Slip Patches Research Articles

Resolving co- and early post-seismic slip variations of the 2021 MW 7.4 Maduo earthquake in east Bayan Har block with a block-wide distributed deformation mode from satellite synthetic aperture radar data

On 21 May 2021 (UTC), an <i>M</i>_W 7.4 earthquake jolted the east Bayan Har block in the Tibetan Plateau. The earthquake received widespread attention as it is the largest event in the Tibetan Plateau and its surroundings since the 2008 Wenchuan earthquake, and especially in proximity to the seismic gaps on the east Kunlun fault. Here we use satellite interferometric synthetic aperture radar data and subpixel offset observations along the range directions to characterize the coseismic deformation of the earthquake. Range offset displacements depict clear surface ruptures with a total length of ~170 km involving two possible activated fault segments in the earthquake. Coseismic modeling results indicate that the earthquake was dominated by left-lateral strike-slip motions of up to 7 m within the top 12 km of the crust. The well-resolved slip variations are characterized by five major slip patches along strike and 64% of shallow slip deficit, suggesting a young seismogenic structure. Spatial–temporal changes of the postseismic deformation are mapped from early 6-day and 24-day InSAR observations, and are well explained by time-dependent afterslip models. Analysis of Global Navigation Satellite System (GNSS) velocity profiles and strain rates suggests that the eastward extrusion of plateau is diffusely distributed across the east Bayan Har block, but exhibits significant lateral heterogeneities, as evidenced by magnetotelluric observations. The block-wide distributed deformation of the east Bayan Har block along with the significant co- and post-seismic stress loadings from the Madoi earthquake imply high seismic risks along regional faults, especially the Tuosuo Lake and Maqên–Maqu segments of the Kunlun fault that are known as seismic gaps.

Insights on fault reactivation during the 2019 November 11, Mw 4.9 Le Teil earthquake in southeastern France, from a joint 3-D geological model and InSAR time-series analysis

SUMMARY The 2019, Mw 4.9 Le Teil earthquake occurred in southeastern France, causing substantial damage in this slow deforming region. Field observations, remote sensing and seismological studies following the event revealed that coseismic slip concentrates at shallow depth along a ∼5 km long rupture associated with surface breaks and a thrusting mechanism. We further investigate this earthquake by combining geological field mapping, 3-D geology, InSAR time-series analysis and a coseismic slip inversion. From structural, stratigraphic and geological data collected around the epicentre, we first produce a 3-D geological model of the region surrounding the rupture using the GeoModeller software. Our model includes the geometry of the geological layers and the main faults, including the La Rouvière Fault, (LRF) the Oligocene normal fault that ruptured during the earthquake. We generate a time-series of surface displacement from Sentinel-1 SAR data ranging from early 2019 January to late 2020 January using the NSBAS processing chain. The spatio-temporal patterns of surface displacement for this time span show neither a clear pre-seismic signal nor significant post-seismic transient deformation. We extract the coseismic displacement pattern from the InSAR time-series, highlighting along-strike variations of coseismic surface slip. The maximum relative displacement along the line of sight is up to ∼16 cm and is located in the southwestern part of the rupture. We invert for the slip distribution on the fault from the InSAR coseismic surface displacement field. Constraining our fault geometry from the geological model, acceptable fault dip ranges between 55° and 60°. Our model confirms the reactivation of LRF, with reverse slip at very shallow depth and two main slip patches reaching, respectively, 30 and 24 cm of slip, both around 500 m depth. We finally discuss how the 3-D fault geometry and geological structure may have impacted the slip distribution and propagation during the earthquake. This study is a step to reassess the seismic hazard of the many faults similar to the La Rouvière one along the Cévennes fault system, in a densely populated area hosting several sensitive nuclear sites.

Complex Slip Distribution of the 2021 Mw 7.4 Maduo, China, Earthquake: An Event Occurring on the Slowly Slipping Fault

Abstract The 21 May 2021 Maduo earthquake occurred on the Kunlun Mountain Pass–Jiangcuo fault (KMPJF), a seismogenic fault with no documented large earthquakes. To probe its kinematics, we first estimate the slip rates of the KMPJF and Tuosuo Lake segment (TLS, ∼75 km north of the KMPJF) of the East Kunlun fault (EKLF) based on the secular Global Positioning System (GPS) data using the Markov chain Monte Carlo method. Our model reveals that the slip rates of the KMPJF and TLS are 1.7 ± 0.8 and 7.1 ± 0.3 mm/yr, respectively. Then, we invert high-resolution GPS and Interferometric Synthetic Aperture Radar observations to decipher the fault geometry and detailed coseismic slip distribution associated with the Maduo earthquake. The geometry of the KMPFJ significantly varies along strike, composed of five fault subsegments. The most slip is accommodated by two steeply dipping fault segments, with the patch of large sinistral slip concentrated in the shallow depth on a simple straight structure. The released seismic moment is ∼1.5×1020 N·m, equivalent to an Mw 7.39 event, with a peak slip of ∼9.3 m. Combining the average coseismic slip and slip rate of the main fault, an earthquake recurrence period of ∼1250−400+1120 yr is estimated. The Maduo earthquake reminds us to reevaluate the potential of seismic gaps where slip rates are low. Based on our calculated Coulomb failure stress, the Maduo earthquake imposes positive stress on the Maqin–Maqu segment of the EKLF, a long-recognized seismic gap, implying that it may accelerate the occurrence of the next major event in this region.

Observations noise and stability of geodetic variable slip fault models

SUMMARY Variable slip models of seismic faults represent ill-posed (or underdetermined) problems with infinity of solutions; a single solution is typically obtained using constraints imposed by the L-curve theory, through an experimentally derived coefficient which optimizes the trade-off between fault roughness (smoothness, mean slip gradient) and mean misfit of observations to the fault model. However, in some cases, diverse variable slip fault models (VSFM) have been presented, even using the same data sets. In this paper, we investigate the problem of stability (sensitivity) of VSFM to input geodetic (GNSS/GPS, INSAR) ground slip measurements, based on statistics of synthetic data: statistically similar sets of synthetic ground displacements were the only degree of freedom in an analysis leading to comparable VSFM, regarded as images of slip amplitude and of rake. Differences of synthetic models from the reference fault model for different levels of noise, both for filtered and for PCA-filtered were examined, and results were summarized in images depicting the differences and the variability (standard deviation) of each fault slip patch, both for slip and rake. From this ‘truth’-based approach, it was found that an increase in observations noise is reflected in increase of variability (instability) of fault models, especially away from the fault centre, higher for slip than in rake. Analysis with a different virtual observations system indicates that stations above the fault tend to show spurious stress concentration areas on the fault surface, in agreement with previous studies. Analysis with synthetic data appears as a promising strategy to validate a VSFM based on geodetic data.

Joint Inversion of Rupture across a Fault Stepover during the 8 August 2017 Mw 6.5 Jiuzhaigou, China, Earthquake

Abstract The 8 August 2017 Mw 6.5 Jiuzhaigou earthquake occurred in a tectonically fractured region in southwest China. We investigate the multifault coseismic rupture process by jointly analyzing teleseismic, strong-motion, high-rate Global Positioning System, and Interferometric Synthetic Aperture Radar (InSAR) datasets. We clearly identify two right-stepping fault segments and a compressional stepover based on variations in focal mechanisms constrained by coseismic InSAR deformation data. The average geometric parameters of the northwest and southeast segments are strike = 130°/dip = 57° and strike = 151°/dip = 70°, respectively. The rupture model estimated from a joint inversion of the seismic and geodetic datasets indicates that the rupture initiated on the southeastern segment and jumped to the northwestern segment, resulting in distinctive slip patches on the two segments. A 4-km-long coseismic slip gap was identified around the stepover, consistent with the aftershock locations and mechanisms. The right-stepping segmentation and coseismic rupture across the compressional stepover exhibited by the 2017 Jiuzhaigou earthquake are reminiscent of the multifault rupture pattern during the 1976 Songpan earthquake sequence farther south along the Huya fault system in three successive Ms∼7 events. Although the common features of fault geometry and stepover may control the similarity in event locations and focal mechanisms of the 2017 and 1976 sequences, the significantly wider (~15 km) stepover in the 1976 sequence likely prohibited coseismic rupture jumping and hence reduced seismic hazard.

Rupture Process of the 2020 Mw7.0 Samos Earthquake and its Effect on Surrounding Active Faults

AbstractOn October 30, 2020, a Mw7.0 normal faulting earthquake struck the eastern Aegean Sea, causing casualties, and substantial damage at Samos island and Izmir province. We constrained the rupture history of the main shock by jointly inverting GPS static offsets, high‐rate GPS recordings, strong motion waveforms, and teleseismic broadband data. The results revealed that the slip distribution was dominated by a slip patch which spans a depth range of 3–13 km and occupies ∼30 km along strike. The rupture propagated toward SW and updip and the maximum slip amplitude reached 4.6 m at 7 km depth. The remarkably low aftershock productivity within the main asperity suggests a near complete stress release. Coseismic Coulomb stress changes effectively explain the entire aftershock sequence implying absence of short‐term postseismic stress transfer mechanisms. The static stress change calculations also suggest that active faults around Sıgacık and Kusadasi Bays have been brought closer to rupture.

Rupture Kinematics of the 11 January 2021 Mw 6.7 Hovsgol, Mongolia, Earthquake and Implications in the Western Baikal Rift Zone

AbstractThe Mongolia plateau is the farthest intracontinental region of the India–Eurasia collision and is a transition zone between north–south convergence to the south in the Tien Shan and northwest–southeast extension to the north in the Baikal rift. Mongolia has experienced four M 8 earthquakes since 1905, but due to limited observations, the mechanism of these strong earthquakes and regional tectonics are poorly understood. The 11 January 2021 Mw 6.7 Hovsgol, Mongolia, earthquake is the largest event that has occurred in the Hovsgol graben, which is noted for being the northernmost convergence region of the India–Eurasia collision and the youngest extension region of the Baikal rift. In this article, the coseismic displacements are retrieved by space geodesy for the first time in this region, providing good constraints for the deformation pattern. We use a finite-fault inversion of InSAR and teleseismic data, and a backprojection analysis to reveal the rupture kinematics of this event. The geometry of the Hovsgol fault is determined as east-dipping with a dip of 45°. The rupture process is characterized by a northwestward propagation with a moderate average rupture velocity of ∼2.0 km/s and a complex slip pattern composed of two major slip patches with dimensions of 40 km×20 km. The oblique slip, illustrated by predominate extension and significant dextral striking, confirms the right-lateral-striking faulting in the Hovsgol rift, which indicates that the eastwardly north–south convergence across the southwest segment of the Baikal rift has decreased.

Structural Controls Over the 2019 Ridgecrest Earthquake Sequence Investigated by High‐Fidelity Elastic Models of 3D Velocity Structures

AbstractWe develop finite element models of the coseismic displacement field accounting for the 3D elastic structures surrounding the epicentral area of the 2019 Ridgecrest earthquake sequence containing two major events of Mw7.1 and Mw6.4. The coseismic slip distribution is inferred from the surface displacement field recorded by interferometric synthetic aperture radar. The rupture dip geometry is further optimized using a novel nonlinear‐crossover‐linear inversion approach. It is found that accounting for elastic heterogeneity and fault along‐strike curvilinearity improves the fit to the observed displacement field and yields a more accurate estimate of geodetic moment and Coulomb stress changes. We observe spatial correlations among the locations of aftershocks and patches of high slip, and rock anomalous elastic properties, suggesting that the shallow crust's elastic structures possibly controlled the Ridgecrest earthquake sequence. Most of the coseismic slip with a peak slip of 7.4 m at 3.6 km depth occurred above a zone of reduced S‐wave velocity and significant post‐Mw7.1 afterslip. This implies that viscous materials or fluid presence might have contributed to the low rupture velocity of the mainshock. Moreover, the zone of high slip on the northwest‐trending fault segment is laterally bounded by two aftershock clusters, whose location is characterized by intermediate rock rigidity. Notably, some minor orthogonal faults consistently end above a subsurface rigid body. Overall, these observations of structural controls improve our understandings of the seismogenesis within incipient fault systems.

Future magnitude 7.5 earthquake offshore Martinique: spotlight on the main source features controlling ground motion prediction

SUMMARY The eastern offshore of Martinique is one of the active areas of the Lesser Antilles Subduction Zone (LASZ). Although its seismicity is moderate compared to other subduction zones, LASZ is capable of generating a M 7+ interplate earthquake and recent studies and historical events, such as the M 8 1839 and M 7–7.5 1946 earthquakes, confirm this possibility. Given the high risk that Martinique can face in case of unpreparedness for such a M 7+ earthquake, and the lack of a regional seismic hazard study, we investigated through numerical modelling how ground motion can vary for a hypothetical Mw 7.5 interplate earthquake. Our main objective is to highlight the major factors related to earthquake source that can cause the highest variation in ground motion at four broad-band seismic stations across Martinique. For this purpose, we generated 320 rupture scenarios through a fractal kinematic source model, by varying rupture directivity, source dimension, slip distribution. We computed the broad-band ground motion (0.5–25 Hz) by convolution of source–time functions with empirical Green’s functions (EGFs), that we selected from the analysis of moderate events (M 4–4.5) recorded in the area of interest since 2016 by the West Indies network. We found that the fault geometry and the spatial extension of the largest slip patch are the most influential factors on ground motion. The significance of the variation of the predicted ground motion with respect to ground motion prediction equations (GMPEs) depends on the evaluated frequency of ground motion and on the station. Moreover, we concluded that the EGF selection can be another significant factor controlling the modelled ground motion depending on station. Our results provide a new insight for the seismic source impact on ground motion across Martinique and can guide future blind seismic hazard assessment studies in different regions.

Correction to: Seismotectonic implications of the 2020 Samos, Greece, Mw 7.0 mainshock based on high‐resolution aftershock relocation and source slip model

The 30 October 2020, Mw 7.0 Samos mainshock took place in the offshore north of Samos Island in eastern Aegean area, previously struck in 1904 with a comparable magnitude earthquake offshore the southern coastline. The investigation of the aftershock seismicity evolution and the properties of the activated fault network was accomplished with aftershock relocation performed with the double-difference and cross-correlation techniques. The highly accurate relocated seismicity illustrates a well-defined E–W activated structure located deeper than 5 km with an average depth of ~ 12 km. Moment tensor solutions indicate mostly normal faulting with an average T-axis ~ 185ο. Strong-motion waveform modeling revealed a N-dipping fault plane with a coseismic slip patch of 36 km $$\times$$ 22 km and a maximum slip equal to 1 m at 12 km depth. The slip is mainly concentrated in a single asperity implying a rupture mode of asperities breaking in isolated earthquakes rather than to cooperate to produce a larger rupture. Coulomb stress calculations unveil increased positive static stress changes values at the locations of the majority of the aftershocks and activation of minor fault segments by stress transfer.

Frequency‐Dependent Rupture Characteristics of the 30 October 2016 Mw 6.5 Norcia, Italy Earthquake Inferred From Joint Multi‐Scale Slip Inversion

AbstractThe relationship between multi‐scale rupture characteristics and different frequency contents of the seismic waveforms has been revealed for moderate to great earthquakes by previous researches. To better understand such a relationship, we propose a two‐step strategy to perform finite fault inversion in the wavelet domain with multi seismic datasets, involving the seismic observations at both teleseismic and near‐field distances. In our two‐step strategy, the larger‐scale pattern of the seismic rupture is first constrained by using the lower‐frequency seismic waveforms and then based on the lower‐frequency rupture pattern, the higher‐frequency rupture features are obtained by performing inversion with higher‐frequency data. We apply this two‐step strategy with multi seismic datasets to study the 30 October 2016 Mw 6.5 Norcia, Italy earthquake, which is the largest event of the 2016–2017 central Italy seismic sequence. By analyzing the recovered slip patterns from different frequency bands, we notice the frequency‐dependent rupture features of this earthquake. The lower‐frequency slip concentrated at the shallower depths above the source, and the higher‐frequency slip appeared near the edges of the lower‐frequency slip patch, working as “barriers”, due to fault complexities in central Italy. The frequency‐dependent slip pattern of the 30 October 2016 Norcia earthquake shows a relation to the unevenly distributed pore fluid pressure, as indicated by Vp/Vs tomography from previous researches. Our results suggest that fault complexities and unevenly distributed fluids contribute to the frequency‐dependent rupture of this earthquake.

Earthquake Source Investigation of the Kanallaki, March 2020 Sequence (North-Western Greece) Based on Seismic and Geodetic Data

The active collision of the Apulian continental lithosphere with the Eurasian plate characterizes the tectonics of the Epirus region in northwestern Greece, invoking crustal shortening. Epirus has not experienced any strong earthquakes during the instrumental era and thus there is no detailed knowledge of the way the active deformation is being expressed. In March 2020, a moderate size (Mw 5.8) earthquake sequence occurred close to the Kanallaki village in Epirus. The mainshock and major aftershock focal mechanisms are compatible with reverse faulting, on NNW-ESE trending nodal planes. We measure the coseismic surface deformation using radar interferometry and investigate the possible fault geometries based on seismic waveforms and InSAR data. Slip distribution models provide good fits to both nodal planes and cannot resolve the fault plane ambiguity. The results indicate two slip episodes for a 337° N plane dipping 37° to the east and a single slip patch for a 137° N plane dipping 43° to 55° to the west. Even though the area of the sequence is very close to the triple junction of western Greece, the Kanallaki 2020 activity itself seems to be distinct from it, in terms of the acting stresses.

Nucleation of frictional sliding by coalescence of microslip

The onset of frictional motion is mediated by the dynamic propagation of a rupture front, analogous to a shear crack. The rupture front nucleates quasi-statically in a localized region of the frictional interface and slowly increases in size. When it reaches a critical nucleation length it becomes unstable, propagates dynamically and eventually breaks the entire interface, leading to macroscopic sliding. The nucleation process is particularly important because it determines the stress level at which the frictional interface fails, and therefore, the macroscopic friction strength. However, the mechanisms governing nucleation of frictional rupture fronts are still not well understood. Specifically, our knowledge of the nucleation process along a heterogeneous interface remains incomplete. Here, we study the nucleation of localized slip patches on linear slip-weakening interfaces with deterministic and stochastic heterogeneous friction properties. Using numerical simulations, we analyze the process leading to a slip patch of critical size for systems with varying correlation lengths of the local friction strength. Our deterministic interface model reveals that the growth of the critical nucleation patch at interfaces with small correlation lengths is non smooth due to the coalescence of neighboring slip patches. Existing analytical solutions do not account for this effect, which leads to an overestimation of global interface strength. Conversely, when the correlation length is large, the growth of the slip patch is continuous and our simulations match the analytical solution. Furthermore, nucleation by coalescence is also observed on stochastic interfaces with small correlation length. In this case, the applied load for a given slip patch size is a random variable. We show that its expectation follows a logistic function, which allows us to predict the strength of the interface well before failure occurs. Our model and observations provide new understanding of the nucleation process and its effect on the static frictional strength.

The 22 July 2020 MW 7.8 Shumagin seismic gap earthquake: Partial rupture of a weakly coupled megathrust

The earthquake potential of the Shumagin seismic gap along the Alaska Peninsula (∼162°W to ∼158.5°W) has been debated for more than 40 years. On 22 July 2020, the eastern half of the gap hosted an MW 7.8 earthquake involving a patchy rupture of the megathrust in the depth range of 20 to 45 km. The space-time slip distribution is determined by joint inversion of teleseismic P and SH waves and static displacements from regional GPS stations. The event initiated near the epicenter of the 10 November 1938 (MW 8.2) event, and ruptured westward, with little/no overlap with the 1938 rupture zone. The main slip patch has peak slip of ∼3.8 m below the Shumagin Islands, and produced ∼30 cm uplift and ∼25 cm SSE horizontal displacement on Chernabura Island. The slip model predicts well the small (<1 cm) tsunami signals persisting for more than ten hours observed at deep-water DART seafloor pressure recordings along the Alaska-Aleutian arc. Aftershocks with depths from 20 to 40 km fringe the large-slip patches, and show westward concentration during the first month after the mainshock. Aftershocks up-dip of the 1948 MW 7.1 event contribute to the high level of modest-size background seismicity extending to the trench in the region of very low seismic coupling (0.0–0.1) in the western Shumagin gap east of the 1 April 1946 (MW 8.6) rupture zone. The 31 May 1917 event is the last major earthquake to rupture the eastern half of the Shumagin gap, and has a lower surface wave magnitude (MSG-R 7.4, horizontal components) compared to the 2020 event (MSG-R 7.7, vertical components). Comparison of instrument-equalized waveforms for the 1917 and 2020 events indicates similar size contrast and differences in overall rupture duration and slip complexity. The 2020 rupture has average slip of ∼1.9 m over the 3600 km2 region with co-seismic slip ≥1 m. This is much less than the ∼6.7 m of potentially accumulated slip deficit since 1917, consistent with geodetic estimates of low average seismic coupling coefficient of 0.1–0.4. The megathrust seaward of the 2020 event has low seismicity and may either be aseismic or capable of comparable size ruptures. Comparisons are made with other subduction zones that have experienced relatively deep megathrust slip in regions with moderate seismic coupling.

A self-similar dynamic rupture model based on the simplified wave-rupture analogy

SUMMARYThe investigation of stresses, faults, structure and seismic hazards requires a good understanding and mapping of earthquake rupture and slip. Constraining the finite source of earthquakes from seismic and geodetic waveforms is challenging because the directional effects of the rupture itself are small and dynamic numerical solutions often include a large number of free parameters. The computational effort is large and therefore difficult to use in an exploratory forward modelling or inversion approach. Here, we use a simplified self-similar fracture model with only a few parameters, where the propagation of the fracture front is decoupled from the calculation of the slip. The approximative method is flexible and computationally efficient. We discuss the strengths and limitations of the model with real-case examples of well-studied earthquakes. These include the Mw 8.3 2015 Illapel, Chile, megathrust earthquake at the plate interface of a subduction zone and examples of continental intraplate strike-slip earthquakes like the Mw 7.1 2016 Kumamoto, Japan, multisegment variable slip event or the Mw 7.5 2018 Palu, Indonesia, supershear earthquake. Despite the simplicity of the model, a large number of observational features ranging from different rupture-front isochrones and slip distributions to directional waveform effects or high slip patches are easy to model. The temporal evolution of slip rate and rise time are derived from the incremental growth of the rupture and the stress drop without imposing other constraints. The new model is fast and implemented in the open-source Python seismology toolbox Pyrocko, ready to study the physics of rupture and to be used in finite source inversions.

Spherical-earth finite element model of coseismic slip distribution during the 2010 Mentawai earthquake

The moment magnitude (MW) 7.8 earthquake occurred along the Mentawai seismic region of Sunda Trench on 25 October 2010, which is classified as the tsunami earthquake. The GPS inversion results under the assumption of simple horizontal layered medium show that the slip near the trench is not apparent, which is inconsistent with our understanding of tsunami earthquake. Here, we construct a spherical-earth finite element model (FEM) to investigate the coseismic slip distribution of the 2010 Mentawai earthquake by geometrically combining the subducting plate with a precise subduction interface. The FEM-based coseismic slip distribution shows that there are three major slip patches on the fault interface, one is located on the shallow region and the other two are located on the deeper part of the fault plane. The largest slip patch is located near the trench with the depth less than 10 km, and the maximum amplitude is about 12 m. This significant near-trench slip is consistent with the tsunami earthquake studies. A more realistic domain of FEM is used to change the coseismic slip distribution, thus significantly improving the fitting degree of the data model. These findings suggest that the FEM-derived Green's function is essential to image a more robust and realistic coseismic slip distribution of the large earthquake in the subduction zone.

Megathrust Slip Behavior for Great Earthquakes Along the Sumatra-Andaman Subduction Zone Mapped From Satellite GOCE Gravity Field Derivatives

During the last two decades, space geodesy allowed mapping accurately rupture areas, slip distribution, and seismic coupling by obtaining refined inversion models and greatly improving the study of great megathrust earthquakes. A better understanding of these phenomena involving large areas of hundreds of square kilometers came from the last gravity satellite mission that allowed detecting mass transfer through the Earth interior. In this work, we performed direct modeling of satellite GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) derived gravity gradients up to degree/order N = 200 of the harmonic expansion and then corrected this by the effect of topography. Cutting off the model up to this degree/order allows inferring mass heterogeneities located at an approximate depth of 31 km, just along the plate interface where most (but not all) significant slip occurs. Then, we compared the vertical gravity gradient to well-constrained coseismic slip models for three of the last major earthquakes along the Sunda interface. We analyzed seismic rupture behavior for recent and for historical earthquakes along this subduction margin and the relationship of the degree of interseismic coupling using the gravity signal. From this, we found that strong slip patches occurred along minima gravity gradient lobes and that the maximum vertical displacements were related quantitatively to the gravity-derived signal. The degree of interseismic coupling also presents a good correspondence to the vertical gravity gradient, showing an inverse relationship, with low degrees of coupling over regions of relatively higher density. This along-strike segmentation of the gravity signal agrees with the along-strike seismic segmentation observed from recent and historical earthquakes. The thermally controlled down-dip ending of the locked fault zone along central Sumatra also presented an inverse relationship with the density structure along the forearc inferred using our modeling. From this work, we inferred different mass heterogeneities related to persistent tectonic features along the megathrust and along the marine forearc, which may control strain accumulation and release along the megathrust. Combining these data with geodetical and seismological data could possibly delimit and monitor areas with a higher potential seismic hazard around the world.

Spatial\u2013temporal properties of afterslip associated with the 2015 Mw 8.3 Illapel earthquake, Chile

In order to characterize the spatial–temporal properties of postseismic slip motions associated with the 2015 Illapel earthquake, the daily position time series of 13 GNSS sites situated at the near-field region are utilized. Firstly, a scheme of postseismic signal extraction and modeling is introduced, which can effectively extract the postseismic signal with consideration of background tectonic movement. Based on the extracted postseismic signal, the spatial–temporal distribution of afterslip is inverted under the layered medium model. Compared with coseismic slip distribution, the afterslip is extended to both deep and two sides, and two peak slip patches are formed on the north and south sides. The afterslip is mainly cumulated at the depth of 10–50 km, and the maximum slip reaches 1.46 m, which is situated at latitude of − 30.50°, longitude of − 71.78°, and depth of 18.94 m. Moreover, the postseismic slip during the time period of 0–30 days after this earthquake is the largest, and the maximum of fault slip and corresponding slip rate reaches 0.62 m and 20.6 mm/day. Whereas, the maximum of fault slip rate during the time period of 180–365 days is only around 1 mm/day. The spatial–temporal evolution of postseismic slip motions suggests that large postseismic slip mainly occurs in the early stage after this earthquake, and the fault tend to be stable as time goes on. Meanwhile, the Coulomb stress change demonstrate that the postseismic slip motions after the Illapel earthquake may be triggered by the stress increase in the deep region induced by coseismic rupture.

Benchmarking the Optimal Time Alignment of Tsunami Waveforms in Nonlinear Joint Inversions for the Mw 8.8 2010 Maule (Chile) Earthquake

Finite-fault models for the 2010Mw8.8 Maule, Chile earthquake indicate bilateral rupture with large-slip patches located north and south of the epicenter. Previous studies also show that this event features significant slip in the shallow part of the megathrust, which is revealed through correction of the forward tsunami modeling scheme used in tsunami inversions. The presence of shallow slip is consistent with the coseismic seafloor deformation measured off the Maule region adjacent to the trench and confirms that tsunami observations are particularly important for constraining far-offshore slip. Here, we benchmark the method of Optimal Time Alignment (OTA) of the tsunami waveforms in the joint inversion of tsunami (DART and tide-gauges) and geodetic (GPS, InSAR, land-leveling) observations for this event. We test the application of OTA to the tsunami Green’s functions used in a previous inversion. Through a suite of synthetic tests we show that if the bias in the forward model is comprised only of delays in the tsunami signals, the OTA can correct them precisely, independently of the sensors (DART or coastal tide-gauges) and, to the first-order, of the bathymetric model used. The same suite of experiments is repeated for the real case of the 2010 Maule earthquake where, despite the results of the synthetic tests, DARTs are shown to outperform tide-gauges. This gives an indication of the relative weights to be assigned when jointly inverting the two types of data. Moreover, we show that using OTA is preferable to subjectively correcting possible time mismatch of the tsunami waveforms. The results for the source model of the Maule earthquake show that using just the first-order modeling correction introduced by OTA confirms the bilateral rupture pattern around the epicenter, and, most importantly, shifts the inferred northern patch of slip to a shallower position consistent with the slip models obtained by applying more complex physics-based corrections to the tsunami waveforms. This is confirmed by a slip model refined by inverting geodetic and tsunami data complemented with a denser distribution of GPS data nearby the source area. The models obtained with the OTA method are finally benchmarked against the observed seafloor deformation off the Maule region. We find that all of the models using the OTA well predict this offshore coseismic deformation, thus overall, this benchmarking of the OTA method can be considered successful.

Stick-slip nucleation triggering of Wenchuan earthquake

The MS8.0 Wenchuan earthquake occurred on May 12th, 2008 on the steeply reverse fault of the Longmen Shan fault zone, Western China. Catastrophic failure and rupture occurred along the listric sliding surface. Several numerical simulations have been performed to reconfigure the dynamic processes of this strong earthquake. However, few scientific studies on the stick-slip of the listric fault have been conducted. In this study, finite element method is utilized to simulate the instant stick-slip fault responses at the triggering moment. The fault surface is a non-continuous and frictional contact interface. The simulation results demonstrated that the sliding friction and sticking friction interact spatially. This interaction occurred temporarily on swarms of slip patches, rather than the entire fault slipping surface. Stick-slip causes serrated energy release, and transient aseismic slip can occur spontaneously. These tremors propagate from the epicenter upwards to the ground surface. Therefore, they may provide important details on earthquake onset rupture.