Shallow Slip Deficit Research Articles

Source mechanism of the 2023 Ms 5.5 earthquake in Subei, Gansu Province revealed by relocated aftershocks and InSAR: complement to the ‘shallow slip deficit’ of the eastern boundary of the Altyn Tagh fault

The Ms 5.5 earthquake struck on 24 October 2023, in Subei County, Gansu Province, China, occurring along the eastern segment of the Altyn Tagh fault. It raises the question of whether this earthquake is linked to the ongoing shortening slip rate along this segment or triggered by other seismic events. Analyzing the fault geometry of the Subei earthquake and understanding the significance of the weakening activity rate for seismic hazards in neighboring regions is crucial. The surface deformation from small- and medium-sized earthquakes (magnitudes less than Mw5.5) is often subtle, and the coseismic deformation detected by interferometric synthetic aperture radar (InSAR) is vulnerable to atmospheric disturbances, leading to significant measurement errors. Moreover, inaccuracies in the regional crustal velocity structure can cause errors in earthquake localization based on seismic data. These challenges complicate the establishment of a rupture model for seismogenic faults and hinder the inversion of fault slip models. To overcome these limitations, we employed the time-series InSAR stacking method and aftershock relocation to determine the fault geometry of the Subei earthquake. A two-step inversion method was utilized to ascertain both the fault geometry and slip distribution. Our modeling indicates that the 2023 Subei earthquake had a thrust mechanism with a component of strike-slip. The rupture did not reach the surface, with the maximum fault slip measuring 0.45 m at a depth of 2.5–3.5 km. The fault dips westward, and the moment magnitude is calculated at 5.4. This earthquake is associated with the ongoing weakening of the left-lateral strike-slip rupture along the Altyn Tagh fault in the Subei region. Furthermore, retrograde thrust tectonics significantly contribute to the absorption of accumulated stress during this process.Our findings highlight the potential of utilizing time-series InSAR images to enhance earthquake catalogs with geodetic observations, offering valuable data for further studies of the earthquake cycle and active tectonics. This approach is also applicable in other tectonically active regions, enhancing understanding of seismic hazards and risk assessment.

Unwrap Intractable C‐Band Coseismic Interferograms: An Improved SNAPHU Method With Range Offset Gradients as Prior Information

AbstractC‐band Interferometric Synthetic Aperture Radar (InSAR) data are widely used to map coseismic deformation. However, phase unwrapping errors are commonly distributed near faults owing to decorrelation and steep phase gradients from short radar wavelengths. Here, we propose an improved SNAPHU phase‐unwrapping algorithm that considers the prior information of the range offset gradients (P‐SNAPHU) to overcome the constraints imposed by the phase continuity assumption. P‐SNAPHU exploits median filtering of homogeneous pixels for initial denoising of range offset and then refines range offset gradients divisionally through saliency extraction. The derived gradients are used to estimate the expected values of the cost functions for the low‐coherence fringes. The synthetic experiments show significant improvements in the phase‐unwrapping accuracy of the P‐SNAPHU method compared with classical unwrapping methods. We apply the P‐SNAPHU method to unwrap Sentinel‐1 coseismic interferograms of three large (Mw > 6.5) strike‐slip earthquake events that the existing classical methods could not successfully unwrap. In the comparison of the unwrapped interferograms with the external global navigation satellite system (GNSS) displacements and those from the classical methods, we find that P‐SNAPHU significantly reduces the phase unwrapping errors with the mean absolute error of 7.2, 2.3, and 1.8 cm for the 2023 Kahramanmaraş earthquake doublet, the 2016 Kumamoto earthquake, and the 2019 Ridgecrest earthquakes, respectively. Based on the unwrapped results derived from P‐SNAPHU, an estimate is made regarding the shallow slip deficit of the 2023 Kahramanmaraş earthquake doublet, which is approximately 7%. Therefore, P‐SNAPHU is useful for developing and applying C‐band InSAR data for large earthquakes and volcanic activity.

Kinematics of the 2023 Kahramanmaraş Earthquake Doublet: Biased Near-Fault Data and Shallow Slip Deficit

Abstract Accurate estimate of the shallow slip deficit (SSD) for large strike-slip events is highly dependent on near-fault data. Previous studies have estimated the SSD of the 2023 Kahramanmaraş earthquake doublet, which may vary from a few percent to about 50%. Whether this reduced shallow slip is real or artificial is crucial for understanding the seismic hazards during and following the earthquake doublet. In this study, we inverted for the kinematic slip of this earthquake doublet with refined near-fault Interferometric Synthetic Aperture Radar observations and compared the results with the source model without further data processing. The model that excludes nonphysical data produced only ∼6% and ∼22% SSD for the M7.8 and 7.6 events, respectively, compared to ∼44% and ∼53% SSD of the model using original data. The increased data gap generally leads to overestimated SSD, but when the data coverage is almost complete, our result shows that the SSD is very sensitive to biased near-fault data which may induce significant artificial SSD. Our results suggest that overestimated SSD may be accompanied by slip migration from shallow toward deeper depths, to meet the total moment or energy release constrained by surface displacement data. The M7.8 event increased static Coulomb stress at where the second M7.6 event nucleated and thus may have triggered the following event.

Mechanism and implications of the post-seismic deformation following the 2021 Mw 7.4 Maduo (Tibet) earthquake

SUMMARY A major earthquake shook the Chinese county of Maduo, located in the Songpan-Ganzi terrane on the Tibetan Plateau, on 21 May 2021. Here, we investigate the post-seismic deformation process of this event, with the aim to understand the fault geometry, friction behaviour and regional rheology. To keep the self-consistency between co- and post-seismic deformation models, we first constrain the fault geometry and coseismic slip model of this event, which are directly used in modelling the post-seismic deformation. The coseimsic slip model reveals that the majority of coseismic slip is confined at the middle (3–15 km) of the brittle layer, leading to significant shallow slip deficit. Secondly, we obtain the post-seismic deformation in the first 450 d following the 2021 Maduo earthquake using the GPS and InSAR displacement time-series data. Thirdly, a combined model incorporating afterslip and viscoelastic relaxation is built to explain the observed post-seismic deformation. Our results suggest that the viscoelastic relaxation effect should be considered in the observation period, in order to avoid the unphysical deep afterslip in the ductile lower crustal layer. Combined analysis on viscosities inferred from this study and previous studies suggests a weak lower crust with steady-state viscosity of 1018–1019 Pa s beneath the Songpan-Ganzi terrane, which may give rise to the distributed shear deformation and the development of subparallel secondary faults within the terrane. Besides, the inferred afterslip on uppermost patches of the middle fault segment suggests a rate-strengthening frictional behaviour that may be related to the coseismic slip deficit and rupture arrest of the Maduo earthquake.

Coseismic slip and deformation mode of the 2022 Mw 6.5 Luding earthquake determined by GPS observation

The Luding Mw 6.5 earthquake on September 5, 2022 occurred in the southeast section of the Xianshuihe Fault, filling a seismic gap previously identified by the earthquakes (M > 6.5) since 1700. The horizontal coseismic displacements are obtained using remeasurement data from global positioning system (GPS) campaign stations over a range of ∼80 km from the epicenter. The overall pattern of displacements is consistent with left-lateral strike-slip. The largest displacement (∼ 227 mm in the north-north-east direction) is observed at station ZD17, located ∼10 km east of the epicenter. Coseismic slip distribution primarily propagates to the south-southeast of the epicenter, corresponding to the interseismic locking area. The lower stress accumulation of the north-northwest direction of the epicenter may arrest the rupture propagation. Slip located at shallower depths (1–3 km) is systematically smaller than that at deeper depths (4–8 km), suggesting a moderate shallow coseismic slip deficit. The distributed inelastic deformation caused by strong ground motion might be the main reason for the shallow slip deficit. The Daofu–Kangding section of the Xianshuihe and the Shimian–Mianning section of the Anninghe faults received a significant Coulomb stress increase caused by the Luding earthquake, and remained unbroken and hazardous.

Rupture imaging of the 2021 Ms 6.4 Yangbi, China, earthquake: Implications for the diffuse deformation in the northern region of the Red River fault

In May 2021, a Ms 6.4 earthquake occurred in the Yangbi Country, Yunnan, China, located in the northern region of the Red River fault, resulting in three deaths. It is a typical foreshock-mainshock-aftershock sequence. However, the kinematic rupture process and the interplay between foreshocks, the mainshock, and aftershocks associated with the Yangbi event remain controversial. Here, we decipher the detailed rupture process associated with this moderate event by jointly inverting the teleseismic body waves, three-component regional waveform, near-field Global Positioning System offsets, and Interferometric Synthetic Aperture Radar data. Results show that the rupture expands as a narrow slip-pulse that propagates southeastward. This event is dominated by the dextral movements with minor normal components, cohere with the tectonic shear and dilation strain partitioning. The high slip is concentrated within the depth range of 5–13 km, spanning ∼12 km along strike. A significant shallow slip deficit is identified, perhaps related to the fault being immature. The foreshocks, coseismic slips, and aftershocks reveal a complementary pattern, together releasing the accumulated stresses on the fault. Their distribution areas and Coulomb stress changes suggest that the Yangbi seismic sequence follows the rupture cascade. The absence of large earthquakes, dispersed seismicity, and diffuse strain rate patterns indicate that strain accumulates over a wide area around the northern region of the Red River fault, leading to small-scale ruptures distributed over the area and a low possibility of M ≥ 6.5 events in the near future.

Slip distribution of the February 6, 2023 Mw 7.8 and Mw 7.6, Kahramanmaraş, Turkey earthquake sequence in the East Anatolian Fault Zone

On February 6, 2023, two large earthquakes occurred near the Turkish town of Kahramanmaraş. The moment magnitude (Mw) 7.8 mainshock ruptured a 310 km-long segment of the left-lateral East Anatolian Fault, propagating through multiple releasing step-overs. The Mw 7.6 aftershock involved nearby left-lateral strike-slip faults of the East Anatolian Fault Zone, causing a 150 km-long rupture. We use remote-sensing observations to constrain the spatial distribution of coseismic slip for these two events and the February 20 Mw 6.4 aftershock near Antakya. Pixel tracking of optical and synthetic aperture radar data of the Sentinel-2 and Sentinel-1 satellites, respectively, provide near-field surface displacements. High-rate Global Navigation Satellite System data constrain each event separately. Coseismic slip extends from the surface to about 15 km depth with a shallow slip deficit. Most aftershocks cluster at major fault bends, surround the regions of high coseismic slip, or extend outward of the ruptured faults. For the mainshock, rupture propagation stopped southward at the diffuse termination of the East Anatolian fault and tapered off northward into the Pütürge segment, some 20 km south of the 2020 Mw 6.8 Elaziğ earthquake, highlighting a potential seismic gap. These events underscore the high seismic potential of immature fault systems.

The 2020 Mw 6.5 Monte Cristo Range, Nevada, Earthquake: Anatomy of a Crossing-Fault Rupture through a Region of Highly Distributed Deformation

ABSTRACT The 15 May 2020 Mw 6.5 Monte Cristo Range earthquake (MCRE) in Nevada, United States, is the largest instrumental event in the Mina deflection—a zone of east-trending left-lateral faults accommodating a right step between northwest-trending right-lateral faults of the Walker Lane. The MCRE ruptured a highly distributed faulting area with muted geomorphic expressions, motivating us to characterize the behavior of an earthquake on a structurally immature fault system. Inverse modeling of Interferometric Synthetic Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS) displacements reveals left-lateral slip on an east-striking, eastern fault and left-lateral–normal slip on an east-northeast-striking, western fault. Unusually, the two faults cross one another and ruptured together in the mainshock. The maximum slip of 1 m occurs at 8–10 km depth, but less than 0.1 m of slip reaches the surficial model fault patches, yielding a pronounced shallow slip deficit (SSD) of 91%. Relocated hypocenters indicate that the mainshock initiated at 9 km depth and that aftershocks span depths of 1–11 km, constraining the local seismogenic thickness. Our new field observations of fracturing and pebble-clearing in the western MCRE characterize a third, shorter, northern fault that is at the resolution limit of the InSAR–GNSS modeling. The segmented and intersecting fault geometry, off-fault aftershocks with variable mechanisms, distributed surface fractures, limited long-term geomorphic offsets, and a 600–700 m (cumulative) bedrock offset are all characteristic of a structurally immature fault system. However, the large SSD is not unusual for an earthquake of this magnitude, and a larger compilation of InSAR models (28 Mw≥6.4 strike-slip events) shows that SSDs correlate with magnitude rather than structural maturity. This study demonstrates the importance of integrating geodesy, seismology, and field observations to capture the full complexity of large earthquakes, and further suggests that seismic hazard assessments in shattered crustal regions consider the potential for multi- and cross-fault rupture.

Kinematic Slip Evolution During the 2022 Ms 6.8 Luding, China, Earthquake: Compatible With the Preseismic Locked Patch

AbstractThe September 2022 Ms 6.8 Luding earthquake broke more than 230 years of seismic calm on the Moxi fault, providing a unique opportunity to understand its seismogenic environment, rupture dynamics, and seismic hazard. Using teleseismic body waves, regional strong‐motion observations, GNSS, and InSAR data, we decipher the spatiotemporal rupture evolution of the mainshock. Combining the elastic dislocation model with surface creep, we find that the coseismic slip correlates closely with a locked patch with a loading rate of 9.7 mm/yr, but the creeping rate is insufficient to make up the shallow slip deficit. Notably, the Luding earthquake ruptured only ∼1/4 of the Moxi seismic gap, and it further increased the stress in the unruptured northern part. We thus argue that the Moxi fault has the potential for magnitude 7+ earthquakes in the near future, although geodetic prediction may overestimate the actual seismic moment released by the coseismic rupture.

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.

Afterslip From the 2020 M 6.5 Monte Cristo Range, Nevada Earthquake

AbstractWe investigate postseismic deformation following the 15 May 2020, Mw 6.5 Monte Cristo Range, Nevada earthquake using geodetic and aftershock data. Seven months of Sentinel 1‐A/B SAR images were used to model deformation as afterslip on two subparallel fault planes outlined by aftershocks. Postseismic deformation fits exponential and logarithmic decay models equally well. For exponential decay, the average decay time is ∼38 days. On the western plane, where most coseismic slip was observed, afterslip was minor, occurred at shallower depths than coseismic slip, and is anticorrelated with aftershock distribution. On the eastern plane, afterslip is significant, exceeds coseismic moment release, occurred at and below coseismic slip, and is correlated with aftershock distribution. On both segments, geodetic moment exceeds seismic moment, suggesting most afterslip occurred aseismically. Aseismic creep does not make up for observed coseismic shallow slip deficit, perhaps related to fault immaturity.

Coseismic and early post-seismic deformations due to the 2019 earthquake sequence in Ridgecrest, California

SUMMARYThe 2019 Ridgecrest earthquake sequence in the Mojave Desert that ruptured a complex orthogonal conjugate fault system offers a rare opportunity to probe the mechanics of the intraplate lithosphere of the central eastern California shear zone (ECSZ). We used space geodetic data to investigate the coseismic and post-seismic deformations attributable to this earthquake sequence. A triangular dislocation inversion scheme was used to derive the coseismic and post-seismic slip distributions on five non-planar faults in a homogeneous elastic crust. A wide range of viscoelastic relaxation models with varying upper mantle viscosities were also tested to constrain the rheological structure. The inferred coseismic slip exhibited a pronounced (approximately 60 per cent) shallow slip deficit (SSD), only a small proportion of which was recovered by early aseismic afterslip. Inversions of GPS and InSAR data suggest that the near-field post-seismic transient was dominated by afterslip at depths of 3–8 km. Viscosity modelling yielded a robust lower bound of approximately 8.0 × 1017 Pa s on the viscosity of the upper mantle. Both the GPS observations and the modelled viscosity at each individual station showed a lateral heterogeneous relaxation process on both northeast and southwest ECSZ. The effective viscosity of the mantle asthenosphere to the northeast was inferred to be approximately two times lower than that to the southwest, which is consistent with the pattern expected for regional heat flow. A comparison among the static coseismically induced Coulomb stress changes, the cumulative distribution of aftershocks and the afterslip distribution suggests that aftershocks and shallow afterslip could be responses to coseismically induced stresses, but the aftershock and afterslip distribution are poorly correlated. We argue that the pronounced SSD and lack of shallow afterslip during the Ridgecrest earthquake sequence are indicative of an immature fault. Furthermore, we suggest similar rheological structures for the southern and central ECSZ and infer a relatively low effective viscosity for the mantle to the northeast because of its high heat flow. We also propose that the afterslip may illuminate the rate-strengthening regions that mostly slip aseismically, but the aftershocks may illuminate fluid-saturated areas near ruptures.

Slip Kinematics of the 2021 Yangbi Earthquake: Fore-Main-Aftershock Sequence Rupture along an Unknown Secondary Fault of the Weixi–Qiaohou Fault

Abstract On 21 May 2021, an earthquake sequence occurred at the Weixi–Qiaohou fault that is considered as a boundary of the Chuandian block in Southwest China, providing a significant opportunity to understand the seismogenic tectonics of the fault and its secondary faults. We used Interferometric Synthetic Aperture Radar and Global Navigation Satellite System observations to obtain surface displacements and slip kinematics of the Mw 6.1 mainshock. The Mw 6.1 event ruptured along a previously unknown secondary fault of the Weixi–Qiaohou fault; this newly identified southwest-dipping fault is dominated by right-lateral strike slip with a small normal component. Fault slip was concentrated between depths of 3 and 8 km, with a maximum slip of ∼0.8 m. Given that there is minor slip and lack of foreshocks and aftershocks rupture in 0–3 km depth, a systemic shallow slip deficit is observed in the depth. In addition, stress change simulations suggest that the Mw 5.4 foreshock promoted the Mw 6.1 event. The increased coulomb stress change region extends approximately 16 km (from 25.65° N to 25.52° N) along the Weixi–Qiaohou fault triggered by the Mw 6.1 mainshock. Combined with modeling results of the Yangbi earthquake sequence and magnetotelluric model across the Weixi–Qiaohou fault, the fore-main-aftershock sequence initiate in depths of 8–16 km (corresponding to relatively low resistivity) and further promote larger slip upward along depths of 3–8 km (relatively high resistivity). Finally, aftershocks rupture and release stress below slip depths of the mainshock.

Coseismic Slip Model of the 2021 Maduo Earthquake, China from Sentinel-1 InSAR Observation

The 2021 Maduo earthquake occurred in the interior of the Bayan Har block of the Tibetan Plateau. We used space-born radar interferometry to study the coseismic deformation of this earthquake. Sentinel-1 InSAR observations along ascending and descending orbits provide the coseismic deformation. Pixel offset tracking method is used to complement InSAR observations near the rupture zone. The surface trace of the ruptured fault can be clearly mapped by InSAR observations. We constructed a three-dimensional coseismic slip model constrained by interferograms and pixel offset tracking in the form of a geodetic inverse problem. The coseismic slip model demonstrates that: (1) the Maduo earthquake was a left-lateral strike-slip event with moment magnitude of 7.4; (2) the peak slip is approximately 8 m and is located at a depth of 4 km; (3) a ‘shallow slip deficit’ of the Maduo earthquake is observed; (4) the ruptured faults are found to be dipping northward with a high dipping angle (80 degrees). This study has important implications on earthquake hazard evaluation of the Bayan Har block.

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.

Hypocentral dependent shallow slip distribution and rupture extents along a strike-slip fault

Natural faults feature heterogeneities in geometry, material properties, and stress distributions, posing great challenges in predicting fault slip behaviors. Among these factors, the stress on the fault is poorly understood due to the lack of direct measurements. Here, we estimate stress distribution from interseismic locking models and derive earthquake scenarios along the Anninghe fault in Sichuan, China, by conducting dynamic rupture simulations. The predicted rupture segmentation and moment magnitudes (Mw 6.9 – 7.3) are well consistent with historical earthquakes. Besides, we have observed models accompanied by continuous surface rupture, with surface offsets comparable to the values identified in field surveys of historical earthquakes. In addition, the surface rupture and the shallow slip deficit (SSD) vary drastically among models with different hypocenters, demonstrating that the earthquake process is indeterministic. The coseismic moment predicted in our models is ∼50% of what was anticipated from the static locking model. Our results highlight that those rupture scenarios derived from the locking-based heterogeneous stress can further contribute to the seismic hazard assessment.

Tectonic and Geometric Control on Fault Kinematics of the 2021 Mw7.3 Maduo (China) Earthquake Inferred From Interseismic, Coseismic, and Postseismic InSAR Observations

AbstractThe 2021 Mw7.3 Maduo (Qinghai, China) earthquake ruptured ∼160 km along a poorly known secondary fault inside the Bayanhar block on the northern Tibetan plateau, which is generally parallel to the Kunlun fault. Here we integrate the interseismic (2015–2020) and coseismic geodetic observations to quantify the interseismic strain rate, fault geometry and coseismic slip distribution. Our results reveal that the seismogenic fault is featured by the low (&lt;20 nanostrain/yr) and nearly undetectable interseismic strain rate. Three‐dimensional displacement fields and coseismic strain maps demonstrate the spatial variations of rupture kinematics due to the change of fault geometry. Our study reveals the steeply north‐dipping fault geometry of the seismogenic fault. The majority of coseismic slip occurred between 0 and 15 km with slight shallow slip deficit, which not penetrates through the inferred elastic upper crust (∼20–25 km). Our study highlights the tectonic and geometric control on fault kinematics of the Maduo earthquake.

Diffuse Deformation and Surface Faulting Distribution from Submetric Image Correlation along the 2019 Ridgecrest, California, Ruptures

ABSTRACT The 2019 Mw 6.4 and 7.1 Ridgecrest, California, earthquake sequence (July 2019) ruptured consecutively a system of high-angle strike-slip cross faults (northeast- and northwest-trending) within 34 hr. The complex rupture mechanism was illuminated by seismological and geodetic data, bringing forward the issue of the interdependency of the two fault systems both at depth and at the surface, and of its effect on the final surface displacement pattern. Here, we use high-resolution (WorldView and Pleiades) optical satellite image correlation to measure the near-fault horizontal and vertical surface displacement fields at 0.5 m ground resolution for the two earthquakes. We point out significant differences with previous geodetic- and geologic-based measurements, and document the essential role of distributed faulting and diffuse deformation in producing the observed surface displacement patterns. We derive strain fields from the horizontal displacement maps, and highlight the predominant role of rotation and shear strain in the surface rupture process. We discuss the segmentation of the rupture based on the fault geometry and along-strike slip variations. We also image several northeast-trending faults with similar orientation to the deeply embedded shear fabric identified in aftershock studies, and show that these cross faults are present all along the rupture, including at a scale &amp;lt;100 m. Finally, we compare our results to kinematic slip inversions, and show that the surface diffuse deformation is primarily associated with areas of shallow slip deficit; however, this diffuse deformation cannot be explained using elastic modeling. We conclude that inelastic processes play an important role in contributing to the total surface deformation associated with the 2019 Ridgecrest sequence.

Rupture Kinematics and Coseismic Slip Model of the 2021 Mw 7.3 Maduo (China) Earthquake: Implications for the Seismic Hazard of the Kunlun Fault

The 21 May 2021 Maduo earthquake was the largest event to occur on a secondary fault in the interior of the active Bayanhar block on the north-central Tibetan plateau in the last twenty years. A detailed kinematic study of the Maduo earthquake helps us to better understand the seismogenic environments of the secondary faults within the block, and its relationship with the block-bounding faults. In this study, firstly, SAR images are used to obtain the coseismic deformation fields. Secondly, we use a strain model-based method and steepest descent method (SDM) to resolve the three-dimensional displacement components and to invert the coseismic slip distribution constrained by coseismic displacement fields, respectively. The three-dimensional displacement fields reveal a dominant left-lateral strike-slip motion, local horizontal displacement variations and widely distributed near-fault subsidence/uplift deformation. We prefer a five-segment fault slip model, with well constrained fault geometry featuring different dip angles and striking, constrained by InSAR observations. The peak coseismic slip is estimated to be ~5 m near longitude 98.9°E at a depth of ~4–7 km. Overall, the distribution of the coseismic slip on the fault is highly correlated to the measured surface displacement offsets along the entire rupture. We observe the moderate shallow slip deficit and limited afterslip deformation following the Maduo earthquake, it may indicate the effects of off-fault deformation during the earthquake and stable interseismic creep on the fault. The occurrence of the Maduo earthquake on a subsidiary fault updates the importance and the traditional estimate of the seismic hazards for the Kunlun fault.

Seismic and Geodetic Analysis of Rupture Characteristics of the 2020 Mw 6.5 Monte Cristo Range, Nevada, Earthquake

ABSTRACT The largest earthquake since 1954 to strike the state of Nevada, United States, ruptured on 15 May 2020 along the Monte Cristo range of west-central Nevada. The Mw 6.5 event involved predominantly left-lateral strike-slip faulting with minor normal components on three aligned east–west-trending faults that vary in strike by 23°. The kinematic rupture process is determined by joint inversion of Global Navigation Satellite Systems displacements, Interferometric Synthetic Aperture Radar (InSAR) data, regional strong motions, and teleseismic P and SH waves, with the three-fault geometry being constrained by InSAR surface deformation observations, surface ruptures, and relocated aftershock distributions. The average rupture velocity is 1.5 km/s, with a peak slip of ∼1.6 m and a ∼20 s rupture duration. The seismic moment is 6.9×1018 N·m. Complex surface deformation is observed near the fault junction, with a deep near-vertical fault and a southeast-dipping fault at shallow depth on the western segment, along which normal-faulting aftershocks are observed. There is a shallow slip deficit in the Nevada ruptures, probably due to the immature fault system. The causative faults had not been previously identified and are located near the transition from the Walker Lane belt to the Basin and Range province. The east–west geometry of the system is consistent with the eastward extension of the Mina Deflection of the Walker Lane north of the White Mountains.