source model for very low‐frequency earthquake signal without detectable associated tremors. We show that a single asperity model with sudden stress drop followed by a rate strengthening effect damps the seismic radiation and increases event duration. We compute synthetic seismograms for our source model. The synthetic signal successfully reproduces the features of observed very low‐frequency earthquakes. Moreover, the synthetic very low‐frequency earthquake signal in 0.02–0.05 Hz is not accompanied by detectable tremor signals at 2–8 Hz. Our results help explain why in some cases we observe very low‐frequency earthquakes without accompanying tremor. Plain Language Summary: Earthquakes are generated wherever faults accommodate their relative motion in a stick‐slip manner. Outside this seismogenic zone, tectonic plates are thought to be moving past each other slowly, continuously, and silently, in a manner known as creep. Currently, a number of anomalous seismic events are observed at the edge of seismogenic zone. They are results of slow, transient movement along the fault, where the plates do not move as fast as in regular earthquakes. These seismic events are called slow earthquakes. So far, different types of slow earthquakes are identified. Very low‐frequency earthquake is one of them. It is still not clear whether different types of slow earthquakes share the same source processes. In this study, we set up a model for very low‐frequency earthquake. Our result suggests that very low‐frequency earthquake could have a different source from other slow earthquakes, such as tremor. Our finding contributes to a better understanding of the relationship between different types of slow seismic events and broad spectrum of fault behaviors. Key Points: Our dynamic rupture model reproduces very low‐frequency earthquake signal without associated nonvolcanic tremorsA single asperity on fault with sudden stress drop followed by a rate strengthening effect can increase moment rate function durationOur results explain why in some cases we observe very low‐frequency earthquake accompanied with tremors, while in some cases, we do not [ABSTRACT FROM AUTHOR]

Whether the final properties of large earthquakes can be inferred from initial observations of rupture (deterministic rupture) is valuable for understanding earthquake source processes and is critical for operational earthquake and tsunami early warning. Initial (P‐wave) characteristics of small to moderate earthquakes scale with magnitude, yet observations of large to great earthquakes saturate, resulting in magnitude underestimation. Whether saturation is inherent to earthquake dynamics or rather is due to unreliable observation of long‐period signals with inertial seismic instrumentation is unclear. Seismogeodetic methods are better suited for broadband observation of large events in the near‐field. In this study, we investigate the deterministic potential of seismogeodetically derived P‐wave amplitude using a dataset of 14 medium‐to‐great earthquakes around Japan. Our results indicate that seismogeodetic P‐wave amplitude is not a reliable predictor of magnitude, opposing the notion of strong determinism in the first few seconds of rupture. Plain Language Summary: Rapid estimation of an earthquake's magnitude is key for the effectiveness of earthquake early warning. Large magnitude earthquakes can take minutes to fully evolve; whether information from the first few seconds of a seismogram is indicative of the final magnitude is therefore of significant interest for early warning operations as well as general understanding of seismic source processes. This study investigates these early rupture observations using a combination of seismic and GPS instrumentation. We find that observation of the first few seconds of rupture is not sufficient to reliably estimate the magnitude of an earthquake greater than magnitude 7.5. Key Points: Seismogeodesy provides a more accurate P‐wave amplitude than high‐pass filtered seismic‐only methods due to its broadband sensitivity.Magnitude saturation from P‐wave characteristics is inherent to large earthquake dynamics and not due to instrumental limitations.P‐wave amplitude measurements are not predictive of final magnitude for medium‐to‐large magnitude earthquakes. [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]

Observations of surface deformation within 1–2 km of a surface rupture contain invaluable information about the coseismic behavior of the shallow crust. We investigate the oblique strike‐slip 2016 M7 Kumamoto, Japan, earthquake, which ruptured the Futagawa‐Hinagu Fault. We solve for variable fault slip in an inversion of differential lidar topography, satellite optical image correlation, and Interferometric Synthetic Aperture Radar (InSAR)‐derived surface displacements. The near‐fault differential lidar pose several challenges. The model fault geometry must follow the surface trace at the sub‐kilometer scale. Integration of displacement datasets with different sensitivities to the 3D deformation field and varying spatial distribution permits additional complexity in the inferred slip but introduces ambiguity that requires careful selection of the regularization. We infer a Mw7.09−0.05+0.03 earthquake. The maximum slip of 6.9 m occurred at 4.5‐km depth, suggesting an on‐fault slip deficit in the upper several kilometers of the crust that likely reflects distributed and inelastic deformation within the shallow fault zone. Plain Language Summary: Coseismic slip inversions quantify fault slip over a fault surface and serve as critical input into research on rupture propagation, earthquake triggering, and seismic hazard. However, coseismic slip distributions are rarely constrained by observations of surface displacement immediately adjacent to the fault rupture. This limits the quality of slip models within the shallowest crust. We solve for the slip field of the 2016 M7 Kumamoto, Japan, earthquake throughout the seismogenic crust using near‐ and far‐field observations from differential lidar topography, satellite optical image correlation, and Interferometric Synthetic Aperture Radar surface displacements. The near‐field differential lidar topography is critical for measuring shallow fault slip. We infer a Mw7.09−0.05+0.03 earthquake and a maximum slip of 6.9 m at 4.5‐km depth. This represents a shallow fault slip deficit where slip is greater at depth than at the surface. The missing shallow along‐fault slip is accommodated as off‐fault and inelastic deformation, presumably along secondary faults and folds in the shallow crust. Future earthquakes are also likely to be measured with different surface displacement data types. Researchers will have a new opportunity to learn about the behavior of the shallow fault zone and will also be presented with technical challenges such as those discussed here. Key Points: We conduct an earthquake slip inversion using differential lidar topography, optical image correlation, and InSAR imageryWe show how each data set constrains the variable fault slipThe apparent shallow slip deficit may reflect shallow off‐fault deformation [ABSTRACT FROM AUTHOR]

The magnitude of postseismic slip is useful for constraining physical models of fault slip. Here we examine the postseismic slip following intermediate‐magnitude (M 4 to 5) earthquakes by systematically analyzing data from borehole strainmeters in central and northern California. We assess the noise in the data and identify 11 earthquakes that generated interpretable strain records. We estimate the earthquakes' postseismic to coseismic moment ratios by comparing the coseismic strain changes with strain changes induced by afterslip in the following 1.5 days. The median estimated postseismic moment is 0.45 times the coseismic moment, with a 90% confidence interval between 0.25 and 0.60. This postseismic moment is slightly larger than typically observed following large (M > 6) earthquakes but smaller than observed following small (M2 to 4) earthquakes. The intermediate‐magnitude postseismic slip suggests a size dependence in the dynamics of earthquakes or in the properties of fault areas that surround earthquakes. Key Points: We examine postseismic slip following M4‐5 earthquakes using borehole strain dataThe median estimated postseismic moment is between 0.25 and 0.6 times the coseismic momentThe postseismic moment ratio is intermediate between estimates for small and large earthquakes [ABSTRACT FROM AUTHOR]

The impact of earthquakes can be severely aggravated by cascading secondary hazards. The 2018 Mw 7.5 Palu, Indonesia, earthquake led to devastating tsunamis and landslides, while triggered submarine landslides possibly contributed substantially to generate the tsunami. The rupture was supershear over most of its length, but its speed was unexpectedly slow for a supershear event, between the S wave velocity VS and Eshelby's speed 2VS, an unstable speed range in conventional theory. Here, we investigate whether dynamic rupture models including a low‐velocity fault zone can reproduce such a steady supershear rupture with a relatively low speed. We then examine numerically how this peculiar feature of the Palu earthquake could have affected the near‐field ground motion and thus the secondary hazards. Our findings suggest that the presence of a low‐velocity fault zone can explain the unexpected rupture speed and may have mitigated the near‐field ground motion and the induced landslides in Palu. Plain Language Summary: Earthquakes are produced by slippage quickly unzipping along faults, causing Earth's vibrations, that is, ground shaking. The impact of the earthquake can become more catastrophic by triggered phenomena, like landslides and tsunamis, as witnessed during the 2018 Palu (Indonesia) earthquake of magnitude 7.5. Generally, the faster the earthquake rupture, the stronger the shaking. The Palu earthquake is among a class of very fast but rare earthquakes whose speed exceeds that of shearing waves in rocks. Theoretically, these so‐called "supershear earthquakes" can propagate steadily only if faster than a speed known as Eshelby's speed. Surprisingly, the Palu earthquake is slower than this limit. How can we explain this unusual speed? Did it affect the triggering of landslides, including submarine landslides that likely contributed to the tsunami? We address these questions through computer simulations, particularly focusing on the possible effect of a "fault damage zone," a layer of softened rocks surrounding faults and caused by rock fracturing accumulated throughout the past fault activity. We found that, if a damage zone exists around the Palu fault, it can explain the unusual speed of this supershear earthquake and may have had the beneficial effect of reducing the shaking and thus its induced landslide and tsunami hazards in Palu. Key Points: The unexpected supershear but sub‐Eshelby rupture speed of the 2018 Palu earthquake can be explained by a fault damage zoneThe reduction of rupture speed by a fault damage zone mitigates the near‐field ground motion and landslide hazardFault zone waves could have amplified ground motions, but not enough to compensate for the mitigation effect of reduced rupture speed [ABSTRACT FROM AUTHOR]

Machine learning regression can predict macroscopic fault properties such as shear stress, friction, and time to failure using continuous records of fault zone acoustic emissions. Here we show that a similar approach is successful using event catalogs derived from the continuous data. Our methods are applicable to catalogs of arbitrary scale and magnitude of completeness. We investigate how machine learning regression from an event catalog of laboratory earthquakes performs as a function of the catalog magnitude of completeness. We find that strong model performance requires a sufficiently low magnitude of completeness, and below this magnitude of completeness, model performance saturates. Plain Language Summary: Seismologists analyze faults in the earth by creating earthquake catalogs‐records of the times, locations, and sizes of earthquakes. For decades, researchers have attempted to use the these catalogs to predict the timing and size of future earthquakes. Recently, researchers have found that machine learning algorithms can forecast the motion of the fault using subtle "creaking" sounds, both in the laboratory and in the real world. These creaking sounds had previously been thought to be noise and were not commonly cataloged as earthquake activity. We installed a very powerful sensor in a laboratory fault and created a very detailed catalog that captures very small quakes—small enough that they would have looked like noise to a less powerful sensor. We then used machine learning on this catalog to try and forecast the large laboratory earthquakes. We found that machine learning model is successful when small‐enough events are part of the catalog. This says that subtle seismic sounds that look like noise may be very small earthquakes that were previously overlooked. These findings suggest that to improve earthquake forecasting, we might broaden our ideas of what signals to label as potential earthquakes and save in catalogs. Key Points: Machine learning can model important characteristics of laboratory fault physics by training on finely resolved catalogs of slip eventsFault physics becomes significantly harder to learn if catalogs are truncated at or above a critical magnitude of completeness [ABSTRACT FROM AUTHOR]

Stress drop scaling relations of earthquakes in intraplate seismic zones are less well constrained, partly due to less dense instrumentation and lower seismicity rate. Here we use new data to estimate the static stress drop values of earthquakes in the Charlevoix Seismic Zone in eastern Canada (MN < 4.5), June 2012 to July 2017. We first perform double‐difference relocation to obtain the hypocentral distribution of 518 events, which highlights a diffuse distribution within the Charlevoix Seismic Zone, probably related to the highly fractured crust by a Devonian meteorite impact. Using spectral ratios, we obtain stress drop values of 47 events ranging from ~2 to 200 MPa, typical of intraplate earthquakes, and observe an invariant stress drop scaling in the magnitude range Mw 2.2–3.8. Events within the impact structure have higher stress drops than those outside, which may indicate differences of fault maturity of the St. Lawrence paleorift system and the presence of a distributed fracture network. Plain Language Summary: Earthquake source parameter studies provide information related to earthquake processes. The scaling of earthquake source parameters such as stress drop versus seismic moment is important for understanding how the fault rupture process varies with event size. For intraplate seismic zones in eastern North America, such a scaling relationship is less well constrained compared to earthquakes along major plate boundaries (interplate). In this study we use newly available seismic data from a densified local array in Charlevoix, eastern Canada, and a spectral ratio method that minimizes nonsource effects to best constrain the stress drop values for earthquakes within the magnitude range ~2–3.8 to be ~2–200 MPa. Our results thus extend the constant stress drop scaling for eastern North America earthquakes from previous studies to a lower magnitude range. We find that stress drop values are higher within the meteorite impact structure than those outside, highlighting influence from local geology. Key Points: We observe high stress drops (2‐200 MPa) for earthquakes in an intraplate seismic zone near Charlevoix, eastern CanadaCombining our results with results from previous studies, we confirm constant stress drop scaling for earthquakes in eastern North America over M ~2‐5Events within the Charlevoix impact structure have higher stress drops than those outside, suggesting a fault maturity contrast [ABSTRACT FROM AUTHOR]

The 3 September 2017 underground nuclear test (mbNEIC = 6.3) is the largest of six announced North Korean explosions. The event generated many P wave seismograms at global broadband seismic stations with good signal‐to‐noise ratio for periods less than ~5 s. Instrument deconvolution provides 435 stable broadband P wave ground displacement records in the period range 0.1 to 5.0 s. These are stacked in 26 azimuth/distance windows to average path and receiver effects. Waveform stacks and average amplitude of 4‐Hz ground displacements are modeled assuming a Mueller‐Murphy explosion source model for a granite source medium. Nonelastic pP delays consistent with burial depths in the mountainous source topography are considered, and explosion yield and an average constant‐Q attenuation operator are estimated by fitting the waveforms. For a source depth of 750 m in heavily damaged environment, the estimated yield = 230 ± 50 kt and t* = 0.78 ± 0.03 s. Plain Language Summary: Seismic P wave ground motions recorded around the world for the 3 September 2017 underground nuclear test at the North Korean test site are modeled to constrain the explosion yield and the average path attenuation parameter. Broadband ground motion waveforms are modeled along with the amplitude of 4‐Hz ground motions, exploring a wide range of attenuation and yield parameters along with allowing for nonelastic behavior of the P wave reflection (pP) from the free surface above the explosion, which travels down through the damage zone surrounding the explosion‐produced cavity. For a source depth of 750 m, which is reasonable for location of the explosion under the mountain peak, and allowing for an extensive damage zone, the waveform modeling provides an estimated yield of 230 ± 50 kt. Yield estimates of smaller events are made based on calibration of magnitude‐yield relations using this result and range from 0.5 to 17.8 kt. Key Points: Teleseismic broadband and high‐frequency P wave displacements are modeled using the Mueller‐Murphy model to constrain the 2017 yieldP wave displacements for frequencies above 4 Hz have little influence from the free surface reflection pPFor a source depth of 750 m and an extensive damage zone that delays pP, the yield is estimated to be 230 ± 50 kt for t* = 0.78 ± 0.03 s [ABSTRACT FROM AUTHOR]

A major challenge for understanding the physics of shallow fault creep has been to observe and model the long‐term effect of stress changes on creep rate. Here we investigate the surface creep along the southern San Andreas fault (SSAF) using data from interferometric synthetic aperture radar spanning over 25 years (ERS 1992–1999, ENVISAT 2003–2010, and Sentinel‐1 2014–present). The main result of this analysis is that the average surface creep rate increased after the Landers event and then decreased by a factor of 2–7 over the past few decades. We consider quasi‐static and dynamic Coulomb stress changes on the SSAF due to these three major events. From our analysis, the elevated creep rates after the Landers can only be explained by static stress changes, indicating that even in the presence of dynamically triggered creep, static stress changes may have a long‐lasting effect on SSAF creep rates. Plain Language Summary: There are two significant conclusions from this study. First, we analyzed 25 years of InSAR measurements over the Southern San Andreas Fault system to document a major increase in the average creep rate following the 1992 Mw 7.3 Landers Earthquake which is then followed by creep rate reductions after the 1999 Mw 7.1 Hector Mine Earthquake and the 2010 Mw 7.2 El Major Cucapah Earthquake. Second, we attribute all these creep rate changes to the Coulomb stress variations from these three major Earthquakes. The dynamic Coulomb stress changes are similar for all three events, contributing to triggered creep on the SSAF. In contrast, the static Coulomb stress changes on the SSAF are positive after the Landers and negative after the Hector Mine and El Major Cucapah, coinciding with the higher average creep rate after the Landers and lower rates after the other two events. An implication of this study is that small but steady Coulomb stress changes have a larger impact on shallow creep than the larger dynamic stress changes associated with passing seismic waves. These results illuminate the significance of time scale‐dependent complexity of shallow fault creep and how these behaviors are communicated by stress perturbations from regional earthquakes. Key Points: We provide detailed new observations that constrain variations in average creep rates following regional earthquakesA new model is proposed for understanding dynamic/static stress changes and short‐ and long‐term creep rate variations on the shallow SSAFOur observations suggest that static stress change has a long‐lasting effect on fault creep, even in the presence of transient processes [ABSTRACT FROM AUTHOR]

Fracture damage patterns around faults induced by dynamic earthquake rupture are an invaluable record to clarify the rupture process on complex fault networks. The 2016 Mw 7.8 Kaikōura earthquake in New Zealand has been reported as one of the most complex earthquakes ever documented that ruptured at least 15 crustal faults. High‐resolution optical satellite image displacement maps provide distinctive profiles of displacement across the faults and help visualize the off‐fault damage pattern. They are combined with field observation and coupled with a numerical tool that captures the dynamics of the rupture and simultaneous activation of off‐fault damage to allow the determination of the most likely rupture scenario. This study demonstrates that complex rupture processes can be explained in a rather simple way via a synergetic combination of state‐of‐the‐art observation and first principle physics‐based numerical modeling of off‐fault damage. Plain Language Summary: In a first‐of‐its‐kind work, we combine optical image correlation at a resolution never achieved before (1.8‐m ground resolution), field observations, and a unique dynamic rupture simulation tool, including off‐fault deformation, to unravel, beyond doubt, part of the Kaikōura rupture process. We show that matching observed off‐fault deformation pattern with rupture simulation helps easily identify the earthquake rupture path to propose a kinematically simple earthquake scenario. Thus, this work shows that when building models of seismic hazard, like in Southern California, systematic consideration of off‐fault damage patterns for past events would help understand earthquake scenario and to model future events. Hence, this presents a great potential to narrow down the multitude of rupture scenarios along complex fault networks that need to be considered in seismic hazard model assessment. Key Points: Matching observed off‐fault deformation pattern with rupture simulation helps identify the earthquake rupture pathSystematic consideration of off‐fault damage patterns for past events would help understand earthquake scenario and to model future eventsThis presents a great potential to narrow down the multitude of rupture scenario on complex fault system [ABSTRACT FROM AUTHOR]