Distributed Slip Model Research Articles

The N-S direction strike-slip activities in the Pamir hinterland under oblique convergence: the 2015 and 2023 earthquakes

SUMMARY The prominent Pamir plateau holds considerable significance in comprehending the processes of Asian continental collisional orogeny. However, due to harsh natural conditions and low seismic activity within the Pamir hinterland, our understanding of this region remains deficient. Recent major events and the accumulation of geodetic observations present a rare opportunity for us to get insights into the tectonic activities and orogenic processes occurring in this region. First, employing Sentinel-1 and Advanced Land Observation Satellite (ALOS)-2 Synthetic Aperture Radar (SAR) images, we acquire coseismic displacements associated with the most recent earthquakes in 2015 and 2023. Subsequently, we conduct the source models inversion with the constraints of surface displacements based on a finite-fault model. Our results reveal displacements ranging from −0.8 to 0.8 m for the 2015 Mw 7.2 Tajik earthquake and −0.25 to 0.25 m for the 2023 Mw 6.9 Murghob event, respectively. The optimal three-segment model for the 2015 event ruptured a fault length of 89 km with a surface rupture extending 59 km along the Sarez–Karakul fault (SKF), characterized predominantly by left-lateral strike-slip motion, with a maximum slip of 3.5 m. Meanwhile, our preferred uniform slip model suggests that the 2023 event ruptured an unmapped fault in the southern Pamir region with a strike angle of 31° and a dip angle of 76.8°. The distributed slip model indicates that the 2023 event ruptured a fault length of 32 km, resulting in an 8 km surface rupture. This event is characterized by left-lateral strike slip, with a peak slip of 2.2 m. Secondly, the Coulomb stress calculations demonstrate that the 2023 event was impeded by the 2015 event. Finally, interseismic Global Positioning System data revel a relative motion of 3.4–5.7 mm yr−1 in the N-S component and 3.2–3.8 mm yr−1 in the E-W component along the SKF in the Pamir hinterland, respectively. These N-S direction strike-slip activities and slip behaviours support an ongoing strong shear and extension in the Pamir regime, which is a response to the oblique convergence between the Indian and Eurasian plates.

A Bayesian Source Model for the 2022 Mw6.6 Luding Earthquake, Sichuan Province, China, Constrained by GPS and InSAR Observations

Until the Mw 6.6 Luding earthquake ruptured the Moxi section of the Xianshuihe fault (XSHF) on 5 September 2022, the region had not experienced an Mw >6 earthquake since instrumental records began. We used Global Positioning System (GPS) and Sentinel-1 interferometric synthetic aperture radar (InSAR) observations to image the coseismic deformation and constrain the location and geometry of the seismogenic fault using a Bayesian method We then present a distributed slip model of the 2022 Mw6.6 Luding earthquake, a left-lateral strike-slip earthquake that occurred on the Moxi section of the Xianshuihe fault in the southwest Sichuan basin, China. Two tracks (T26 and T135) of the InSAR data captured a part of the coseismic surface deformation with the line-of-sight displacements range from ∼−0.16 m to ~0.14 m in the ascending track and from ~−0.12 m to ~0.10 m in the descending track. The inverted best-fitting fault model shows a pure sinistral strike-slip motion on a west-dipping fault plane with a strike of 164.3°. We adopt a variational Bayesian approach and account for the uncertainties in the fault geometry to retrieve the distributed slip model. The inverted result shows that the maximum slip of ~1.82 m occurred at a depth of 5.3 km, with the major slip concentrated within depths ranging from 0.9–11 km. The InSAR-determined moment is 1.3 × 1019 Nm, with a shear modulus of 30 GPa, equivalent to Mw 6.7. The published coseismic slip models of the 2022 Luding earthquake show apparent differences despite the use of similar geodetic or seismic observations. These variations underscore the uncertainty associated with routinely performed source inversions and their interpretations for the underlying fault model.

Geodetic constraints of the 2015Mw6.5 Alor, East Indonesia earthquake: a strike-slip faulting in the convergent boundary

SUMMARYOn 2015 November 4, an Mw 6.5 earthquake struck the east region of Alor Island, eastern Indonesia. Here, we use Sentinel-1 and ALOS-2 data to explore the coseismic surface displacement of this earthquake. Based on the ALOS-2 coseismic interferograms, the first fault model and coseismic slip distribution of the 2015 Alor earthquake are presented in this study. The preferred slip model links to a blind south-southeast striking, south-southwest dipping strike-slip fault with amount of normal slip component and a peak slip of 2.09 m at 2.34 km of depth. Considering the fact that the ascending and descending InSAR prediction of the distributed slip model does not fit the InSAR observations at the northwest and southeast tips with the simple planar fault model. We tried to construct a strike-variable fault model to further improve data fit. The results of the calculated Coulomb stress change imply that the regions with positive CFS changes mainly located at the northwest and southeast extremities of the rupture of the Alor earthquake, and in the lobes north and south of the rupture. The most striking discovery from the InSAR observations of the Alor earthquake is that most of the displacements occurred on a fault whose existence was unknown before the earthquake.

Coseismic and Early Postseismic Deformation of the 2020 Mw 6.4 Petrinja Earthquake (Croatia) Revealed by InSAR

The largest earthquake (Mw 6.4) in northwestern Croatia ruptured the faults near the city of Petrinja on 29 December 2020, at 11:19 UTC. The epicenter was located ~3 km southwest of Petrinja, ~40 km southeast of Zagreb, the capital of the Republic of Croatia. Here we investigated the geometric and kinematic properties of the 2020 Mw 6.4 Petrinja earthquake using a joint inversion of ascending and descending interferograms from three tracks of Sentinel-1 Single-Look Complex (SLC) images. The coseismic and early postseismic surface displacements associated with the Petrinja earthquake were imaged using standard DInSAR and SBAS time-series InSAR methods, respectively. The distributed slip model was inverted based on the ground surface displacements with maximum slip patch in 5 km depth. The early postseismic deformation occurred on the northwestern extent of coseismic slip, and it cannot be well modeled by the coseismic model. We thus suggested that the postseismic deformation was caused by a combined effect of the postseismic afterslips and aftershocks occurring in this area. Based on the inverted slip model, we calculated the Coulomb stress change in the region, and found a good correlation between positive Coulomb failure stress ∆CFS and the distribution of aftershocks. Based on these results, we identified which faults are more active, and then better estimated the seismic hazards in the region.

Coseismic displacement fields and the slip mechanism of the 2021 Mw 6.7 Hovsgol earthquake in Mongolia constrained by Sentinel-1 and ALOS-2 InSAR

ABSTRACT On 11 January 2021, an Mw 6.7 earthquake, which involved a complex rupture mechanism, occurred in Lake Hovsgol, Mongolia. This study measured the earthquake’s 3-D coseismic surface displacement fields using interferometric synthetic aperture radar (InSAR) pairs of three radar look directions: Sentinel-1B from the descending path and Advanced Land Observing Satellite-2 from the ascending and descending paths. The three DInSARs showed that the maximum coseismic surface displacement appeared east of the Northern Hovsgol Fault (NHF), where the displacement components were 18 cm, 5 cm, and 33 cm in the east–west, north–south, and up–down directions, respectively (precision: 1.7 cm, 20.4 cm, and 5.2 cm). However, the 3-D displacements indicated that although the earthquake induced a combination of normal faulting with strike slip motions, the displacements in the north–south direction had very large uncertainty owing to the similar geometry of the InSARs in the descending path. Later, we performed inversions of the DInSAR-measured coseismic surface displacement fields in the line-of-sight direction, which assumes a fault plane’s uniform and distributed slips in analyzing the slip mechanism. We conducted slip distribution estimations on the ruptured fault plane in accordance with the optimal fault geometry and source parameters determined by the uniform slip model. Investigations revealed that, with a correlation of 95.3%, the simulated displacements from the best-fitted distributed slip model were consistent with the observed displacements from DInSARs. Besides, our slip distribution model showed two distinctive slip patches, which include differences in their magnitudes and directions on the fault plane. We also observed ruptured faults experiencing a predominant right-lateral strike slip with a significant dip slip, according to the slip distribution, which caused two distinct slips due to the dramatic bending of the fault strike. Then, by analyzing the Coulomb stress change, our findings proposed that the seismic risk potential of active faults in the Hovsgol Basin increased after the earthquake. Overall, the great potential of multi-track DInSAR observations in the identification of complex slip mechanisms was demonstrated.

GPS derived finite source mechanism of the 30 October 2020 Samos earthquake, Mw = 6.9, in the Aegean extensional region

A submarine area close to the Turkish and Greek border between the cities of Samos-Greece and Seferihisar-Turkey has been shaked on October 30, 2020 by a Mw= 6.9 earthquake. In this study, the finite source mechanism of the Samos earthquake was investigated using geodetic methods and the coseismic behavior of the earthquake was modeled. The observed coseismic displacements at 62 sites were inverted for the fault geometry and the slips. The mainshock did not generate an on-land surface rupture. However, the uniform slip modeling shows a finite source of 43.1 km long and 16 km wide rupture, which slips 1.42 m along a north dipping normal fault extending from the Aegean Sea floor to a depth down to ~13 km. While the uniform slip model is consistent with the seismological solutions and provides a sufficient fit to the far field coseismic offsets, a distributed slip model is necessary to account for the near field coseismic displacements.

The 2019 September 24, Mw = 6, Mirpur earthquake, NW Himalaya: Geodetic evidence for shallow, near-horizontal décollement rupture of the Main Himalayan Thrust

We present a source model for the 2019 Mw = 6 Mirpur earthquake, NW Himalaya using Interferometric Synthetic Aperture Radar (InSAR) measurements. Bayesian inversion of InSAR data from both ascending and descending orbits suggests that the earthquake ruptured a shallow (Depth ~ 5 km), near-horizontal (Dip ~2.5°) up-dip portion of the décollement of the Main Himalayan Thrust (MHT). The distributed slip model suggests a compact rupture terminating the up-dip end at the base of the Main Frontal Thrust (MFT) with higher slip (> 0.4 m) around the hypocentre, equivalent to a moment magnitude of Mw = 6. A shallow up-dip rupture of the MHT through a moderate magnitude earthquake is atypical as Himalayan earthquakes generally originate at the down-dip portion of the MHT and propagate towards south. We estimate a low effective coefficient of friction of 0.06 ± 0.02 from the slip model and suggest that high pore fluid pressures and/or a weak, lubricated portion of décollement could have caused a local, near-horizontal rupture at the base of the MFT. The 2019 Mirpur earthquake released a small fraction of the accumulated strain energy since the 1555 Kashmir earthquake. Coseismic Coulomb stress change analysis suggests a significant increase in stress on the locked, up-dip portion of the MHT and the frontal fold-thrust system. These findings compel a revisit of the seismic hazard assessment of the northwestern Himalaya.

Sentinel-1 InSAR observations of co- and post-seismic deformation mechanisms of the 2016 Mw 5.9 Menyuan Earthquake, Northwestern China

On January 21, 2016, a Mw 5.9 earthquake occurred in Menyuan, Qinghai Province, in northwestern China, which is an earthquake-prone region located along the northeastern margin of the Tibetan Plateau. However, the source parameters, deformation mechanism, especially the influence of this earthquake on the surrounding regions and on post-seismic deformation, remains poorly understood. In this study, Sentinel-1 images from ascending and descending tracks were used to investigate the co- and post-seismic deformation fields. Uniform and distributed slip models were used to determine the source parameters. Finally, the co- and post-earthquake two-dimensional (2D) deformation decomposition, coulomb failure stress changes caused by the 2016 Menyuan earthquake, and the influence of this event on the surrounding region and active faults were analyzed. The results suggest that the 2016 Menyuan earthquake was triggered by a concomitant buried reverse fault in the northeastern part of the Northern Lenglongling Fault, which is a SW dipping sinistral thrust fault with a strike angle of 140°. The fault properties are consistent with the results of Interferometric Synthetic Aperture Radar (InSAR) 2D decompositions. The co-seismic deformation of the two satellite tracks have maximum line-of-sight uplifts of 55 mm and 65 mm, without notable subsidence. The post-seismic effect of the earthquake mainly affected the area between the Lenglongling Fault and the Northern Lenglongling Fault. The earthquake caused stress loading on the southwest of the western segment of the Lenglongling Fault and on the southeast of the eastern segment of the Northern Lenglongling Fault, as well as along these two faults.

Present-day orogenic processes in the western Kalpin nappe explored by interseismic GNSS measurements and coseismic InSAR observations of the 2020Mw 6.1 Kalpin event

SUMMARYAs the largest and most active intracontinental orogenic belt on Earth, the Tien Shan (TS) is a natural laboratory for understanding the Cenozoic orogenic processes driven by the India–Asia collision. On 19 January 2020, a Mw 6.1 event stuck the Kalpin region, where the southern frontal TS interacts with the Tarim basin. To probe the local ongoing orogenic processes and potential seismic hazard in the Kalpin region, both interseismic and instantaneous deformation derived from geodetic observations are used in this study. With the constraint of interseismic global navigation satellite system (GNSS) velocities, we estimate the décollement plane parameters of the western Kalpin nappe based on a 2-D dislocation model, and the results suggest that the décollement plane is nearly subhorizontal with a dip of ∼3° at a depth of 24 km. Then, we collect both Sentinel-1 and ALOS-2 satellite images to capture the coseismic displacements caused by the 2020 Kalpin event, and the interferometric synthetic aperture radar (InSAR) images show a maximum displacement of 7 cm in the line of sight near the epicentral region. With these coseismic displacement measurements, we invert the source parameters of this event using a finite-fault model. We determine the optimal source mechanism in which the fault geometry is dominated by thrust faulting with an E–W strike of 275° and a northward dip of 11.2°, and the main rupture slip is concentrated within an area 28.0 km in length and 10.3 km in width, with a maximum slip of 0.3 m at a depth of 6–8 km. The total released moment of our preferred distributed slip model yields a geodetic moment of 1.59 × 1018 N$ \cdot $m, equivalent to Mw 6.1. The contrast of the décollement plane depth from interseismic GNSS and the rupture depth from coseismic InSAR suggests that a compression still exists in the Kalpin nappe forefront, which is prone to frequent moderate events and may be at risk of a much more dangerous earthquake.

The Relationship Between Seismic and Aseismic Slip on the Philippine Fault on Leyte Island: Bayesian Modeling of Fault Slip and Geothermal Subsidence

AbstractDelineating seismic and aseismic slip on faults allows the exploration of the complex relationship between these different modes of slip. Further, quantifying them helps in the assessment of seismogenic potential. We present a distributed slip model of the rate of aseismic slip along the Leyte island section of the Philippine Fault and make comparisons with the extent of seismic slip from the latest significant earthquakes in July 2017. We derived both coseismic and aseismic slip distributions from kinematic inversions of synthetic aperture radar interferometric (InSAR) observations using a probabilistic (Bayesian) framework. Velocity maps from stacking and time series analysis of ALOS interferograms spanning 4 years (2007–2011) show a step change in ground deformation right on the fault. Inverting for slip at depth reveals along‐strike variations of aseismic slip rate over ∼100 km. Aseismic slip on the surface reaches rates of more than 3 cm/yr, equivalent to the long‐term slip rate (3.3 ± 0.2 cm/yr). Over the same period, a 20‐km segment in Tongonan appears to be locked. This segment ruptured in Mw 6.5 and Mw 5.8 earthquakes on 6 and 10 July 2017, respectively, as constrained by Sentinel‐1 and ALOS‐2 InSAR data. Seismic slip appears to be restricted within the Tongonan segment, with up to 152 ± 21 cm of left‐lateral displacement. The slip budget and complementarity between the extents of interseismic and coseismic slip suggest that a seismogenic asperity exists in Tongonan. The presence of active hydrothermal systems and rate‐strengthening materials provide physical conditions that can promote aseismic slip.

The 2019 Mw 5.9 Torkaman chay earthquake in Bozgush mountain, NW Iran: A buried strike-slip event related to the sinistral Shalgun-Yelimsi fault revealed by InSAR

On 7 November 2019, an Mw 5.9 earthquake with a left-lateral strike-slip mechanism struck the Bozgush mountain in NW Iran, marking the largest event in the mountain area in the past 44 years. Because no surface rupture associated with the earthquake has been reported, the causative fault of the earthquake is unknown. This event provides an opportunity to gain insight into the active tectonics of the region. Here, we acquire the coseismic deformation associated with this earthquake through Sentinel-1A interferometric synthetic aperture radar (InSAR) data and further invert the fault geometry and the detailed coseismic slip of this event based on the planar fault model. By a joint analysis of the inversion results, aftershocks sequence and local tectonics, we propose that the buried branch fault splaying from the Shalgun-Yelimsi fault (SYF) at depth may be responsible for this earthquake. Both the uniform and distributed slip models infer a smaller fault rupture size than predicted by classical fault scaling relation. We suggest that the subsurface fault geometry of the SYF may stop the rupture propagation further down to southeast at depth, thus resulting in high slip and stress drop at such a small fault plane. Considering the proximity of the epicenter to the SYF, we also invert for the distributed slip model on an anti-listric fault with the purpose to explore the possible complexity of rupture fault. However, due to the lack of obvious improvement to fit the observed data and the limited slip in the deep fault segment, the planar fault model is still regarded as the optimal one. According to the calculated coseismic Coulomb stress changes, we find that 55 % of aftershocks are triggered by the statics stress and that the northern segment of SYF and the eastern segment of SBF have a potential rupture risk. The occurrence of this earthquake, together with the prominent offset feature and slip style of surrounding active fault, indicates an active conjugate fault system, which may accommodate the small-scale northeastward lateral extrusion.

Sentinel-1 observation of 2019 Mw 5.7 Acipayam earthquake: A blind normal-faulting event in the Acipayam basin, southwestern Turkey

Rapid disaster response will provide significant information to relevant responding agencies. On 20 March 2019, an Mw 5.7 earthquake struck the Acipayam basin in southwestern Turkey, which is the largest event to occur in the basin. Here, we use Sentinel-1A interferometric synthetic aperture radar (InSAR) data to investigate the cosesimic deformation related to the Acipayam earthquake. The ascending and descending interforegrams suggest the normal faulting during this earthquake, with the maximum line of sight (LOS) displacements of 4.6 cm and 4.8 cm, respectively. The fault geometric parameters and a detailed coseismic distributed slip model of this event are retrieved by using geodetic data for the first time. Though the coseismic interferograms don’t show enough polarity changes to ascertain the dip orientation of the rupture fault, we regard the northeast-dipping fault as the causative fault by a joint analysis of the residuals plots, 2.5 dimensional (2.5D) deformation field, field investigation results, regional geomorphic feature and overall distribution of aftershock (4.0 < Mw < 4.9). The inversion results show that coseismic rupture was mainly dominated by normal faulting with minor sinistral strike-slip components. The rupture fault is a blind normal fault and the main slip is concentrated in the depth range of 3∼9.5 km with a maximum slip of approximately 0.33 m at a depth of ∼6 km. The InSAR-determined geodetic seismic moment is 3.96 × 1017 Nm corresponding to Mw 5.7. Our model resolution test supports these interpretations for the Acipayam earthquake. Based on the coseismic Coulomb stress change, we find that this 2019 event increases the stress loading in the northwestern and southeastern segment of the rupture fault plane and possibly triggers the subsequent Mw 4.8 and Mw 4.9 earthquakes. The mechanism of the determined seismogenic fault may be helpful to evaluate seismic hazards and understand the evolution of the Acipayam basin.

Source Characteristics of the 28 September 2018 Mw 7.4 Palu, Indonesia, Earthquake Derived from the Advanced Land Observation Satellite 2 Data

On 28 September 2018, an Mw 7.4 earthquake, followed by a tsunami, struck central Sulawesi, Indonesia. It resulted in serious damage to central Sulawesi, especially in the Palu area. Two descending paths of the Advanced Land Observation Satellite 2 (ALOS-2) synthetic aperture radar (SAR) data were processed with interferometric synthetic aperture radar (InSAR) and pixel tracking techniques to image the coseismic deformation produced by the earthquake. The deformation measurement was used to determine the fault geometry and the coseismic distributed slip model with a constrained least square algorithm based on the homogeneous elastic half-space model. We divided the fault into four segments (named AS, BS, CS and DS, from the north to the south) in the inversion. The BS segment was almost parallel to the DS segment, the CS segment linked the BS and DS segments, and these three fault segments formed a fault step-over system. The Coulomb failure stress (CFS) change on the causative fault was also calculated. Results show that the maximum SAR line-of-sight (LOS) and horizontal deformation were −1.8 m and 3.6 m, respectively. The earthquake ruptured a 210-km-long fault with variable strike angles. The ruptured pattern of the causative fault is mainly a sinistral slip. Almost-pure normal characteristics could be identified along the fault segment across the Palu bay, which could be one of the factors resulting in the tsunami. The main slip area was concentrated at the depths of 0–20 km, and the maximum slip was 3.9 m. The estimated geodetic moment of the earthquake was 1.4 × 1020 Nm, equivalent to an earthquake of Mw 7.4. The CFS results demonstrate that the fault step-over of 5.3 km width did not terminate the rupture propagation of the main shock to the south. Two M&gt;6 earthquakes (the 23 January 2005 and the 18 August 2012) decreased CFS along CS segment and the middle part of DS segment of the 2018 main shock. This implies that the stress release during the previous two earthquakes may have played a vital role in controlling the coseismic slip pattern of the 2018 earthquake.

Fault Slip Model of the 2018 Mw 6.6 Hokkaido Eastern Iburi, Japan, Earthquake Estimated from Satellite Radar and GPS Measurements

In this study, Sentinel-1 and Advanced Land Observation Satellite-2 (ALOS-2) interferometric synthetic aperture radar (InSAR) and global positioning system (GPS) data were used to jointly determine the source parameters and fault slip distribution of the Mw 6.6 Hokkaido eastern Iburi, Japan, earthquake that occurred on 5 September 2018. The coseismic deformation map obtained from the ascending and descending Sentinel-1 and ALOS-2 InSAR data and GPS data is consistent with a thrust faulting event. A comparison between the InSAR-observed and GPS-projected line-of-sight (LOS) deformation suggests that descending Sentinel-1 track T046D, descending ALOS-2 track P018D, and ascending ALOS-2 track P112A and GPS data can be used to invert for the source parameters. The results of a nonlinear inversion show that the seismogenic fault is a blind NNW-trending (strike angle ~347.2°), east-dipping (dip angle ~79.6°) thrust fault. On the basis of the optimal fault geometry model, the fault slip distribution jointly inverted from the three datasets reveals that a significant slip area extends 30 km along the strike and 25 km in the downdip direction, and the peak slip magnitude can approach 0.53 m at a depth of 15.5 km. The estimated geodetic moment magnitude released by the distributed slip model is 6.16 × 10 18 N · m , equivalent to an event magnitude of Mw 6.50, which is slightly smaller than the estimates of focal mechanism solutions. According to the Coulomb stress change at the surrounding faults, more attention should be paid to potential earthquake disasters in this region in the near future. In consideration of the possibility of multi-fault rupture and complexity of regional geologic framework, the refined distributed slip and seismogenic mechanism of this deep reverse faulting should be investigated with multi-disciplinary (e.g., geodetic, seismic, and geological) data in further studies.

Source Model and Stress Disturbance of the 2017 Jiuzhaigou Mw 6.5 Earthquake Constrained by InSAR and GPS Measurements

Seismogenic fault geometry, especially for a blind fault, is usually difficult to derive, based only on the distribution of aftershocks and interference fringes of Interferometric Synthetic Aperture Radar (InSAR). To better constrain the fault geometry of the 2017 Jiuzhaigou Mw 6.5 earthquake, we first carried out a nonlinear inversion for a single fault source using multi-peak particle swarm optimization (MPSO), Monte Carlo (MC), and Markov Chain Monte Carlo (MCMC) algorithms, respectively, with constraints of InSAR data in multiple SAR viewing geometries. The fault geometry models retrieved with different methods were highly consistent and mutually verifiable, showing that a blind faulting with a strike of ~154° and a dip angle of ~77° was responsible for the Jiuzhaigou earthquake. Based on the optimal fault geometry model, the fault slip distribution jointly inverted from the InSAR and Global Positioning System (GPS) data by the steepest descent method (SDM) and the MC method showed that the slip was mainly concentrated at the depth of 1–15 km, and only one slip center appeared at the depth of 5–9 km with a maximum slip of about 1.06 m, some different from previous studies. Taking the shear modulus of μ = 32 GPa, the seismic moment derived from the distributed slip model was about 7.85 × 1018 Nm, equivalent to Mw 6.54, which was slightly larger than that from the focal mechanism solutions. The fault spatial geometry and slip distribution could be further validated with the spatial patterns of the immediate aftershocks. Most of the off-fault aftershocks with the magnitude &gt; M2 within one year after the mainshock occurred in the stress positive stress change area, which coincided with the stress triggering theory. The static Coulomb stress, triggered by the mainshock, significantly increased at the Tazang fault (northwest to the epicenter), and at the hidden North Huya fault, and partial segments of the Minjiang fault (west of the epicenter).

‘Two go together’: Near-simultaneous moment release of two asperities during the 2016 Mw 6.6 Muji, China earthquake

On 25 November 2016, a Mw 6.6 earthquake ruptured the Muji fault in western Xinjiang, China. We investigate the earthquake rupture independently using geodetic observations from Interferometric Synthetic Aperture Radar (InSAR) and regional seismic recordings. To constrain the fault geometry and slip distribution, we test different combinations of fault dip and slip direction to reproduce InSAR observations. Both InSAR observations and optimal distributed slip model suggest buried rupture of two asperities separated by a gap of greater than 5 km. Additional seismic gaps exist at the end of both asperities that failed in the 2016 earthquake. To reveal the dynamic history of asperity failure, we inverted regional seismic waveforms for multiple centroid moment tensors and construct a moment rate function. The results show a small centroid time gap of 2.6 s between the two sub-events. Considering the >5 km gap between the two asperities and short time interval, we propose that the two asperities failed near-simultaneously, rather than in a cascading rupture propagation style. The second sub-event locates ∼39 km to the east of the epicenter and the centroid time is at 10.7 s. It leads to an estimate of average velocity of 3.7 km/s as an upper bound, consistent with upper crust shear wave velocity in this region. We interpret that the rupture front is propagating at sub-shear wave velocities, but that the second sub-event has a reduced or asymmetric rupture time, leading to the apparent near-simultaneous moment release of the two asperities.

The 2015 Mw7.2 Sarez Strike‐Slip Earthquake in the Pamir Interior: Response to the Underthrusting of India's Western Promontory

The Pamir orogen, Central Asia, is the result of the ongoing northward advance of the Indian continent causing shortening inside Asia. Geodetic and seismic data place the most intense deformation along the northern rim of the Pamir, but the recent 7 December 2015, Mw7.2 Sarez earthquake occurred in the Pamir's interior. We present a distributed slip model of this earthquake using coseismic geodetic data and postseismic field observations. The earthquake ruptured an ∼80 km long, subvertical, sinistral fault consisting of three right‐stepping segments from the surface to ∼30 km depth with a maximum slip of three meters in the upper 10 km of the crust. The coseismic slip model agrees well with en échelon secondary surface breaks that are partly influenced by liquefaction‐induced mass movements. These structures reveal up to 2 m of sinistral offset along the northern, low‐offset segment of modeled rupture. The 2015 event initiated close to the presumed epicenter of the 1911 Mw∼7.3 Lake Sarez earthquake, which had a similar strike‐slip mechanism. These earthquakes highlight the importance of NE trending sinistral faults in the active tectonics of the Pamir. Strike‐slip deformation accommodates shear between the rapidly northward moving eastern Pamir and the Tajik basin in the west and is part of the westward (lateral) extrusion of thickened Pamir plateau crust into the Tajik basin. The Sarez‐Karakul fault system and the two large Sarez earthquakes likely are crustal expressions of the underthrusting of the northwestern leading edge of the Indian mantle lithosphere beneath the Pamir.

Retrieving Precise Three-Dimensional Deformation on the 2014 M6.0 South Napa Earthquake by Joint Inversion of Multi-Sensor SAR

We reconstructed the three-dimensional (3D) surface displacement field of the 24 August 2014 M6.0 South Napa earthquake using SAR data from the Italian Space Agency’s COSMO-SkyMed and the European Space Agency’s Sentinel-1A satellites. Along-track and cross-track displacements produced with conventional SAR interferometry (InSAR) and multiple-aperture SAR interferometry (MAI) techniques were integrated to retrieve the east, north, and up components of surface deformation. The resulting 3D displacement maps clearly delineated the right-lateral shear motion of the fault rupture with a maximum surface displacement of approximately 45 cm along the fault’s strike, showing the east and north components of the trace particularly clearly. These maps also suggested a better-constrained model for the South Napa earthquake. We determined a strike of approximately 338° and dip of 85° by applying the Okada dislocation model considering a single patch with a homogeneous slip motion. Using the distributed slip model obtained by a linear solution, we estimated that a peak slip of approximately 1.7 m occurred around 4 km depth from the surface. 3D modelling using the retrieved 3D maps helps clarify the fault’s nature and thus characterize its behaviour.

A study on the seismogenic structure of the 2016 Zaduo, Qinghai Ms6.2 earthquake using InSAR technology

On October 17th, 2016, a Ms6.2 earthquake occurred in Zaduo County of Qinghai Province, China. The aim of this study is to use synthetic aperture radar (SAR) technology aboard the Sentinel-1A satellite to obtain high-resolution co-seismic surface displacement data and then to confirm the geometric parameters of the fault and slip distribution model. To this end, linear and non-linear inversion algorithms based on an elastic half-space dislocation model were used. The results showed that a distributed slip model can explain the surface deformation field measured by InSAR very well. The surface deformation field caused by the earthquake was an oval-shaped region of subsidence with a maximum displacement of 5 cm along the line of sight of the radar waves. This earthquake was mainly the result of a normal-slip fault process with 72°N strike and 65° dip. The slip was mainly concentrated at depths of 9–15 km. The maximum slip was 0.17 m, located at a depth of 12 km. The moment magnitude given by inversion was Mw5.9. This was basically in agreement with the moment magnitudes and surface magnitudes measured by USGS and CENC.

An improved geodetic source model for the 1999Mw6.3 Chamoli earthquake, India

An improved geodetic source model for the 1999<i>M</i>_w6.3 Chamoli earthquake, India