Afterslip Model Research Articles

Coseismic slip model and early post-seismic deformation processes of the 2024 M7.5 Noto Peninsula, Japan earthquake revealed by InSAR and GPS observations

SUMMARY On 2024 January 1, an Mw7.5 earthquake ruptured the shallow reverse fault in the Noto Peninsula, which provides opportunities to better constrain the distributed coseismic slip of this earthquake and explore the early post-seismic deformation processes following this earthquake. We first utilized the coseismic displacements to invert the preferred coseismic rupture, including the optimal fault geometry and slip distribution. Our results indicated that the distributed slip mode on the simple fault plane with the dip and strike angles of 35o and 51o of this event is mainly featured by thrust slip. Three main rupture zones with ∼100 km length and 0–20 km depths have averaged slip of ∼3 m and maximum slip of ∼5.2 m, which has a seismic moment of 2.18 × 1020 Nm (Mw7.49). We first estimate the 3-D surface deformation due to the viscoelastic relaxation in the lower crust and upper mantle through the finite-element simulation method, and obtain the residual displacements at these GPS stations through removing its deformation effects from the observed post-seismic displacements. Based on the above fault geometry, we then inverted the first 138-day afterslip evolution following this earthquake by fitting the above residual displacements. The afterslip model mainly occurred the gap between two shallow slip asperities, which has a peak slip of up to ∼1.2 m and a seismic moment of 2.36 × 1019 Nm (Mw6.85). A number of aftershocks mainly ruptured on the surroundings of the coseismic slip and afterslip zones, suggesting that the aftershocks are mostly driven by the combined stress changes from the coseismic and post-seismic slip of the fault. The viscoelastic relaxation in the lower crust and upper mantle and the afterslips of the fault play the dominant roles in the early deformation processes, which also contribute to the post-seismic surface deformation following this earthquake. We simulate the deformation due to the poroelastic rebound in the top of upper crust. Model results indicate that the poroelastic rebound produces the centimetre-scale surface subsidence in the near-field area within ∼50 km around the hypocentre of the earthquake.

Co- and post-seismic deformation of the 2022 Menyuan Mw6.6 earthquake from InSAR and GPS observations

SUMMARY This study combines InSAR (Interferometric Synthetic Aperture Radar) and GPS (Global Positioning System) data to examine the coseismic slip and post-seismic deformation of the 2022 Menyuan Mw 6.6 earthquake, shedding light on the fault structure, slip distribution and rheology of the northeastern Tibetan Plateau. InSAR coseismic deformation revealed fault dip angle of 82° for the Lenglongling fault and 84° for the Tuolaishan fault. The optimal coseismic slip model, derived from InSAR, GPS and near-field pixel-offset data, indicates a maximum slip of 3.3 m, concentrated in the upper 3 km of the fault. We propose a three-step procedure to comprehensively invert the post-seismic deformation, accounting for both afterslip and viscoelastic relaxation, constrained by the InSAR and GPS observations. The optimal afterslip model indicates a cumulative afterslip of 9 cm over two years, primarily concentrated in the deeper sections of the coseismic slip zone. Incorporating viscoelastic relaxation improved the spatial distribution of the afterslip on the Tuolaishan fault. The afterslip released a seismic moment of 1.96 × 10¹⁸ N·m, accounting for approximately 15 per cent of the coseismic moment release. The viscosity of the lower crust in the region was estimated to range from 1.5 to 5 × 1018 Pa·s, with an optimal value of 2 × 10¹⁸ Pa·s. As the distance from the epicentre increased, viscoelastic relaxation became the dominant post-seismic mechanism, contributing up to 80 per cent of the deformation observed at the QHGC and GSMI stations. Additionally, the earthquake increased Coulomb stress on the ‘Tianzhu Seismic Gap’ and nearby faults, raising seismic hazard in the region. These findings highlight the importance of simultaneously considering both afterslip and viscoelastic relaxation mechanisms in post-seismic deformation analysis, and accounting for post-seismic effects when evaluating seismic stress changes.

Co-seismic and post-seismic slip associated with the 2021 Mw5.9 Arkalochori, Central Crete (Greece) earthquake constrained by geodetic data and aftershocks

The co-seismic and post-seismic deformation field associated with the Mw5.9 Arkalochori main shock that occurred in central Crete (Greece) on 27 September 2021 is analyzed using Copernicus Sentinel-1A & 1B images, GNSS measurements and seismological data. The fault geometry is constrained through the joint inversion of multiple datasets and the slip distribution for the co-seismic and post-seismic period is obtained using a homogeneous half-space elastic model and the Steepest Descent Method. The results indicate a blind normal fault striking 215° with a 55° dip to the northwest and the co-seismic slip model suggests a nearly circular main slip patch (8 × 6 km2) with a maximum slip of 0.98 m. Post-seismic displacements started rapidly after the main shock followed by a gradual decay as highlighted by the calculated InSAR time series. The temporal evolution of post-seismic slip is described by a simple logarithmic function, decaying faster at the southwest part of the fault. The cumulative afterslip model suggests that the maximum post-seismic slip of 0.23 m occurred within a similar depth range compared to the co-seismic one, yet with a shift towards the southwest. Post-seismic slip inside the main shock rupture area is sustained, highlighting the slow recovery of locking in the co-seismic slip region. Afterslip (seismic or aseismic) played a dominant role in the early post-seismic period acting complementarily to the main rupture. Indications suggest that the spatiotemporal evolution of the productive aftershock sequence may be driven afterslip, alongside other potential factors.

Ramp-Flat and Splay Faulting Illuminated by Frictional Afterslip Following the 2017 Mw 7.3 Sarpol-e Zahab Earthquake

Abstract As the largest instrumentally recorded earthquake in the fold-and-thrust belt of the northwestern Zagros mountain so far, the fault structure of the 2017 Mw 7.3 Sarpol-e Zahab earthquake and its contribution to regional crustal shortening remain controversial. Here, we utilize the integration of Interferometric Synthetic Aperture Radar observations and 2D finite element models incorporating various fault geometries such as planar faults, ramp-flat faults, and the combined models of ramp-flat and splay faults to explore frictional afterslip process due to coseismic stress changes following the mainshock. Our findings suggest that a ramp-flat frictional afterslip model, characterized by the maximum afterslip of ∼1.0 m and frictional variations (Δμ) of ∼0.001 and ∼0.0002 for the up-dip and down-dip portions, respectively, better explains the long-wavelength postseismic deformation than planar fault models. However, an integration model of a ramp-flat and a splay fault further improves the model fit, although the splay fault’s frictional slip is limited to <0.2 m, which is much smaller than that on the ramp-flat part (∼0.9 m). Considering the relocated aftershocks and structural cross-sections, the combined model could be best attributed to fault slip on the blind Mountain Front fault. Our findings thus suggest the complexity of the fault interactions between the basement and sedimentary cover in the Zagros, and that this largest basement-involved event in the region contributes to both thick- and thin-skinned shortening via seismic and aseismic behaviors, respectively.

Resolving a ramp-flat structure from combined analysis of co- and post-seismic geodetic data: an example of the 2015 Pishan Mw 6.5 earthquake

SUMMARY Previous studies have shown that it is difficult to determine whether the 2015 Pishan earthquake occurred on a uniform fault or a ramp-flat fault with variable dip angles due to the similar goodness of data fit to coseismic and afterslip models on these two fault models. Here, we first present the InSAR deformation obtained from both ascending and descending orbits, covering the coseismic period and cumulative 5-yr period after the 2015 Pishan earthquake. We then determine the preferred fault geometry by the spatial distributions between the positive Coulomb failure stress change triggered by main shock and the afterslip. Based on the preferred fault model, we finally use a combined model to determine the contributions of elastic and viscoelastic deformation in the post-seismic deformation. We find that the Pishan earthquake prefers to occur on a ramp-flat fault, and the coseismic slip is mainly distributed at a depth of 9–13 km, with a maximum slip of about 1.3 m. The post-seismic deformation is primarily governed by afterslip, as the poroelastic rebound-induced deformation fails to account for the observed post-seismic deformation and the contributions from the viscoelastic relaxation mechanism can be considered negligible in the combined model. Moreover, the modelled stress-driven afterslip and observed kinematic afterslip have good consistency, and the difference between the root mean square error of the two afterslip models is only 4.3 mm. The results from the afterslip model indicate that both of the updip and downdip directions distribute the afterslip, and slip in the updip direction is greater than that of the downdip direction. Meanwhile, the maximum cumulative afterslip after 5 yr is approximately 0.26 m which is equivalent to a released seismic moment of a Mw 6.47.

Afterslip on Conjugate Faults of the 2020 Mw 6.3 Nima Earthquake in the Central Tibetan Plateau: Evidence from InSAR Measurements

ABSTRACT Afterslip could help to reveal seismogenic fault structure. The 2020 Mw 6.3 Nima earthquake happened in a pull-apart basin within the Qiangtang block, central Tibetan plateau. Previous studies have explained the coseismic and early (<6 mo) postseismic deformation by rupture and afterslip on a normal fault bounding the western side of the basin. Here, we resolved the 19-month Interferometric Synthetic Aperture Radar-measured sequences of postseismic displacements that revealed a second postseismic displacement center ~12 km to the east of the main fault. Fitting the postseismic displacement requires afterslip on both the main fault and an antithetic fault that probably forms a y-shaped pair of conjugate faults in a negative flower structure. Stress-driven afterslip models suggest that the required afterslip on the antithetic fault could be triggered by coseismic rupture of the main fault or by a simultaneous rupture on the antithetic fault. The afterslip on both faults occurred mainly up-dip to the coseismic slip and has released moment ~15%–19% of that by the coseismic rupture. These results provide insights into active extension in the central Tibetan plateau and highlight the complex nature of fault rupture and afterslip.

Afterslip of the Mw 8.3 2015 Illapel Earthquake Imaged Through a Time‐Dependent Inversion of Continuous and Survey GNSS Data

AbstractWe use continuous and survey GNSS data to image the spatial and temporal evolution of afterslip during the 2 months following the Mw 8.3 2015 Illapel earthquake. Our approach solves for the incremental daily slip at the subduction interface using nonnegative least squares with spatial and temporal Laplacian regularization constraints. We find that afterslip developed at three specific areas at the megathrust, surrounding the coseismic rupture. In addition, well resolved afterslip also occurs within the coseismic rupture area that experienced ∼4 m of seismic slip. Our afterslip model shows striking correlations with the spatial distribution of aftershocks and repeating earthquakes. We capture the local afterslip triggered by a Mw 6.8 and two 6.9 aftershocks that ruptured downdip and north of the coseismic rupture, respectively. The latter ones were possibly triggered by the afterslip that developed north of the rupture. We also find a pulse of enhanced aseismic slip lasting a few days south of the rupture that spatially and temporally correlates with a seismicity burst. We finally find that areas of enhanced afterslip spatially correlates with areas having experienced regular seismic swarms observed during the years prior to the Illapel earthquake. This correlation supports the view of localized fluid high pore pressure areas behaving aseismically and surrounding a highly locked asperity, preventing the seismic rupture to propagate into them.

A two-step inversion for fault frictional properties using a temporally varying afterslip model and its application to the 2019 Ridgecrest earthquake

The stress status and frictional parameters of a fault system control its slip behaviors over earthquake cycles. These parameters generally exhibit large spatial variations in a real fault system, and resolving such variations is of critical importance for realistic assessments of the seismic hazard. Slowly evolving afterslip reflects the slip response of a fault system to the coseismic stress loading, which is commonly observed following large earthquakes, thus providing information for constraining the fault stress and frictional parameters. In this study, we demonstrate that two independent and determinable frictional properties (i.e., σ(a−b) and Rinit=log⁡(vinit/v0), where σ, (a−b), vinit and v0 are the effective normal stress, velocity-dependence frictional parameters, initial slip velocity, and reference velocity, respectively) can be obtained from an afterslip evolution process. We propose a two-step strategy that uses the temporally segmented afterslip models to invert for the independent parameters. After validating the performance of our inversion procedure by synthetic tests, we apply it to the 2019 Ridgecrest earthquake afterslip process. Our results show that σ(a−b) in the main afterslip area is 0.2∼0.5 MPa. The spatial distribution of this parameter suggests a significant difference of pore pressure at the two ends of the coseismic rupture. The depth profile of the effective normal stress in the afterslip concentration area reveals that the pore fluid pressure is hydrostatic above 5 km depth and increases to lithostatic from 5 to 20 km, indicating a gradual permeability decrease in this depth range. Our analysis also shows that the afterslip models which assume a uniform slip evolution function cannot be directly used for the estimation of the frictional properties.

Relative Afterslip Moment Does Not Correlate With Aftershock Productivity: Implications for the Relationship Between Afterslip and Aftershocks

AbstractAseismic afterslip has been proposed to drive aftershock sequences. Both afterslip moment and aftershock number broadly increase with mainshock size, but can vary beyond this scaling. We examine whether relative afterslip moment (afterslip moment/mainshock moment) correlates with several key aftershock sequence characteristics, including aftershock number and cumulative moment (both absolute and relative to mainshock size), seismicity rate change, b‐value, and Omori decay exponent. We select Mw ≥ 4.5 aftershocks for 41 tectonically varied mainshocks with available afterslip models. Against expectation, relative afterslip moment does not correlate with tested aftershock characteristics or background seismicity rate. Furthermore, adding afterslip moment to mainshock moment does not improve predictions of aftershock number. Our findings place useful empirical constraints on the link between afterslip and potentially damaging Mw ≥ 4.5 aftershocks and raise questions regarding the role afterslip plays in aftershock generation.

Depth‐Varying Friction on a Ramp‐Flat Fault Illuminated by ∼3‐Year InSAR Observations Following the 2017 Mw 7.3 Sarpol‐e Zahab Earthquake

AbstractWe use interferometric synthetic aperture radar observations to investigate the fault geometry and afterslip evolution within 3 years after a mainshock. The postseismic observations favor a ramp‐flat structure in which the flat angle should be lower than 10°. The postseismic deformation is dominated by afterslip, while the viscoelastic response is negligible. A multisegment, stress‐driven afterslip model (hereafter called the SA‐2 model) with depth‐varying frictional properties better explains the spatiotemporal evolution of the postseismic deformation than a two‐segment, stress‐driven afterslip model (hereafter called the SA‐1 model). Although the SA‐2 model does not improve the misfit significantly, this multisegment fault with depth‐varying friction is more physically plausible given the depth‐varying mechanical stratigraphy in the region. Compared to the kinematic afterslip model, the mechanical afterslip models with friction variation tend to underestimate early postseismic deformation to the west, which may indicate more complex fault friction than we expected. Both the kinematic and stress‐driven models can resolve downdip afterslip, although it could be affected by data noise and model resolution. The transition depth of the sedimentary cover basement interface inferred by afterslip models is ∼12 km in the seismogenic zone, which coincides with the regional stratigraphic profile. Because the coseismic rupture propagated along a basement‐involved fault while the postseismic slip may activate the frontal structures and/or shallower detachments in the sedimentary cover, the 2017 Sarpol‐e Zahab earthquake may have acted as a typical event that contributed to both thick‐ and thin‐skinned shortening of the Zagros in both seismic and aseismic ways.

Earthquake Cycle Deformation Associated With the 2021 MW 7.4 Maduo (Eastern Tibet) Earthquake: An Intrablock Rupture Event on a Slow‐Slipping Fault From Sentinel‐1 InSAR and Teleseismic Data

AbstractIn the continents, the importance of earthquakes that occur away from major block‐bounding faults is still debated. The 21 May 2021 MW ∼ 7.4 Maduo earthquake occurred on a secondary fault away from previously‐identified major block boundaries. Here we use 7 years of Sentinel‐1 Interferometric Synthetic Aperture Radar (InSAR) time series (between October 2014 and November 2021) to determine the distribution of coseismic slip and early postseismic afterslip following the Maduo earthquake, and the preceding interseismic strain accumulation. We devised a 13‐segment 3‐D fault geometry constrained by the SAR range offsets and the distribution of relocated aftershocks and used a Bayesian method incorporating von Karman regularization to solve for coseismic slip and afterslip models. We also used teleseismic waveforms as a standalone inversion to show the rupture evolution in space and time during the earthquake, finding that it propagates bilaterally with three notable rupture episodes. Our preferred coseismic self‐similar slip model shows a moderate shallow slip deficit, with the majority of moment release occurring in the depth interval of 1–10 km. The coseismic slip deficit is taken up in part by afterslip at shallow (<4 km) depths that grows linearly with time during the first ∼6 months, and at >10 km depths where afterslip grows logarithmically with time. We suggest that this heterogeneity is likely controlled by spatial variations in fault friction related to lithology. We discuss the implications for seismic hazard away from major tectonic block boundaries in light of our observations of the earthquake cycle on this intrablock fault.

Postseismic Process Inversion Using Full Time Series of Surface Deformation: Full Time‐Series Inversion (FTI) Theory and Its Application to the 2017 Sarpol‐e Zahab Earthquake

AbstractPostseismic processes provide important opportunities to probe into and investigate the frictional, viscous, and porous properties of the seismogenic fault and the surrounding Earth media. To accommodate the temporal and spatial resolutions and long‐term baseline stability of different deformation data, we develop a full time‐series inversion (FTI) technique, which jointly inverts for afterslip patterns using full time series of Global Navigation Satellite System, SAR, and strainmeter data. The FTI linearizes the inversion problem with a prescribed source evolution function to achieve efficient inversion. We conduct synthetic tests to validate the spatial and temporal resolution of the FTI algorithm. FTI outperforms static inversion techniques in terms of inversion stability under high noise level. We apply different parameterization strategies to evaluate its resolution for slip evolution parameters. The tests show that FTI can discriminate spatially separated afterslip with distinct evolution functions. Finally, we apply FTI to investigate the afterslip process following the 2017 Mw 7.3 Sarpol‐e Zahab earthquake that occurred along the Iran‐Iraq border in northwestern Zagros using Synthetic Aperture Radar Interferometry time series derived from the Sentinel‐1 observations 1 year after the mainshock. Similar to the synthetic tests, the algorithm is capable to discriminate afterslip with different evolution functions in the up‐ and downdip portions of the coseismic rupture zone. By comparing with the stress‐driven afterslip model simulated using rate‐strengthening frictional law, we demonstrate the stability of FTI in resolving the afterslip process. We emphasize the importance of incorporating early postseismic observations for deciphering afterslip evolution and frictional parameters.

Extension in the West Kunlun Mountains, NW Tibet: Insights from seismicity and analytical modeling

Normal faulting in orogens is usually associated with gravitational potential energy (GPE) due to elevation differences and contrasting material rheology between mountains and foreland areas. The 2008–2020 Yutian normal earthquake sequence in the West Kunlun Mountains has been closely studied over the last decade, but the mechanisms of normal faulting in the region are still an open question as both gravitational force and releasing step-overs have been proposed to explain the extension in West Kunlun. We investigated the normal faulting mechanism through (1) co- and postseismic kinematic models constrained by interferometric synthetic aperture radar (InSAR) and (2) analytical force balance modeling with a thin viscous approximation. We used the InSAR time series technique to map the postseismic deformation within 3 years following the 2008 Yutian earthquake. The InSAR-derived afterslip model revealed a clear slip polarity transition from strike-slip to normal faulting, suggesting a tight connection between the normal faulting and bounding strike-slip faults. The global navigation satellite system (GNSS)-constrained thin viscous sheet model gave an estimate of ~1022 Pa∙s for the effective viscosity of the West Kunlun lithosphere, which is generally in line with previous estimates that used a similar methodology. The stress field predicted by the thin viscous model primarily features north–south compression rather than east–west extension, suggesting that normal faulting is caused not by the GPE contrast across the margin of West Kunlun, but instead by the releasing step-overs bounded by strike-slip faults. The main contribution of this paper is therefore identifying the primary causes of normal faulting in West Kunlun and improving our understanding of the active tectonics of the NW Tibetan Plateau.

Transient poroelastic response to megathrust earthquakes: a look at the 2015Mw 8.3 Illapel, Chile, event

SUMMARYLarge earthquakes can alter regional groundwater pressure, resulting in fluid flow, and the process of restoring hydrostatic equilibrium would in turn lead to observable surface deformation, termed poroelastic rebound, which is one of the most important post-seismic mechanisms for stress transfer and triggering. To constrain the poroelastic contributions to the early post-seismic deformation, we model the hydrologic response within 1.5 months following the 2015 Mw 8.3 Illapel earthquake and remove its effects from the observed geodetic signals. Results demonstrate the post-seismic fluid-flow patterns from the co-seismic high-slip region to the north and south sides, and the northern poroelastic effects are remarkably stronger than those on the south side, verified by northern liquefaction phenomena. Therefore, previous pure afterslip models overestimate the asperities on both flanks of the co-seismic rupture zone and underestimate the middle region, with local errors of more than 50 per cent. It highlights the importance of considering the poroelastic effects, when modelling the transient post-seismic deformation.

Afterslip Moment Scaling and Variability From a Global Compilation of Estimates.

Aseismic afterslip is postseismic fault sliding that may significantly redistribute crustal stresses and drive aftershock sequences. Afterslip is typically modeled through geodetic observations of surface deformation on a case‐by‐case basis, thus questions of how and why the afterslip moment varies between earthquakes remain largely unaddressed. We compile 148 afterslip studies following 53 M w 6.0–9.1 earthquakes, and formally analyze a subset of 88 well‐constrained kinematic models. Afterslip and coseismic moments scale near‐linearly, with a median Spearman's rank correlation coefficient (CC) of 0.91 after bootstrapping (95% range: 0.89–0.93). We infer that afterslip area and average slip scale with coseismic moment as Mo2/3 and Mo1/3, respectively. The ratio of afterslip to coseismic moment (M rel ) varies from <1% to >300% (interquartile range: 9%–32%). M rel weakly correlates with M o (CC: −0.21, attributed to a publication bias), rupture aspect ratio (CC: −0.31), and fault slip rate (CC: 0.26, treated as a proxy for fault maturity), indicating that these factors affect afterslip. M rel does not correlate with mainshock dip, rake, or depth. Given the power‐law decay of afterslip, we expected studies that started earlier and spanned longer timescales to capture more afterslip, but M rel does not correlate with observation start time or duration. Because M rel estimates for a single earthquake can vary by an order of magnitude, we propose that modeling uncertainty currently presents a challenge for systematic afterslip analysis. Standardizing modeling practices may improve model comparability, and eventually allow for predictive afterslip models that account for mainshock and fault zone factors to be incorporated into aftershock hazard models.

Coseismic and Postseismic Fault Kinematics of the July 22, 2020, Nima (Tibet) Ms6.6 Earthquake: Implications of the Forming Mechanism of the Active N‐S‐Trending Grabens in Qiangtang, Tibet

AbstractThe N‐S‐trending grabens, the most prominent active structures in the Tibetan plateau, contain important information about the evolution and deformation mechanism of the plateau. The Ms6.6 earthquake that occurred in Nima, Qiangtang on July 22, 2020, provides an opportunity to investigate the kinematics of the grabens. Here, we use field survey and interferometric synthetic aperture radar to study the structural style, coseismic and postseismic deformation, fault slip, and stress changes of the Nima earthquake. The N‐S‐trending Qomo Ri graben (QRG) and the normal faults on the east and west sides are kinematically correlated with the NE‐trending sinistral Jiangaizangbu River and West Yibug Caka faults. The QRG is characterized by an extensional strike‐slip duplex and negative flower structure. The maximum coseismic deformation of the Nima earthquake is 29.4 cm in the line‐of‐sight direction observed by Sentinel‐1 ascending images. The coseismic slip model demonstrates that the NE20–30°‐striking fault caused the Nima earthquake has a maximum slip of 1.39 m and a main slip area at 4–12 km depth. The seismological fault may transit from a moderately dipping normal fault near the surface to a steeper strike‐slip fault at depth. Obvious postseismic deformation was observed in the stress‐enhancement area. The afterslip model shows that the mainshock fault is correlated with parallel normal faults to its west. We infer that the QRG was formed by local E‐W‐trending extension at the joints of the left‐stepping sinistral fault system, and the active normal faulting in the Qiangtang block favors stress localization.

InSAR Constrained Downdip and Updip Afterslip Following the 2015 Nepal Earthquake: New Insights into Moment Budget of the Main Himalayan Thrust

We use ALOS-2 and Sentinel-1 data spanning 2015–2020 to obtain the post-seismic deformation of the 2015 Mw 7.8 Nepal earthquake. ALOS-2 observations reveal that the post-seismic deformation was mainly distributed in four areas. A large-scale uplift deformation occurred in the northern subsidence area of the co-seismic deformation field, with a maximum uplift of ~80 mm within 4.5 yr after the mainshock. While in the southern coseismic uplift area, the direction of the post-seismic deformation is generally opposite to the co-seismic deformation. Additionally, two notable deformation areas are located in the region around 29° N, and near the MFT, respectively. Sentinel-1 observations reveal post-seismic uplift deformation on the north side of the co-seismic deformation field with an average rate of ~20 mm/yr in line-of-stght. The kinematic afterslip constrained by InSAR data shows that the frictional slip is distributed in both updip and downdip areas. The maximum cumulative afterslip is 0.35 m in downdip areas, and 0.2 m in the updip areas, constrained by the ALOS measurements. The stress-driven afterslip model shows that the afterslip is distributed in the downdip area with a maximum slip of 0.3 m during the first year after the earthquake. Within the 4.5 yr after the mainshock, the estimated moment released by afterslip is ~1.5174 × 1020 Nm,about 21.2% of that released by the main earthquake.

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.

Coseismic and postseismic deformation of the 2016 Mw 6.0 Petermann ranges earthquake from satellite radar observations

Continental earthquakes, especially shallow earthquakes, can cause disasters in populated areas. On 20 May 2016, a moderate magnitude earthquake (Mw 6.0) ruptured on Northern Territory of Australia. With the aim of seismic hazard assessment, coseismic and postsesmic deformation of the Petermann earthquake has been determined using ALOS-2 ascending and Sentinel-1A descending images. The coseismic deformation field suggests that the earthquake ruptured in the NNW direction with a length of about 20 km, a rake angle of 57.1° and a moment magnitude of Mw 6.06. The maximum slip of 1.31 m can be observed at the depth of 1.25 km. The small baseline subset (SBAS) InSAR was utilized to obtain the postseismic deformation from 69 Sentinel-1A images in a descending orbit. The spatiotemporal evolution of the postseismic deformation presents a gradually decaying trend, with a maximum displacement of 5.4 cm within 858 days after the mainshock. Our afterslip model suggests that the maximum slip of 0.28 m occurred at the depth of 0.25 km with a slight shift to the northwest compared to the coseismic slip. The static Coulomb failure stress (CFS) changes at various depths were calculated, indicating the consistency between high loaded stress zones and aftershock distribution.

The 2019 Ridgecrest, California earthquake sequence: Evolution of seismic and aseismic slip on an orthogonal fault system

Cascade-up and/or slow-slip processes are commonly believed to control interactions between foreshocks, mainshocks and aftershocks, but their relative contributions remain poorly resolved. Discrimination between these processes will shed light on the understanding of earthquake physics, which requires exceptional observations of earthquake sequences. The well-recorded July 2019 Ridgecrest, California foreshock-mainshock-aftershock earthquake sequence provides such an opportunity. We perform simultaneous inversion of the July 4th MW 6.4 foreshock and July 5th MW 7.1 mainshock kinematic rupture models using SAR, strong motion, and GPS data. We also invert for afterslip models following the MW 6.4 foreshock and the mainshock, respectively, by developing an inversion method that utilizes strainmeter, SAR and daily GPS time series. The inversion results show that the overall sequence involves no less than six fault segments, which include a main northwest-trending fault and secondary faults with sub-parallel and orthogonal geometry to the main fault. Co-seismic slip and afterslip have complementary patterns on the faults. During the early post-seismic period following the MW 6.4 foreshock and the mainshock, moment release on the southwest-trending fault is dominated by aseismic slip, in contrast to the predominantly seismic slip on the northwest-trending fault. The mainshock appears to be triggered by a cascade migration of foreshocks on a northwest-trending fault. Slip on the southwest-trending fault migrates from the fault junction at the northeast end (following the MW 6.4 foreshock) to the southwest end (following the mainshock) during the afterslip interval. The dual-mode (seismic versus aseismic) slip phenomena appear to be driven by co-seismic stress changes produced by the major events.