Search

The article discusses the physical process and seismogenic mechanism of the Yangbi Ms 6.4 earthquake that occurred in Dali, Yunnan Province in 2021. The earthquake did not occur on a pre-existing active fault but on a buried fault in the eastern boundary of Baoshan sub-block. The study uses various seismological methods to analyze the earthquake sequence, seismogenic environment, and focal parameter characteristics, providing insights into the earthquake's source characteristics and trigger mechanism. [Extracted from the article]

This document is a compilation of various research papers and abstracts related to seismic activity and earthquake research in Croatia and Hungary. The papers cover topics such as seismic site amplification, site response analysis, local site effects, lithosphere structure, gravity anomaly models, and seismic performance assessment of buildings. The research provides valuable insights into earthquake risk and mitigation strategies in the region. The document also includes links to additional information and resources for further exploration. [Extracted from the article]

Anthropogenic fluid injections at depth induce seismicity which is generally organized as swarms, clustered in time and space, with moderate magnitudes. Earthquake swarms also occur in various geological contexts such as subduction zones, mountain ranges, volcanic, and geothermal areas. While some similarities between anthropogenic and natural swarms have already been observed, whether they are driven by the same mechanism, or by different factors, is still an open question. Fluid pressure diffusion or aseismic deformation processes are often proposed to explain observations of hypocenters migration during swarms, while recent models suggest that swarm seismicity is rather triggered by fluid‐induced aseismic fault slip. Here, using a global compilation of 22 natural and anthropogenic swarms, we observe that duration, migration velocity, and total moment scale similarly for all swarms. This supports a common driving process for both natural and induced swarms. The scaling relations are similar to those found for slow slip events. These observations highlight the prevalence of fluid‐induced aseismic slip as main driver of earthquakes migration during swarms. After quantifying aseismic slip released in the swarms, we propose an approach to estimate the seismic‐to‐total moment ratio, which we then compare to a theoretical estimation that depends on the migration velocity of the swarm and the effective stress drop. Our findings lead to a generic explanation of the process driving earthquake swarms that might open new possibilities to monitor seismic swarms. Plain Language Summary: Earthquake swarms are a particular type of seismic activity, during which a sequence of many earthquakes occurs without being initiated by a larger one. Swarms can be induced by anthropic hydraulic injections at depth, like during geothermal power exploitation and massive storage of diverse fluids (i.e., wastewater, CO2) in porous reservoir formations. Natural earthquake swarms are also observed in a large variety of geological contexts. Previous works showed that natural and injection‐induced swarms share some similarities, like the migration of seismicity. However, their underlying processes remain unclear. Here, we explain the observed similarities in both types of swarms by a model in which earthquakes are triggered by the propagation of an aseismic slip transient, which in turn is induced by pressurized fluid circulation. This model reconciles a suite of independent observations made over different length and time scales and provides a generic explanation of the driving process for the migration of earthquake swarms. Key Points: Scaling laws support that injection‐induced and natural earthquake swarms have the same driving mechanismAseismic slip is a key feature of earthquake swarms, although its contribution differs from one swarm to anotherWe introduce a simple model based on fluid‐induced aseismic slip propagation to relate observables to physical parameters [ABSTRACT FROM AUTHOR]

The Nordland region, Northern Norway, situated in an intraplate continental setting, has the highest seismicity rate in mainland Norway. However, the exact cause of seismicity in this region is still debated. Better understanding of factors that influence the seismicity in Nordland can help increase knowledge of intraplate seismicity in general. Here, we address this problem with the aid of a new high-resolution 3-D VP and VP / VS ratio images of the crust in Nordland using seismic traveltime tomography. These images show the existence of a localized, 10–15 km Moho step that runs parallel to the coast. The north–south extent of this step coincides with the region that exhibits the highest rates of seismicity. Focal mechanisms of selected earthquakes computed in this study are dominated by normal and oblique-normal, indicating a coast-perpendicular extension. The coast-perpendicular extensional stress regime deviates from the regional compression imposed by the ridge push from the North Atlantic. This deviation is thought to stem from the additional interference with local flexural stress caused by sediment redistribution and glacial isostatic adjustment, and possibly exacerbated by gravitational potential energy stress associated with the Moho step. The deformation due to the extensional regime is localized on pre-existing faults and fractures along the coastline. The tomography result shows that two distinct seismic swarms occurred in the coastal area with low VP and variable VP / VS ratio anomalies, pointing towards fractured crust and possibly the presence of fluids. The existence of fluids here can change the differential stress and promote seismic rupture. [ABSTRACT FROM AUTHOR]

Understanding the extensional processes in tectonic context at the transition from continental to oceanic spreading is fundamental to obtain new insights about formations of new oceans. To that scope, we study a large and long‐lived earthquake swarm occurring in 2020–2021 in a back‐arc rift (the Bransfield Basin) south of the South Shetland Islands, Antarctica. We make use of one local seismological station to detect more than 36,000 small earthquakes, occurring from August 2020 to June 2021. Together with the occurrence of earthquakes, we observe a significant, geodetic deformation at a nearby GPS station. By joint interpretation of b‐value, spatiotemporal evolution of seismicity and geodetic deformation, we infer a volcanic origin for this swarm that takes place close to the ridge axis. Our study suggests that beyond the 7 mm/yr deformation reported at the Bransfield Basin ridge, transient deformation episodes localized at the ridge axial volcanic structure also modulate the extension. Plain Language Summary: Understanding the extensional tectonics in places at the transition from continental to oceanic spreading, can provide new insights about the extensional processes leading to formation of new oceans. We report on a long‐lived (∼1 year) earthquake swarm in the Bransfield Basin, just south of the South Shetland islands, in Antarctica, that brings new observations to such tectonics. This basin represents a ridge separating two tectonic plates and is characterized by extensional tectonics at the transition from back‐arc rifting to ocean spreading. By detection and characterization of more than 36,000 earthquakes and observation of associated geodetic deformation, we inferred the significant role played by volcanic processes occurring at the ridge axis, in modulating extension of the basin. This observation differs from models of purely tectonic extensional processes involving rift bounding border faults. Key Points: We characterized one of the largest swarms ever recorded in a spreading ridgeAnalysis of seismological and geodetic data suggest volcanic origin for this swarmWe infer a significant role played by axial volcanic structures in opening of spreading ridges [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]

Wave‐induced fluid pressure diffusion (FPD) represents an important mechanism of seismic energy dissipation in fractured media. The associated effects on wave propagation are typically studied considering idealized fracture geometries. Here, we study fracture‐geometry‐related effects on FPD by numerically computing the frequency‐dependent and angle‐dependent seismic velocity and attenuation on models having fractures of realistic geometries. The geometry of the models is derived from microcomputed tomography images of a fractured Berea sandstone. By comparing the numerical results with those for an equivalent thin‐planar‐layer analytical model, we isolate fracture‐geometry effects. We found that discrepancies on the anisotropic behavior of P and S waves with respect to the simple analytical model are small except for the S wave attenuation. This is associated with the pressure gradients induced by S waves in fractures exhibiting a mild curvature. Part of this dissipation occurs inside the fracture, parallel to its walls, and is thus controlled by its permeability, which points to a possible perspective of inferring fracture hydraulic properties from S waves attenuation. Plain Language Summary: The attenuation and velocity dispersion of seismic waves propagating in rocks can be used to infer and to characterize rock heterogeneities and fluid content. Fluid pressure diffusion is an important cause of such attenuation and dispersion and can be estimated using analytical and numerical solutions based on simple representations of fractures. A broadly used approach is to represent fractures as thin‐planar poroelastic layers. However, the effects of complex fracture geometry, such as rough and curved walls and contact areas, remain largely unexplored. In this study, we numerically calculate the seismic energy dissipation due to fluid pressure diffusion in a medium containing a fracture of realistic geometry. We compare these results with those of a thin‐planar layer to study the limitations of the simple model. We show that the S wave response for normal incidence can be strongly attenuated in part due to fluid pressure diffusion occurring inside the fracture and parallel to its walls triggered by a mild fracture curvature. Such effect cannot be accounted for by a thin‐planar model. This points to the potential of estimating hydraulic fracture properties from the S wave attenuation. Key Points: The anisotropic velocity of P and S waves traveling across real fractures can be reproduced using a planar‐thin‐layer modelThe attenuation of S waves is highly affected by a squirt‐type flow prevailing inside fractures with mild curvatureOur observations can help improve the interpretation and inversion of mechanical and hydraulic properties of fractures from seismic data [ABSTRACT FROM AUTHOR]

Since 2011, seafloor temperatures, pressures, and seismic ground motions have been measured by the seafloor cabled Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) on the Nankai margin. These measurements, high‐resolution bathymetry, and abundant contextual information make the DONET region seem ideally suited to provide constraints on seismic shaking‐triggered sediment slope failures and gravity flows, particularly since numerous published studies have linked paleo‐ to modern earthquakes to failures and flows within the DONET. The occurrences of the local 2016 M6.0 Mie‐ken and regional M7.0 Kumamoto earthquakes within and at regional distances, respectively, from the DONET data set provided an opportunity to explore this potential. We used DONET seismic recordings of the posited triggering shaking and to search for submarine slide signals and continuous temperature and pressure data to detect pulses of warm and densified water indicative of passing flows. We developed and applied a variety of analytical methods to eliminate signals generated by water column processes, while leaving slope failures and sediment gravity flow anomalies as residuals. Our explorations yielded no evidence that earthquake shaking initiated either phenomenon, which we suggest reflects the finicky nature both of the detection of and the physical processes that contribute to slope failures and flows (i.e., both require satisfying precise suites of conditions). Nonetheless, this negative result, our analyses, and the estimates of physical properties we derived for them, provide useful lessons and inputs for future studies. Plain Language Summary: Since 2011, measurements of both seafloor displacements caused by deformation within the Earth and motions of the water column above the seafloor have been made by the seafloor cabled Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) in the offshore region of the Nankai subduction zone, Japan. These measurements, the occurrence in 2016 of a magnitude M6.0 earthquake within the DONET footprint and another M7.0 in southern Japan, and a rich repository of other information make the DONET region seem ideally suited to learn about how earthquake shaking triggers submarine landslides and rapidly flowing currents that carry sediments downslope. We analyzed DONET measurements but found no evidence that earthquake shaking initiated either phenomenon, which we suggest reflects the finicky nature both of detecting them and the physical processes that control their initiation. Nonetheless, our results provide useful lessons and inputs for future studies. Key Points: New processing approaches reduce water column temperature and pressure influence, producing coherent, but propagating signaturesGeneration and detection of slope failures and sediment gravity flows require satisfying multiple physical and observational criteriaNo evidence of these phenomena within the Nankai Dense Oceanfloor Network system for Earthquakes and Tsunamis array despite plausible triggering shaking, environment, and history [ABSTRACT FROM AUTHOR]

Determining conditions for earthquake slip on faults is a key goal of fault mechanics highly relevant to seismic hazard. Previous studies have demonstrated that enhanced dynamic weakening (EDW) can lead to dynamic rupture of faults with much lower shear stress than required for rupture nucleation. We study the stress conditions before earthquake ruptures of different sizes that spontaneously evolve in numerical simulations of earthquake sequences on rate‐and‐state faults with EDW due to thermal pressurization of pore fluids. We find that average shear stress right before dynamic rupture (aka shear prestress) systematically varies with the rupture size. The smallest ruptures have prestress comparable to the local shear stress required for nucleation. Larger ruptures weaken the fault more, propagate over increasingly under‐stressed areas due to dynamic stress concentration, and result in progressively lower average prestress over the entire rupture. The effect is more significant in fault models with more efficient EDW. We find that, as a result, fault models with more efficient weakening produce fewer small events and result in systematically lower b‐values of the frequency‐magnitude event distributions. The findings (a) illustrate that large earthquakes can occur on faults that appear not to be critically stressed compared to stresses required for slip nucleation; (b) highlight the importance of finite‐fault modeling in relating the local friction behavior determined in the lab to the field scale; and (c) suggest that paucity of small events or seismic quiescence may be the observational indication of mature faults that operate under low shear stress due to EDW. Plain Language Summary: Understanding the evolution of stress on faults over periods of slow and fast motion is crucial for assessing how earthquakes start, grow, and ultimately stop. Here we use computer models to explore the stress conditions required for simulated earthquake ruptures to occur. We find that the critical stress conditions for rupture propagation depend on the size of the rupture and how efficiently the fault shear resistance weakens during fast slip. In particular, larger earthquakes on faults that experience increasingly more efficient weakening during rupture can propagate under systematically lower stress conditions than those required for rupture nucleation. As a result, we find that faults that exhibit efficient weakening can host predominantly large earthquakes at the expense of smaller earthquakes. Our findings illustrate how large earthquakes can occur on faults that appear to be understressed compared to expected conditions for rupture nucleation. Moreover, our findings support a body of work suggesting that the scarcity of small earthquakes on some major mature fault segments, like the central section of the San Andreas Fault, may indicate that they experience substantial weakening during large earthquakes, a consideration that may be particularly useful for earthquake early warning systems. Key Points: Local shear prestress varies significantly within and among ruptures, being close to the quasi‐static fault strength in nucleation regionsEfficient weakening allows rupture propagation over areas of lower prestress, leading to lower average prestress over larger rupture areasFault models with more efficient dynamic weakening produce fewer smaller events and result in systematically lower b‐values [ABSTRACT FROM AUTHOR]

For a shallow or ground surface source and receiver at the same level or close depth, it is very difficult or computationally inefficient to simulate seismoelectric or electroseismic wave‐fields in stratified porous media by current reflectivity methods, such as the Luco‐Apsel‐Chen generalized reflection and transmission method (LAC GRTM). In this work, the peak‐trough averaging method which has been proved effective and efficient in dealing with this kind of computational problem is adopted to update the seismoelectric and electroseismic modeling algorithm based on LAC GRTM. After thoroughly verifying the accuracy and computational efficiency of the updated algorithm, we utilize it to numerically investigate both the electroseismic and seismoelectric couplings. Snapshots of electroseismic wave‐fields indicate evanescent electroseismic conversion, a reverse process of evanescent seismoelectric conversion, dominates at relatively larger ratios of seismic wavelength to interface depth, whereas the interfacial radiation electroseismic conversion is more prominent for the opposite situation. Our seismoelectric modeling results demonstrate that electric signals can arrive at the ground surface a few milliseconds earlier than their causative seismic signals due to evanescent seismoelectric conversion. This is the first modeling result considering source‐receiver geometries on the surface capable of explaining similar phenomena reported in geophysical field observations of seismically induced electrokinetic effects over a long history. The updated algorithm offers an accurate and efficient tool for forward modeling and will benefit interpretations of field observations as well as future inversion studies. Plain Language Summary: For the case of active seismoelectric or electroseismic measurements with source and receivers located on or near the ground surface, it is computationally difficult for current reflectivity methods, such as Luco‐Apsel‐Chen generalized reflection and transmission method (LAC GRTM), to simulate the wave‐fields in stratified porous media. In this study, the peak‐trough averaging method is adopted to update the LAC‐GRTM‐based seismoelectric and electroseismic modeling algorithms to solve this computational problem. We verify the accuracy and efficiency of the updated algorithm and apply it to investigate the electroseismic and seismoelectric couplings. Two kinds of electroseismic conversions were identified by our modellings. When the ratio of seismic wavelength to interface depth is large, evanescent electroseismic conversion will be dominant; otherwise, the interfacial radiation electroseismic conversion will become prominent. In our seismoelectric modellings, it is found electric signals recorded by ground surface receivers could be several milliseconds earlier than their causative seismic signals due to evanescent seismoelectric conversion. This is the first modeling result capable of explaining similar phenomena that have been reported in seismoelectric field measurements over a long history. Generally speaking, the updated algorithm is a useful forward modeling tool, which could benefit interpretations of field data and future inversion studies. Key Points: An updated algorithm is proposed to handle the modeling of active seismoelectric surveys with the source and receivers on the surfaceEvanescent electroseismic conversion dominant at relatively low frequencies is recognized by numerical simulationsEvanescent seismoelectric waves can explain the arrival of electric signals milliseconds prior to causative seismic arrivals [ABSTRACT FROM AUTHOR]

From California to British Columbia, the Pacific Northwest coast bears an omnipresent earthquake and tsunami hazard from the Cascadia subduction zone. Multiple lines of evidence suggests that magnitude eight and greater megathrust earthquakes have occurred ‐ the most recent being 321 years ago (i.e., 1700 A.D.). Outstanding questions for the next great megathrust event include where it will initiate, what conditions are favorable for rupture to span the convergent margin, and how much slip may be expected. We develop the first 3‐D fully dynamic rupture simulations for the Cascadia subduction zone that are driven by fault stress, strength and friction to address these questions. The initial dynamic stress drop distribution in our simulations is constrained by geodetic coupling models, with segment locations taken from geologic analyses. We document the sensitivity of nucleation location and stress drop to the final seismic moment and coseismic subsidence amplitudes. We find that the final earthquake size strongly depends on the amount of slip deficit in the central Cascadia region, which is inferred to be creeping interseismically, for a given initiation location in southern or northern Cascadia. Several simulations are also presented here that can closely approximate recorded coastal subsidence from the 1700 A.D. event without invoking localized high‐stress asperities along the down‐dip locked region of the megathrust. These results can be used to inform earthquake and tsunami hazards for not only Cascadia, but other subduction zones that have limited seismic observations but a wealth of geodetic inference. Plain Language Summary: The largest earthquakes on Earth occur along faults that develop between two tectonic plates that come into contact. Termed megathrust earthquakes, these catastrophic events are responsible for generating both strong ground‐shaking and tsunamis. The Cascadia megathrust fault straddles the Pacific coastline of North America and from evidence in both the United States and Japan, we know this fault last slipped in 1700 A.D. We have combined models of strain buildup (geodetic coupling models) with state‐of‐the‐art 3‐D computer simulations to understand the potential hazard of a future earthquake in Cascadia and show what factors might lead to the fault slipping its entire length. We compare our simulations to geologic measurements of permanent ground movement from 1700 A.D. Our results demonstrate that no matter where the earthquake is allowed to start, coupling models showing strain accumulation to the top of the fault easily leads to big earthquakes. We also look into what 1700 A.D. event may have looked like and show several scenarios that fit the geologic data very closely. This work represents the first set of 3‐D simulations that use the laws of physics to see what may control the size of future earthquakes in Cascadia. Key Points: We design the first fully dynamic 3‐D earthquake simulations based on geodetic coupling models for the Cascadia megathrustSegmentation in the stress drop is needed to produce subsidence amplitudes consistent with observed megathrust earthquakesDynamic rupture simulations demonstrate how fault friction and stress levels may control margin‐wide rupture [ABSTRACT FROM AUTHOR]

Supershear rupture is crucial in determining ground motion characteristics and its various transition mechanisms is in key connection with rupture dynamics, geometrical complexities and stress heterogeneities. Using 3D dynamic rupture models with numerous depth‐dependent stress regimes, we show that depth‐dependent stress plays a major role in controlling the occurrence of free‐surface‐induced (FSI) supershear ruptures. The effect of stress gradient, initial normal stress at the free surface, fault width and critical depth of depth‐dependent stress regime are examined. The 2D tangent rupture velocity distributions show that generation of a sustained FSI supershear rupture prefers a larger stress gradient and initial normal stress at the free surface. The truncation of fault width could transit the rupture to a sub‐Rayleigh rupture. We propose a dimensionless stress parameter representing a measure of the overall level of depth‐dependence in the initial normal stress, and a larger moment magnitude is more likely to trigger a sustained FSI supershear rupture. By comparing the near‐field ground motions, three distinct ground motion characteristics are presented using the sustainability of Mach fronts and displacement polarization. This work helps to understand the occurrence of FSI supershear ruptures and its relationship with underground depth‐dependent stresses. Plain Language Summary: The Earth's free surface is shown to be a key factor in inducing earthquake rupture speed greater than the shear wave speed on strike‐slip faults. However, depth‐dependent stress can greatly increase the variety of rupture behaviors in strike‐slip faults. The existence of Mach fronts is an important criterion for supershear ruptures, with which three distinct ground motion patterns are displayed based on numerous dynamic rupture simulations with depth‐dependent stress regimes. A small earthquake, even if the rupture reaches the free surface, is unlikely to trigger an observable FSI supershear rupture on a strike‐slip fault. Key Points: Depth‐dependent stress plays a key role in determining the occurrence of free‐surface‐induced (FSI) supershear ruptureThe FSI supershear rupture is likely to occur if the moment magnitude is sufficiently largeStress tapering at the fault bottom has great influence in the rupture behavior at the free surface [ABSTRACT FROM AUTHOR]

Most large tsunamis are generated by earthquakes on offshore plate boundary megathrusts. The primary factors influencing tsunami excitation are the seismic moment, faulting geometry, and depth of the faulting. Efforts to provide rapid tsunami warning have emphasized seismic and geodetic methods for quickly determining the event size and faulting geometry. It remains difficult to evaluate the updip extent of rupture, which has significant impact on tsunami excitation. Teleseismic P waves can constrain this issue; slip under deep water generates strong pwP water reverberations that persist as ringing Pcoda after the direct P phases from the faulting have arrived. Event‐averaged Pcoda/P amplitude measures at large epicentral distances (>80°), tuned to the dominant periods of deep water pwP (~12–15 s), correlate well with independent models of whether slip extends to near the trench or not. Data at closer ranges (30° to 80°) reduce the time lag needed for inferring the updip extent of rupture to <15 min. Arrival of PP and PPP phases contaminates closer distance Pcoda measures, but this can be suppressed by azimuthal or distance binning of the measures. Narrowband spectral ratio measures and differential magnitude measures of Pcoda and direct P (mB) perform comparably to broader band root‐mean‐square (RMS) measures. Pcoda/P levels for large nonmegathrust events are also documented. Rapid measurement of Pcoda/P metrics after a large earthquake can supplement quick moment tensor determinations to enhance tsunami warnings; observation of large Pcoda levels indicates that shallow submarine rupture occurred and larger than typical tsunami (for given MW) can be expected. Key Points: Teleseismic Pcoda amplitude generated by water reverberations (pwP) increases relative to the direct P for shallow slip under deep waterMeasurement of high or low Pcoda/P ratios in the distance range from 30° to 80° can reliably identify events with or without shallow slipTsunami warning for subduction zone thrust earthquakes can be improved by rapidly measuring Pcoda to constrain the updip extent of slip [ABSTRACT FROM AUTHOR]

The deformation of a circular fault in a thin floating ice plate imposed by a slow rotational displacement is investigated. Temporal changes in shear strength, as a proxy for the resistance of the fault as a whole, are monitored by the torque required to impose a constant displacement rate. Micro‐seismic monitoring is used to study the relationship between fault average resistance (torque) and micro‐ruptures. The size distribution of ruptures follows a power‐law scaling characterized by an unusually high exponent (b≃3), characteristic of a deformation driven by small ruptures. In strong contrast to the typical brittle dynamics of crustal faults, an 'apparently aseismic' deformation regime is observed in which small undetected seismic ruptures, below the detection level of the monitoring system, control the slip budget. Most (≃71%) of the detected ruptures are organized in bursts with highly similar waveforms, suggesting that these ruptures are only a passive by‐product of apparently aseismic slip events. The seismic signature of this deformation regime has strong similarities with crustal faulting in settings characterized by high temperature and with non‐volcanic tremors. Key Points: A micro‐seismic monitoring of a fault is setup to monitor its deformationSmall undetected fractures control the slip and energy budgets of the faultFractures are mostly a by‐product of the imposed slip, similarly to swarm‐seismicity [ABSTRACT FROM AUTHOR]

Actively deforming crust and upper mantle around the obliquely convergent Pacific-Australia plate boundary in the northern South Island of New Zealand have been investigated by seismic tomography using data from a temporary c. 50 station network plus GEONET. The Alpine-Wairau Fault (principal component of the plate boundary at the surface) transects the study area, separating the Buller-Nelson (BN) province of active compressional inversion involving steep reverse faulting to the northwest, from the Marlborough fault system (MA) dominated by dextral strike-slip faulting to the southeast. In the course of this study, MA hosted the 2013 Seddon earthquake sequence (M 5.9, 6.6) and the 2016 M 7.8 Kaikoura earthquake. Active fault structures and present seismicity are associated with a heterogeneous distribution of low-velocity and Vp/Vs anomalies. Elsewhere, there is a general association between crustal seismicity and low-velocity zones along major fault structures within both the MA and BN seismotectonic provinces along which high Vp/Vs anomalies are locally conspicuous. Areas of active crustal seismicity are also generally characterized by high Vp/Vs , for instance the hypocentre and aftershocks of the 2016 M 7.8 Kaikoura earthquake overlie a low-velocity region with high Vp/Vs. The coincidence of anomalously low Vp and Vs and anomalously high Vp/Vs with zones of high electrical conductivity defined by a previous MT transect is consistent with the notion that upward migration of overpressured hydrothermal fluid from the subducting slab at depth leads to a heterogeneous distribution of overpressured fluid in and around the base of the crustal seismogenic zone, weakening the overlying crust and promoting seismic rupture along major fault systems. It seems possible that the Association of mid-crustal low-velocity zones with anomalously high Vp/ Vs may diagnose rupture preparation zones where frictional strength is being lowered by the build-up of fluid overpressure concurrent with accumulating shear stress, so that eventual fault failure is 'dual-driven'. • Seismic low-velocity zones beneath both the Buller-Nelson inversion province and the Marlbourough fault system • Hypocenter of the 2016 Kaikoura earthquake within low velocity zone • Seismic low-velocity can be interpreted as fluid overpressured area. [ABSTRACT FROM AUTHOR]

Deciphering the relationship between lateral growth of faults and along‐strike deformation (i.e., shortening and uplift) in the Earth's upper crust remains a challenge. Here we gain insight into the relation between these processes by studying the Kashi anticline, an asymmetric, doubly plunging thrust‐fault‐related fold located in the southwest Tian Shan, China. We use seismic interpretation and field observations, together with 2‐D trishear and excess area methods, to quantify the distribution of shortening along this structure. The shortening distribution along strike of the Kashi anticline is nonlinear and has a peaked, asymmetric, bell shape, with a maximum value of 5.9 ± 0.2 km. After comparing the 3‐D structural model of the Kashi anticline and our trishear models, we propose that lateral propagation‐to‐maximum shortening ratio, initiation fault length, and lateral propagation rate control the lateral fault propagation process and the fold terminations. Moreover, the 3‐D fault morphology and the ages of the growth strata suggest that the Kashi anticline experienced two stages of lateral growth with propagation rates of 60 km/Ma between 1.4 ± 0.2 Myr and 0.9 ± 0.3 Ma, and ~67 km/Myr from 0.9 ± 0.3 Ma to present. These observations highlight the relation between the evolution of lateral fault growth and the along‐strike shortening distribution, allowing us to use the latter (which we can measure) to infer the former (which we cannot). These novel insights from the Kashi anticline can be used to understand lateral growth of thrust and normal faults worldwide. Key Points: Structural interpretations reveal that the western segment of Kashi anticline is a trishear fault propagation fold, whereas the eastern segment is a detachment foldVariations of lateral fault growth produced along‐strike shortening distribution with a peaked, nonlinear bell shapeKashi anticline experienced two stages of lateral growth with propagation rates of 60 km/Myr between 1.4 ± 0.2 and 0.9 ± 0.3 Ma, and ~67 km/Myr from 0.9 ± 0.3 Ma to present [ABSTRACT FROM AUTHOR]

In this paper we investigate the dynamic behavior of a system of interconnected faults in the Brawley Seismic Zone (BSZ) in southern California. The system of faults includes the southern San Andreas Fault (SSAF), the Imperial Fault (IF), and a set of cross faults in the BSZ that may serve as connecting structures between the two larger faults. Geological and seismic evidence imply that the SSAF and IF may have buried extensions that link them together in a large‐scale step over, with the cross faults in the BSZ cutting between them. Such a configuration poses the question of whether through‐going rupture across the step over is possible in this region, leading to large, plate‐boundary scale earthquakes. We investigate potential earthquakes in this region through 3‐D dynamic finite element spontaneous rupture modeling. We find that under multiple assumptions about fault stress and fault geometry, through‐going rupture is possible, both from north to south and south to north. Participation of the cross faults is facilitated by two factors: absence of rupture on one of the main two faults and a contrast in prestress between the main faults and the cross faults, leading to slow propagation speed on the main faults while maintaining ease of failure on the cross faults. The pattern of rupture propagation and slip is strongly affected by fault‐to‐fault dynamic stress interactions during the rupture process. The results may have implications for both potential earthquakes in this region, as well as for understanding the dynamics of geometrically complex/branched faults in general. Key Points: Based on our current assumptions about fault geometry and prestress, through‐going rupture might be possible in the Brawley Seismic ZoneSlip on the cross faults can hinder rupture propagation on the main faultsCross‐fault rupture appears to be facilitated by a contrast in stress initial conditions with the main fault segments [ABSTRACT FROM AUTHOR]

In Mogul west of Reno, Nevada, USA, in late February 2008 an earthquake sequence occurred that culminated in a magnitude 4.9 mainshock after a foreshock‐rich period lasting approximately 2 months on previously unidentified fault structures. In this article, we show that the foreshock sequence may have been driven by a fluid pressure intrusion. We use 1,082 previously calculated earthquake focal mechanisms to infer the local stress field as well as 1,408 relocated foreshock events to determine the required excess fluid pressure field in the source region of the Mogul earthquake sequence to trigger these events. A model of nonlinear pore pressure diffusion is used to model the fluid flow in a highly fractured subsurface. We find a strong correlation between high fluid pressure fronts and foreshock hypocenters, suggesting a natural fluid‐driven earthquake sequence. Key Points: Stress inversion and overpressure estimates are well suited to evaluate the feasibility of fluid pressure triggeringA strong correlation between high fluid pressure fronts and foreshock hypocenters suggests a natural fluid-driven earthquake sequenceThe lack of dominant upward migration of seismicity does not exclude fluid pressure as driving mechanism [ABSTRACT FROM AUTHOR]

Field measurements of coseismic fault slip often differ from surface slip models derived from satellite geodesy. Quantifying these differences is challenging as many geodetic techniques inadequately image near‐fault deformation. We use an iterative closest point algorithm to difference preearthquake and postearthquake terrestrial laser scanning point clouds to reveal centimeter‐scale patterns of surface deformation caused by shallow fault slip in the 2016 Mw 6.6 Norcia (Central Italy) earthquake. Terrestrial laser scanning offsets are constant along the fault and match average field measurements. Eighty‐four percent of vertical displacement occurs on a discrete fault zone, with 16% of deformation distributed across a narrow zone <4 m wide. In contrast, horizontal deformation is distributed over an 8‐m‐wide zone with approximately 50% of extension accommodated as off‐fault deformation (OFD). The centimeter‐scale observation of deformation shows that horizontal and vertical coseismic OFD is partitioned—in this case, OFD is dominated by horizontal deformation. Plain Language Summary: During an earthquake, slip on a fault plane creates discrete offsets at depth and at the surface. The pattern and size of offsets at the surface can help to understand what happened in the earthquake and also leaves a record of each event in the landscape. This record is used to infer past earthquake activity and forecast the potential likelihood of future earthquakes. We captured a preearthquake image of a fault that caused the 2016 magnitude 6.6 Norcia earthquake in Central Italy. By reimaging the same fault after the earthquake, we measured the pattern of ground movement during the event to millimeter precision to understand in unprecedented detail how much earthquake slip occurs on the fault itself. This uniquely precise map of surface deformation has never been captured before using a terrestrial laser scanner. We find that the vertical motion of the fault is mainly focused on the fault itself. In contrast, the horizontal motion is distributed over an 8‐m‐wide zone, with approximately 50% of the movement occurring away from the fault—known as off‐fault deformation. Our results have implications for how evidence of past earthquakes preserved in the landscape are interpreted for forecasting future seismic hazard. Key Points: The first known example of an earthquake rupture imaged with preearthquake and postearthquake terrestrial laser scanningWe differenced the laser scans using an iterative closest point algorithm refined for accurately resolving centimeter‐scale offsetsWe find that off‐fault deformation is strongly partitioned between the horizontal and vertical components of displacement [ABSTRACT FROM AUTHOR]

We use the discrete element method to create numerical analogs to subduction megathrusts with natural roughness and heterogeneous fault friction. Boundary conditions simulate tectonic loading, inducing fault slip. Intermittently, slip develops into complex rupture events that include foreshocks, mainshocks, and aftershocks. We probe the kinematics and stress evolution of the fault zone to gain insight into the physical processes that govern these phenomena. Prolonged, localized differential stress drops precede dynamic failure, a phenomenon we attribute to the gradual unlocking of contacts as the fault dilates prior to rupture. Slip stability in our system appears to be governed primarily by geometrical phenomena, which allow both slow and fast slip to take place at the same areas along the fault. Similarities in slip behavior between simulated faults and real subduction zones affirm that modeled physical processes are also at work in nature. Plain Language Summary: Relatively little is known about how earthquakes start, what kind of behavior precedes them, and what physical processes control them. Earthquake precursors could serve as early warning signals and improve our ability to predict upcoming events. However, it is impossible to make direct observations on the earthquake initiation process in nature due to the fact that the earthquake source is buried many kilometers beneath the Earth's surface. To overcome this limitation, we use computer models to simulate earthquakes and make direct observations on the rupture process. We find that slow slip and stress changes take place just prior to large earthquake rupture on our simulated fault. We argue that these processes are controlled by relatively simple geometrical phenomena. Key Points: Slow slip, dilation, and differential stress drop precede dynamic failureFault roughness plays a dominant role in emergent rupture complexityStress transfer can initiate long‐duration nucleation processes that ultimately precipitate mainshocks [ABSTRACT FROM AUTHOR]

Abstract The present study is based on the use of Empirical Green's Function (EGF) deconvolution technique to retrieve the slip distribution of the 2014 M w 4.2 Jizan earthquake, Saudi Arabia. Two datasets of complex Source Time Functions (STFs) were retrieved using two appropriate EGF events. We inverted the STF datasets to recover the slip distribution over both nodal planes, using the Bayesian modeling followed by a linear least-squares method. The inversion was performed assuming both planes as the fault plane and examined the goodness of fit for each nodal plane. Based on a series of finite-source inversions using different rupture velocities, we resolved the rupture velocity at 2.7–2.8 km/s and the fault plane of NNW trending; paralleling the Red Sea rift. Using the estimated rupture velocities and the preferred fault plane, we imaged quite similar slip models, exhibiting two slip patches located to the updip and downdip directions from the hypocentre. The spatiotemporal slip distributions revealed a complex rupture history of such small-sized earthquake is likely to that reported for large-sized earthquakes. A seismic moment of 2.8–3.2E+15 NM and a corresponding moment magnitude of 4.2-4-3 are inferred. The stress drops obtained from the slip distribution models were 2.2–2.5 MPa; indicating a typical value that characterized the plate-boundary earthquakes. Highlights • Use EGF deconvolution technique to retrieve the STFs. • Invert STFS to obtain slip distribution model over the fault plane. • Two slip distribution models imaged two patches located to the up-dip and downdip directions from the hypocenter. • Low stress drops of 2.2–2.5 MPa indicated a typical value for plate-boundary earthquakes. [ABSTRACT FROM AUTHOR]

Drilling investigations and structural analysis of drill cores reveal that a fault gouge zone of 10–30 cm in width was observed at depths of ~260 to 900 m in nine drill holes that intersected the Nojima Fault (NF), on which the 1995 Mw 6.9 Kobe (Japan) earthquake occurred. Logging data and an analysis of mesostructures and microstructures in drill cores show that (i) a ~60‐m wide fault damage zone containing a 10‐ to 30‐cm‐thick fault gouge zone developed in the NF, (ii) the fault gouge zone can be divided into 11–20 thin layers of different color, and (iii) the individually colored layers contain different color breccias of fault gouge that are offset and/or cut by cracks and crack‐filled calcite and quartz veinlets. Our results reveal that the fault gouge zone probably records more than 11–20 paleoseismic faulting events along the NF during the late Pleistocene‐Holocene. Plain language Summary: Drilling investigations and structural analyses of drill cores reveal that a ~60‐m wide fault damage zone containing a 10‐ to 30‐cm‐thick fault gouge zone developed along the Nojima Fault (NF), on which the 1995 Mw 6.9 Kobe (Japan) earthquake occurred. The fault gouge zone was observed at depths of ~260 to 900 m in nine drill holes that intersected the NF. Our findings show that (i) the fault gouge zone observed at different depths in the nine drill cores is the principal fault slip zone of the NF, and (ii) the individual colored layers of the fault gouge zone provide a record of repeated seismic slip events within the NF and may therefore represent earthquake fossils, as do pseudotachylyte veins and fault scarps that record the systematic offset of valleys and terrace risers along the NF during the late Pleistocene‐Holocene. Key Points: Drilling investigations were carried out on the Nojima Fault that triggered the 1995 Mw 6.9 Kobe (Japan) earthquakeA 10‐ to 30‐cm‐thick fault gouge zone was observed in nine drill holes at depths of 260‐900 m, containing 11‐20 thin layers of different colors11‐20 thin layers of the gouge zone probably records more than 11‐20 seismic faulting events during the late Pleistocene‐Holocene [ABSTRACT FROM AUTHOR]

We analyze Global Navigation Satellite System (GNSS), Interferometric Synthetic Aperture Radar, and accelerometer data within the San‐in Shear Zone in order to clarify the coseismic and postseismic slip distributions associated with the Mw6.2 2016 Central Tottori earthquake. Inversion of the coseismic displacement data to estimate the slip distribution on the rupture fault shows a patch of large slip to the northwest of the hypocenter of the mainshock location. Relocated aftershocks and off‐fault seismicity 1 month after the mainshock are in agreement with stress change patterns caused by the mainshock fault. Inversion of near‐field GNSS displacements in 7 months following the earthquake under the assumption of afterslip does not show a preferred slip patch but rather a smooth distribution of the slip at shallow depths. Restricted slip propagation of afterslip on the 2016 event might suggest that inland faults in the San‐in Shear Zone are immature. Limited resolution of the GNSS data might inhibit us from finding the slip at depth. Key Points: Coseismic and postseismic slip distributions of the 2016 Central Tottori earthquake are retrieved from data inversionCoseismic slip is found northwest of the mainshock hypocenter over an 8‐km × 12‐km area, while afterslip is observed at shallow depthsRestricted slip propagation in the postseismic period suggests immature faults in the San‐in Shear Zone [ABSTRACT FROM AUTHOR]

The 15 April 2015 Mw 7.8 Nepal Gorkha earthquake occurred on a shallowly dipping portion of the Main Himalayan Thrust (MHT). Notable features of the event include (1) the dominance of a slip pulse of about 6‐s duration that unlocked the lower edge of the MHT and (2) the near‐horizontal fault geometry, which, combined with proximity of the free surface, allows surface‐reflected phases to break the across‐fault symmetries of the seismic wavefield. Our dynamic rupture simulations in an elastoplastic medium yield earthquake parameters comparable to those deduced from kinematic inversions, including seismic moment and rupture velocity. The simulations reproduce pulse‐like behavior predicting pulse widths in agreement with those kinematic studies and supporting an interpretation in which the pulse‐like time dependence of slip is principally controlled by rupture geometry. This inference is strongly supported by comparison of synthetic ground velocity with the near‐field high‐rate GPS recording at station KKN4, which shows close agreement in pulse width, amplitude, and pulse shape. That comparison also constrains the updip extent of rupture and disfavors significant coseismic slip on the shallow ramp segment. Over most of the rupture length, the simulated rupture propagates at a near‐constant maximum velocity (~90% of the S wave speed) that is controlled by the antiplane geometry and off‐fault plastic yielding. Simulations also reveal the role of reflected seismic waves from the free surface, which may have contributed ~30% elongation of the slip pulse, and show the potential for significant free‐surface interaction effects in shallow events of similar geometry. Key Points: Dynamic rupture model is consistent with the 2015 Gorkha earthquake slip‐pulse duration and propagation speedTime‐dependent features of the Gorkha earthquake are influenced by rupture geometry and free‐surface reflectionsConstraints from near‐field GPS recording imply no significant rupture of a shallow ramp [ABSTRACT FROM AUTHOR]

Summary: Pulse‐like records are well recognized for their potential to impose higher demands on structures when compared with ordinary records. The increased severity of the structural response usually caused by pulse‐like records is commonly attributed to the spectral increment around the pulse period. By comparing the building response to sets of spectrally equivalent pulse‐like and ordinary records, we show that there are characteristics of pulse‐like records beyond the shape of the acceleration response spectrum that affect the results of nonlinear dynamic analysis. Nevertheless, spectral shape together with the ratio of pulse period to the first‐mode structural period, Tp/T1, are confirmed as "sufficient" predictors for deformation and acceleration response metrics in a building, conditioned on the seismic intensity. Furthermore, the average spectral acceleration over a period range, AvgSA, is shown to incorporate to a good proxy for spectral shape, and together with Tp/T1, form an efficient and sufficient intensity measure for response prediction to pulse‐like ground motions. Following this latter route, we propose a record selection scheme that maintains the consistency of Tp with the hazard of the site but uses AvgSA to account for the response sensitivity to spectral shape. [ABSTRACT FROM AUTHOR]

The moment evolution of large earthquakes is a subject of fundamental interest to both basic and applied seismology. Specifically, an open problem is when in the rupture process a large earthquake exhibits features dissimilar from those of a lesser magnitude event. The answer to this question is of importance for rapid, reliable estimation of earthquake magnitude, a major priority of earthquake and tsunami early warning systems. Much effort has been made to test whether earthquakes are deterministic, meaning that observations in the first few seconds of rupture can be used to predict the final rupture extent. However, results have been inconclusive, especially for large earthquakes greater than Mw7. Traditional seismic methods struggle to rapidly distinguish the size of large‐magnitude events, in particular near the source, even after rupture completion, making them insufficient to resolve the question of predictive rupture behavior. Displacements derived from Global Navigation Satellite System data can accurately estimate magnitude in real time, even for the largest earthquakes. We employ a combination of seismic and geodetic (Global Navigation Satellite System) data to investigate early rupture metrics, to determine whether observational data support deterministic rupture behavior. We find that while the earliest metrics (~5 s of data) are not enough to infer final earthquake magnitude, accurate estimates are possible within the first tens of seconds, prior to rupture completion, suggesting a weak determinism. We discuss the implications for earthquake source physics and rupture evolution and address recommendations for earthquake and tsunami early warning. Key Points: GNSS observations in addition to seismic observations are critical to rapid and reliable earthquake magnitude estimatesGNSS‐derived displacements can reliably estimate earthquake magnitude prior to rupture completionEvidence of weak determinism suggests that near‐fault instrumentation is the best method for improving rapid magnitude estimations [ABSTRACT FROM AUTHOR]

Fault bends, and associated changes in fault dip, play a key role in explaining the scatter in maximum offset versus surface rupture length fault scaling relationships. Detailed field measurements of the fault geometry and magnitude of slip in the 2016–2017 Central Italy earthquake sequence, alongside three examples from large historical normal‐faulting earthquakes in different tectonic settings, provide multiple examples in which coseismic throw increases across bends in fault strike where dip also increases beyond what is necessary to accommodate a uniform slip vector. Coseismic surface ruptures produced by two mainshocks of the 2016–2017 Central Italy earthquake sequence (24 August 2016 Mw 6.0 and 30 October 2016 Mw 6.5) cross a ~0.83‐km amplitude along‐strike bend, and the coseismic throws for both earthquakes increase by a factor of 2–3, where the strike of the fault changes by ~28o and the dip increases by 20–25o. We present similar examples from historical normal faulting earthquakes (1887, Sonora earthquake, Mw 7.5; 1981, Corinth earthquakes, Mw 6.7–6.4; and 1983, Borah Peak earthquake, Mw 7.3). We demonstrate that it is possible to estimate the expected change in throw across a bend by applying equations that relate strike, dip, and slip vector to horizontal strain conservation along a nonplanar fault for a single earthquake rupture. The calculated slip enhancement in bends can explain much of the scatter in maximum displacement (Dmax) versus surface rupture length scaling relationships. If fault bends are unrecognized, they can introduce variation in Dmax that may lead to erroneous inferences of stress drop variability for earthquakes, and exaggerate maximum earthquake magnitudes derived from vertical offsets in paleoseismic data sets. Key Points: In throughgoing normal fault ruptures, such as the 2016 central Italy earthquakes, throw maxima occur at fault bendsFault growth and linkage result in steeper dips within bends, conservation of strain along strike results in throw maxima across bendsEnhanced slip at bends contributes to scatter in fault scaling relationships and overestimation bias of magnitude, moment and stress drop [ABSTRACT FROM AUTHOR]

Abstract: The role of seafloor roughness on the seismogenic behavior of subduction zones has been increasingly addressed over the past years, although their exact relationship remains unclear. Do subducting features like seamounts, fracture zones, or submarine ridges act as barriers, preventing ruptures from propagating, or do they initiate megathrust earthquakes instead? We address this question using a global approach, taking into account all oceanic subduction zones and a 117‐year time window of megathrust earthquake recording. We first compile a global database, SubQuake, that provides the location of a rupture epicenter, the overall rupture area, and the region where the largest displacement occurs (the seismic asperity) for MW ≥ 7.5 subduction interplate earthquakes. With these data, we made a quantitative comparison with the seafloor roughness seaward of the trench, which is assumed to be a reasonable proxy for the subduction interface roughness. We compare the spatial occurrence of megathrust ruptures, seismic asperities, and epicenters, with two roughness parameters: the short‐wavelength roughness RSW (12–20 km) and the long‐wavelength roughness RLW (80–100 km). We observe that ruptures with MW ≥ 7.5 tend to occur preferentially on smooth subducting seafloor at long wavelengths, which is especially clear for the MW > 8.5 events. At both short and long wavelengths, seismic asperities show a more amplified relation with smooth seafloor than rupture segments in general. For the epicenter correlation, we see a slight difference in roughness signal, which suggests that there might be a physical relationship between rupture nucleation and subduction interface roughness. [ABSTRACT FROM AUTHOR]

The 1957, M W 8.1, Gobi-Altai earthquake, Southern Mongolia, produced a ~360-km-long surface rupture along the Eastern Bogd fault. Cumulative offsets of geomorphic features suggest that the Eastern Bogd fault might produce characteristic slip over the last seismic cycles. Using orthophotographs derived from a dataset of historical aerial photographs acquired in 1958, we measured horizontal offsets along two thirds (~170 km) of the 1957 left-lateral strike-slip surface rupture. We propose a new empirical methodology to extract the average slip for each past earthquake that could be recognized along successive fault segments, to determine the slip distribution associated with successive past earthquakes. Our results suggest that the horizontal slip distribution of the 1957 Gobi-Altai earthquake is fairly flat, with an average offset of 3.5 m ± 1.3 m. A combination of our lateral measurements with vertical displacements derived from the literature, allows us to re-assess the magnitude of the Gobi-Altai earthquake to be between M W 7.8 and M W 8.2, depending on the depth of the rupture, and related value of the shear modulus. When comparing this magnitude to magnitudes derived from seismic data, it suggests that the rupture may have extended deeper than the 15 km to 20 km usually considered for the seismogenic crust. We observe that some fault segments are more likely than others to record seismic deformation through several seismic cycles, depending on the local rupture complexity and geomorphology. Additionally, our results allow us to model the horizontal slip function for the 1957 Gobi-Altai earthquake and for three previous paleoseismic events along 70% of the studied area. Along about 50% of the fault sections where we could recognize three past earthquakes, our results suggest that the slip per event was similar for each earthquake. [ABSTRACT FROM AUTHOR]

The spatio-temporal slip distribution of the earthquake that occurred on 8 August 2017 in Jiuzhaigou, China, was estimated from the teleseismic body wave and near-field Global Navigation Satellite System (GNSS) data (coseismic displacements and high-rate GPS data) based on a finite fault model. Compared with the inversion results from the teleseismic body waves, the near-field GNSS data can better restrain the rupture area, the maximum slip, the source time function, and the surface rupture. The results show that the maximum slip of the earthquake approaches 1.4 m, the scalar seismic moment is ~ 8.0 × 1018 N·m (Mw ≈ 6.5), and the centroid depth is ~ 15 km. The slip is mainly driven by the left-lateral strike-slip and it is initially inferred that the seismogenic fault occurs in the south branch of the Tazang fault or an undetectable fault, a NW-trending left-lateral strike-slip fault, and belongs to one of the tail structures at the easternmost end of the eastern Kunlun fault zone. The earthquake rupture is mainly concentrated at depths of 5-15 km, which results in the complete rupture of the seismic gap left by the previous four earthquakes with magnitudes > 6.0 in 1973 and 1976. Therefore, the possibility of a strong aftershock on the Huya fault is low. The source duration is ~ 30 s and there are two major ruptures. The main rupture occurs in the first 10 s, 4 s after the earthquake; the second rupture peak arrives in ~ 17 s. In addition, the Coulomb stress study shows that the epicenter of the earthquake is located in the area where the static Coulomb stress change increased because of the 12 May 2017 Mw7.9 Wenchuan, China, earthquake. Therefore, the Wenchuan earthquake promoted the occurrence of the 8 August 2017 Jiuzhaigou earthquake. [ABSTRACT FROM AUTHOR]

We present a seismological technique for determining the down-dip shape of seismogenic normal faults. Synthetic models of non-planar source geometries reveal the important signals in teleseismic P and SH waveforms that are diagnostic of down-dip curvature. In particular, along-strike SH waveforms are the most sensitive to variations in source geometry, and have significantly more complex and larger-amplitudewaveforms for curved source geometries than planar ones. We present the results of our forward-modelling technique for 13 earthquakes. Most continental normal-faulting earthquakes that rupture through the full seismogenic layer are planar and have dips of 30°-60°. There is evidence for faults with a listric shape from some of the earthquakes occurring in two regions; Tibet and East Africa. These ruptures occurred on antithetic faults, or minor faults within the hanging walls of the rifts affected, which may suggest a reason for the down-dip curvature. For these earthquakes, the change in dip across the seismogenic part of the fault plane is ≤30°. [ABSTRACT FROM AUTHOR]

On 2016 November 13, an Mw 7.8 earthquake occurred in the northeast of the South Island of New Zealand near Kaikoura. The earthquake caused severe damages and great impacts on local nature and society. Referring to the tectonic environment and defined active faults, the field investigation and geodetic evidence reveal that at least 12 fault sections ruptured in the earthquake, and the focal mechanism is one of the most complicated in historical earthquakes. On account of the complexity of the source rupture, we propose a multisegment fault model based on the distribution of surface ruptures and active tectonics. We derive the source rupture process of the earthquake using the kinematic waveform inversion method with the multisegment fault model from strong-motion data of 21 stations (0.05-0.35 Hz). The inversion result suggests the rupture initiates in the epicentral area near the Humps fault, and then propagates northeastward along several faults, until the offshore Needles fault. The Mw 7.8 event is a mixture of right-lateral strike and reverse slip, and the maximum slip is approximately 19 m. The synthetic waveforms reproduce the characteristics of the observed ones well. In addition, we synthesize the coseismic offsets distribution of the ruptured region from the slips of upper subfaults in the fault model, which is roughly consistent with the surface breaks observed in the field survey. [ABSTRACT FROM AUTHOR]

We have investigated several relationships between ML, M(NEIC) and Mw for the earthquakes locally recorded in the Saint Peter and Saint Paul Archipelago (SPSPA), Equatorial Atlantic. Because we only have one station in the area, we could not derive attenuation relations for events recorded at different distances at different stations. Our approach was then to compare our ML estimates with magnitudes reported by NEIC. This approach produced acceptable results particularly for epicentral distance smaller than 100 km. For distances greater that 100 km, there is a systematic increase in the residuals probable due to the lack of station correction and our inability to accurately estimate Q. We also investigate the Mw—M(NEIC) relationship. We find that Mw estimates using S-wave produce smaller residuals when compared with both M(NEIC). Finally, we also investigate the ML—Mw relationship and observe that given the data set we have, the 1:1 holds. We believe that the use of the present methodologies provide consistent magnitude estimates between all the magnitudes investigated that could be used to better assess seismic hazard in the region. [ABSTRACT FROM AUTHOR]

The article discusses the students' classroom activity in which the students use seismographs to detect, observe and evaluate earthquakes and other ground motions and create their own seismicity maps. It states that the activity helps the students in deriving necessary data that are used as evidence-based hypotheses about geologic processes.

Earthquake ruptures perturb stress within the surrounding crustal volume and as it is widely accepted now these stress perturbations strongly correlates with the following seismicity. Here we have documented five cases of the mainshock-aftershock sequences generated by the strike-slip faults from different tectonic environments of world in order to demonstrate that the stress changes resulting from large preceding earthquakes decades before effect spatial distribution of the aftershocks of the current mainshocks. The studied mainshock-aftershock sequences are the 15 October 1979 Imperial Valley earthquake ( Mw = 6.4) in southern California, the 27 November 1979 Khuli-Boniabad ( Mw = 7.1), the 10 May 1997 Qa'enat ( Mw = 7.2) and the 31 March 2006 Silakhor ( Mw = 6.1) earthquakes in Iran and the 13 March 1992 Erzincan earthquake ( Mw = 6.7) in Turkey. In the literature, we have been able to find only these mainshocks that are mainly characterized by dense and strong aftershock activities along and beyond the one end of their ruptures while rare aftershock occurrences with relatively lower magnitude reported for the other end of their ruptures. It is shown that the stress changes resulted from earlier mainshock(s) that are close in both time and space might be the reason behind the observed aftershock patterns. The largest aftershocks of the mainshocks studied tend to occur inside the stress-increased lobes that were also stressed by the background earthquakes and not to occur inside the stress-increased lobes that fall into the stress shadow of the background earthquakes. We suggest that the stress shadows of the previous mainshocks may persist in the crust for decades to suppress aftershock distribution of the current mainshocks. Considering active researches about use of the Coulomb stress change maps as a practical tool to forecast spatial distribution of the upcoming aftershocks for earthquake risk mitigation purposes in near-real time, it is further suggested that the background mainshocks along the neighbouring faults should be taken into account in producing the stress change maps for commenting on aftershock occurrences. [ABSTRACT FROM AUTHOR]

Field investigations and analyses of satellite images and aerial photographs reveal that the 2016 M 7.1 (Mj 7.3) Kumamoto earthquake produced a ∼40-km surface rupture zone striking NE-SW on central Kyushu Island, Japan. Coseismic surface ruptures were characterized by shear faults, extensional cracks, and mole tracks, which mostly occurred along the pre-existing NE-SW-striking Hinagu-Futagawa fault zone in the southwest and central segments, and newly identified faults in the northeast segment. This study shows that (i) the Hinagu-Futagawa fault zone triggered the 2016 Kumamoto earthquake and controlled the spatial distribution of coseismic surface ruptures; (ii) the southwest and central segments were dominated by right-lateral strike-slip movement with a maximum in-site measured displacement of up to 2.5 m, accompanied by a minor vertical component. In contrast, the northeast segment was dominated by normal faulting with a maximum vertical offset of up to 1.75 m with a minor horizontal component that formed graben structures inside Aso caldera; (iii) coseismic rupturing initiated at the jog area between the Hinagu and Futagawa faults, then propagated northeastward into Aso caldera, where it terminated. The 2016 M 7.1 Kumamoto earthquake therefore offers a rare opportunity to study the relationships between coseismic rupture processes and pre-existing active faults, as well as the seismotectonics of Aso volcano. [ABSTRACT FROM AUTHOR]

In August and October 2016, two normal fault earthquakes (M w 6.0 and M w 6.5, respectively) struck the Amatrice-Norcia area in the central Apennines, Italy. The mainshocks nucleated at depths of ~ 7–9 km with the co-seismic slip propagating upward along the Mt. Gorzano Fault (MGF) and Mt. Vettore Fault System (MVFS). To recognize possible weakening mechanisms along the carbonate-hosted seismogenic faults that generated the Amatrice-Norcia earthquakes, the fresh co-seismic fault exposure (i.e., “nastrino”) exposed along the Mt. Vettoretto Fault was sampled and analyzed. This exposed fault belongs to the MVFS and was exhumed from ~ 2–3 km depth. Over the fresh fault surface, phyllosilicates concentrated and localized along mm- to μm-thick layers, and truncated clasts and fluid-like structures were found. At the nano-scale, instead of their common platy-lamellar crystallographic texture, the analyzed phyllosilicates consist of welded nm-thick nanospherules and nanotubes similar to phyllosilicates deformed in rotary shear apparatus at seismic velocities or altered under high hydrothermal temperatures (> 250 °C). Moreover, the attitude of the Mt. Vettoretto Fault and its kinematics inferred from exposed slickenlines are consistent with the co-seismic fault and slip vectors obtained from the focal mechanisms computed for the 2016 mainshocks. All these pieces of evidence suggest that the Mt. Vettoretto Fault slipped seismically during past earthquakes and that co-seismic slip was assisted and facilitated at depths of < 3 km by phyllosilicate-rich layers and overpressured fluids. The same weakening processes may also have been decisive in facilitating the co-seismic slip propagation during the 2016 M w 6.0 Amatrice and M w 6.5 Norcia earthquakes. The microstructures found along the Mt. Vettoretto Fault, which is certainly a seismogenic fault, provide a realistic synoptic picture of co-seismic processes and weakening mechanisms that may occur in carbonate-hosted seismogenic faults. [ABSTRACT FROM AUTHOR]

Normal-mode summation is the most rapidly used method in calculating synthetic seismograms. However, normal-mode summation is mostly applied to point sources. For earthquakes triggered by faults extending for as long as several 100 km, the seismic waves are usually simulated by point source summation. In this paper, we attempt to follow a different route, i.e., directly calculate the excitation of each mode, and use normal-mode summation to obtain the seismogram. Furthermore, we assume the finite source to be a 'line source' and numerically calculate the transverse component of synthetic seismograms for vertical strike-slip faults. Finally, we analyze the features in the Love waves excited by finite faults. [ABSTRACT FROM AUTHOR]

On 25 April 2015, an M 7.8 earthquake occurred on the Main Himalaya Thrust fault with a dip angle of ~ 7° about 77 km northwest of Kathmandu, Nepal. This Nepal Gorkha event is the largest one on the Himalayan thrust belt since 1950. Here we use the compressive sensing method in the frequency domain to track the seismic radiation and rupture process of this event using teleseismic P waves recorded by array stations in North America. We also compute the distribution of static shear stress changes on the fault plane from a coseismic slip model. Our results indicate a dominant east-southeastward unilateral rupture process from the epicenter with an average rupture speed of ~3 km s. Coseismic radiation of this earthquake shows clear frequency-dependent features. The lower frequency (0.05-0.3 Hz) radiation mainly originates from large coseismic slip regions with negative coseismic shear stress changes. In comparison, higher frequency (0.3-0.6 Hz) radiation appears to be from the down-dip part around the margin of large slip areas, which has been loaded and presents positive coseismic shear stress changes. We propose an asperity model to interpret this Nepal earthquake sequence and compare the frequency-dependent coseismic radiation with that in subduction zones. Such frequency-dependent radiation indicates the depth-varying frictional properties on the plate interface of the Nepal section in the main Himalaya thrust system, similar to previous findings in oceanic subduction zones. Our findings provide further evidence of the spatial correlation between changes of static stress status on the fault plane and the observed frequency-dependent coseismic radiation during large earthquakes. Our results show that the frequency-dependent coseismic radiation is not only found for megathrust earthquakes in the oceanic subduction environment, but also holds true for thrust events in the continental collision zone. [ABSTRACT FROM AUTHOR]

Modelling the seismic potential of active faults is a fundamental step of probabilistic seismic hazard assessment (PSHA). An accurate estimation of the rate of earthquakes on the faults is necessary in order to obtain the probability of exceedance of a given ground motion. Most PSHA studies consider faults as independent structures and neglect the possibility of multiple faults or fault segments rupturing simultaneously (Fault to Fault -FtF- ruptures). The latest Californian model (UCERF-3) takes into account this possibility by considering a system level approach rather than an individual fault level approach using the geological, seismological and geodetical information to invert the earthquake rates. In many places of the world seismological and geodetical information long fault networks are often not well constrained. There is therefore a need to propose a methodology relying only on geological information to compute earthquake rate of the faults in the network. In this methodology, similarly to UCERF-3, a simple distance criteria is used to define FtF ruptures and consider single faults or FtF ruptures as an aleatory uncertainty. Rates of earthquakes on faults are then computed following two constraints: the magnitude frequency distribution (MFD) of earthquakes in the fault system as a whole must follow an imposed shape and the rate of earthquakes on each fault is determined by the specific slip-rate of each segment depending on the possible FtF ruptures. The modelled earthquake rates are then confronted to the available independent data (geodetical, seismological and paleoseismological data) in order to weigh different hypothesis explored in a logic tree. The methodology is tested on the Western Corinth Rift, Greece (WCR) where recent advancements have been made in the understanding of the geological slip rates of the complex network of normal faults which are accommodating the ~15 mm/yr North-South extension. Modelling results show that geological, seismological extension rates and paleoseismological rates of earthquakes cannot be reconciled with only single fault rupture scenarios and require hypothesising a large spectrum of possible FtF rupture sets. Furthermore, in order to fit the imposed regional Gutenberg-Richter MFD target, some of the slip along certain faults needs to be accommodated either with interseismic creep or as post-seismic processes. Furthermore, individual fault's MFDs differ depending on the position of each fault in the system and the possible FtF ruptures associated with the fault. Finally, a comparison of modelled earthquake rupture rates with those deduced from the regional and local earthquake catalogue statistics and local paleosismological data indicates a better fit with the FtF rupture set constructed with a distance criteria based on a 5 km rather than 3 km, suggesting, a high connectivity of faults in the WCR fault system. [ABSTRACT FROM AUTHOR]

The 2015 M w 7.8 Nepal earthquake occurred on the segment of the main Himalayan thrust fault between the Indian and Eurasian plates, and caused serious casualties. This earthquake may produce a profound impact on the evolution of the Tibetan Plateau and have brought a stress loading to faults within the plateau. In this paper, a high-resolution slip distribution of the 2015 Nepal earthquake is inverted from the InSAR and GPS data in the near field, and is used to compute the evolution of the cumulative Coulomb stress changes on faults in the earthquake-prone zone in the Tibetan Plateau. In the given reasonable parameters, the calculated co- and post-seismic stress changes on faults do not exceed 1.0 kPa at the north of latitude 32° in the Tibetan Plateau. The largest positive stress changes occur on the South Tibet Detachment fault, and the magnitudes are much less than 100 kPa. The estimated seismicity rate change on the segment of the South Tibet Detachment fault can be up to a level of two hundred thousandths. This indicates that there is a high hazard of earthquake triggering in the South Tibet Detachment fault and its adjacent regions. In the northern and eastern Tibetan Plateau, the estimated seismicity rate changes are lower than a level of one thousandth. However, some faults with a relative high background seismicity rate, such as the Xianshuihe and Longmenshan faults, may have a high hazard of earthquake triggering in the future. [ABSTRACT FROM AUTHOR]

We estimate coseismic offsets at 20 sites in India due to the 25 April 2015 Gorkha, Nepal (Mw 7.8) earthquake. Only four sites in the Indian region, immediately to the south of the rupture, showed discernible coseismic horizontal offsets ranging between 3 and 7 mm toward north. We invert these offsets along with 13 other offsets at GPS sites in Nepal and 33 offsets at sites in China, for the estimation of slip distribution on the causative rupture. We assume that rupture occurred on the Main Himalayan Thrust (MHT). In our estimated slip model, high slip reaching ∼5 m occurred east of the mainshock epicenter, and the slip on rupture terminated close to the Main Boundary Thrust (MBT). Thus the rupture for this earthquake remained blind, increasing the potential for future earthquake in the shallow, updip unruptured part of the MHT. [ABSTRACT FROM AUTHOR]

The 2015 Gorkha earthquake produced displacement on the lower half of a shallow decollement that extends 100 km south, and upward from beneath the High Himalaya and Kathmandu to where it breaks the surface to form the trace of the Himalayan Frontal Thrust (HFT), leaving unruptured the shallowest ∼50 km of the decollement. To address the potential of future earthquakes along this section of the HFT, we examine structural, stratigraphic, and radiocarbon relationships in exposures created by emplacement of trenches across the HFT where it has produced scarps in young alluvium at the mouths of major rivers at Tribeni and Bagmati. The Bagmati site is located south of Kathmandu and directly up dip from the Gorkha rupture, whereas the Tribeni site is located ∼200 km to the west and outside the up dip projection of the Gorkha earthquake rupture plane. The most recent rupture at Tribeni occurred 1221–1262 AD to produce a scarp of ∼7 m vertical separation. Vertical separation across the scarp at Bagmati registers ∼10 m, possibly greater, and formed between 1031–1321 AD. The temporal constraints and large displacements allow the interpretation that the two sites separated by ∼200 km each ruptured simultaneously, possibly during 1255 AD, the year of a historically reported earthquake that produced damage in Kathmandu. In light of geodetic data that show ∼20 mm/yr of crustal shortening is occurring across the Himalayan front, the sum of observations is interpreted to suggest that the HFT extending from Tribeni to Bagmati may rupture simultaneously, that the next great earthquake near Kathmandu may rupture an area significantly greater than the section of HFT up dip from the Gorkha earthquake, and that it is prudent to consider that the HFT near Kathmandu is well along in a strain accumulation cycle prior to a great thrust earthquake, most likely much greater than occurred in 2015. [ABSTRACT FROM AUTHOR]

Faults are complex structures that substantially influence the mechanical behavior and hydraulic connectivity of rock formations. Therefore, studying faults is important for a variety of disciplines such as geoscience, civil, geotechnical, reservoir engineering, and material science among others. Researchers from these disciplines have considered different aspects of faults, namely geometry, petrophysical properties and mechanics. Until now, these studies have evolved separately and at different scales, making it difficult to connect the geometric development of fault structure to its mechanics. The current understanding of fault geometry and growth is based on fracture mechanics and on many qualitative and quantitative studies on outcrop and seismic reflection surveys among other datasets. The application of fracture mechanics theory is mostly confined to simple geometries: elliptical models for a single fault plane and uniform properties. These applications predict the maximum displacement at the center of the fault, which is not in agreement with the new findings from 3D seismic and outcrop studies. These fracture mechanics models emphasize fault propagation along strike (in 2D). Although they can include the presence of a process zone at the fault tip, the models fail to explain the development of cross-fault damage zones and localization within the fault core as well as fault segmentation and displacement partitioning. Therefore, it is timely to revise the existing applications of fracture mechanics to simple fault geometries and to develop a data-driven fault mechanics possessing closer agreement with real, observed subsurface heterogeneity. This would allow better prediction of fault geometry, propagation, and growth in 3D. We suggest recent advances in non-destructive numerical characterization of faults and application of Deep Neural Networks (DNN) to map fault geometry and predict its properties from seismic data enable us for the first time to extract simultaneously faults' geometrical and mechanical properties at an unprecedented speed and accuracy, thus resolving the 3D fault shape and properties in ways that were unthinkable just a decade ago. [ABSTRACT FROM AUTHOR]

We present a thermodynamically based formulation for modelling dynamic rupture processes in the brittle crust using a continuum damage-breakage rheology. The model combines aspects of a continuum viscoelastic damage framework for brittle solids with a continuum breakage mechanics for granular flow within dynamically generated slip zones. The formulation accounts for the density of distributed cracking and other internal flaws in damaged rocks with a scalar damage parameter, and addresses the grain size distribution of a granular phase in the slip zone with a breakage parameter. A dynamic brittle instability is associated with a critical level of damage in the solid, leading to loss of convexity of the solid strain energy, localization and transition to a granular phase associated with lower energy level. The continuum damagebreakage rheology model treats the localization to a slip zone at the onset of dynamic rupture and post-failure recovery process as phase transitions between solid and granular states. The model generates sub- and supershear rupture velocities and pulse-type ruptures seen also in frictional models, and additional important features such as strong dynamic changes of volumetric strain near the rupture front and diversity of nucleation mechanisms. The propagation of rupture front and slip accumulation at a point are correlated with sharp dynamic dilation followed by a gradual decay to a level associated with the final volumetric change associated with the granular phase transition in the slipping zone. The local brittle failure process associated with the solid-granular transition is expected to produce isotropic radiation in addition to the deviatoric terms. The framework significantly extends the ability to model brittle processes in complex geometrical structures and allows analysing the roles of gouge thickness and other parameters on nucleation, rupture and radiation characteristics. [ABSTRACT FROM AUTHOR]

We present a mathematically consistent numerical algorithm for the simulation of earthquake rupture with rate-and-state friction. Its main features are adaptive time stepping, a novel algebraic solution algorithm involving nonlinear multigrid and a fixed point iteration for the rate-and-state decoupling. The algorithm is applied to a laboratory scale subduction zonewhich allows us to compare our simulations with experimental results. Using physical parameters from the experiment, we find a good fit of recurrence time of slip events as well as their rupture width and peak slip. Computations in 3-D confirm efficiency and robustness of our algorithm. [ABSTRACT FROM AUTHOR]

In this study we present a detailed analysis of natural stresses in the Northern Tien Shan crust averaged in a window of 10-15 km obtained from seismological data of the local KNET network. The transformation of focal mechanism data into the parameters of the stress tensor was based on the method of cataclastic analysis of rupture displacement elaborated by Yu.L. Rebetsky (Institute of Physic of the Earth, Moscow). The results, including the orientation of the principal stress axes and the reduced stresses, are presented for four depth layers. It was shown that the central part of the study area is dominated by horizontal compression, while multiple domains characterized by horizontal shear and superimposed compression or pure horizontal shear are also present (uppermost layers in the eastern part of the Chuya depression, Suusamyr depression and adjoining regions, in the central part of the Kyrgyz Range). There are also several large domains of high and low effective confining pressure, which defines the corresponding deviator stress, according to the Coulomb-Mohr law. It was shown that relatively strong earthquakes are correlated with zones with low levels of effective pressure where the ruptures are characterized by lower resistance to brittle fracturing, i.e., Coulomb friction stresses. It was also shown that a distinct segment of the ~ 60 km E-W striking fault on the northern slope of the Kyrgyz Range generates a uniform distribution of stresses, corresponding to a dextral slip along of its edges. [ABSTRACT FROM AUTHOR]

This study investigates the effects of near and far fault ground motion on the seismic behavior of historical arch bridges through a combined numerical and experimental evaluation. The approach undertaken begins with finite element modeling of the arch bridge and identification of the most significant vibration modes of the bridge through ambient vibration testing. Uncertain parameters of the finite element model are then revised through systematic comparisons of the measured vibration models to those that are predicted by the model. The revised finite element model is used to predict the time history response for displacements and stresses through which the effect of the finite element model updating on model predictions are evaluated. Furthermore, displacements and stresses obtained considering both near and far fault ground motions are then compared. Results indicate that near fault ground motion imposes higher seismic demand on the arch bridge observed in both higher displacements and stresses. [ABSTRACT FROM AUTHOR]

The western Anatolia extension region consists of major E-W trending normal faults and numerous subsidiary faults aligned obliquely to major fault systems. In this paper, the NW-SE trending Mugla Fault was studied through an archaeoseismological analysis of the Lagina sacred area, which was supported by geological and geomorphological field evidence collected along the fault zone. The sacred area is cut by fractures that have caused extensive deformations and displacements in ruins along the fault. The orientations of collapsed columns, folding on the grounds, dilation, and tilting of the walls are systematic. The axes of the observed deformations are perpendicular to the Mugla Fault and could be related to coseismic effects. Although there are no historical records of a large earthquake on the Mugla Fault, the results of thermoluminescence and radiocarbon dating in the Lagina sacred area indicate that a large event occurred in the 4th c. ad or slightly later. Thus, considering the field evidence that has been collected along the Mugla Fault, it can be concluded that subsidiary faults aligned obliquely to major normal fault systems have strike-slip components and may be associated with a significant portion of the recent dynamics in western Anatolia. [ABSTRACT FROM AUTHOR]