Huya Fault Research Articles

Stress heterogeneity in the eastern Tibetan Plateau and implications for the present-day plateau expansion

The eastward expansion of the Tibetan Plateau has resulted in different earthquake types in the eastern Tibetan Plateau, but the mechanism remains unclear. Here, we construct a three-dimensional visco-elastoplastic finite element model considering the topography to investigate the influence of fault geometry and rheological heterogeneity on stress fields. In our best-fitting model, the minimum principal stress is nearly vertical around the southern Huya fault zone, which is adjacent to the Longmen Shan fault zone, due to the significant mid-lower crust lateral rheological heterogeneity, and the thrust stress regime accounts for the reverse fault and thrust-dominated earthquakes. In this scenario, the eastward horizontal motion of the mid-lower crust is obstructed and facilitates thrust faulting, suggesting the limited eastward expansion of the Tibetan Plateau. In contrast, the northern Huya fault zone, one of the terminal branches of the East Kunlun fault, accommodates the continuous eastward extrusion of the East Kunlun fault, where the stress regime under a more homogenized crust favors the strike-slip faulting process, along with the dominant strike-slip earthquakes. Moreover, the best-fitting of stress regime explains the thrust-dominated 2008 Ms. 8.0 Wenchuan and 2013 Ms. 7.0 Lushan earthquakes on the Longmen Shan fault zone. Combining geophysical and geodetic observations and model analyses, we propose that the hybrid deformation mode in the eastern Tibetan Plateau is accommodated by upper crustal shear and thrusting deformation and mid-lower crustal thickening driven by the gravitational potential energy gradient. Our results elucidate the mechanism for differences in strong historical earthquakes and, more importantly, isolate the effect of fault geometry from those of heterogeneous viscosity on crustal deformation and stress heterogeneity in the eastern Tibetan Plateau.

Kinematics Along the Qingchuan Fault and Deformation Pattern in the Eastern Tibetan Plateau

AbstractThe accommodation of the substantial eastward crustal motion of the Bayan Har block characterizes the dynamics of faults located at the eastern Tibetan Plateau. However, uncertainties persist concerning the manner and amount of deformation distributed on these faults, with slip senses and rates constituting critical factors. In the northeastern segment of the Longmenshan thrust zone, the contentious activity and the unknown geologic slip rate present challenges. This study, focusing on the Qingchuan fault, the predominant fault within the northeastern Longmenshan, employed satellite imagery interpretation, displaced fluvial terraces surveying, displacement measurements, and chronological analysis to comprehensively characterize its fault activity. Our investigation robustly demonstrates the Qingchuan fault has been active since the late Quaternary, and is primarily marked with a pronounced dextral slip at a rate of 0.6–1.0 mm/year. By quantitatively assessing the deformation rates of the faults at the eastern Tibetan Plateau, we propose that they sufficiently accommodate the entire eastward crustal movement of the Bayan Har block; thereby no additional deformation propagates beyond the Qingchuan fault. Furthermore, we introduce a subblock model to elucidate the regional crustal deformation pattern, wherein the eastward movement of the Bayan Har block transfers to the northeastward movement of the Bikou subblock. This movement results in reverse slip patterns for the Minjiang and Huya faults, while the Beichuan and Qingchuan faults predominantly experience dextral displacements. The complex strain partitioning within the northern Longmenshan range underscores the observed variations in slip patterns across different segments of the Longmenshan thrust zone, advancing our understanding of fault behavior and the orchestration of crustal deformation in this intricate tectonic framework.

Channel Profiles Reveal Fault Activity along the Longmen Shan, Eastern Tibetan Plateau

Assessing fault activity in regions lacking Quaternary sedimentary constraints remains a global challenge. In this study, we used channel slope distribution to examine variations in rock uplift along faults. By comparing channel steepness with published low-temperature thermochronology and paleo-seismic data, we identified deformation changes both perpendicular to and along the Longmen Shan at various time scales. Our data revealed distinct fault segments displaying distinct thrust activities along the Longmen Shan’s strike. In the southern segment, the Dachuan fault exhibited the highest activity, and its movement had persisted for millions of years. In the central segment, the Wenchuan fault was active during theearly Quaternary but has become dormant since the late Pleistocene. Within the past millions of years, the Yingxiu and Pengguan faults displayed significant vertical displacement. Fault activity in the northern Longmen Shan was relatively weak, with the Qingchuan fault transitioning from thrust movement during the Neogene to pure strike-slip activity since the Pleistocene. Overall, the Dachuan and Huya faults exhibited deformation patterns similar to the Yingxiu fault during the Quaternary. Similar to the Yingxiu fault, which triggered the Wenchuan earthquake, the Dachuan and Huya faults possess the capacity to produce significant earthquakes in the future. The variations in deformation perpendicular to and along the Longmen Shan fault system underscore the importance of upper crustal shortening in shaping the rock uplift patterns and topography of the eastern Tibetan Plateau margin.

Seismic potential in the southeastern margin of Bayanhar block after 2017 Jiuzhaigou earthquake, China – insight from stress evolution and earthquake probability estimation

The southeastern margin of the Bayanhar block (BHB) is located at the junction of the Bayanhar block and the southern China block (SCB). Strong earthquakes occurred frequently in this region because of the intensive tectonic movement. The stress changes caused by strong earthquakes and the observed background seismicity can be used to forecast strong earthquake probability in this region. Therefore, we construct a 3D (three Dimension) viscoelastic finite element model that includes the complex fault system, inhomogeneous medium, tectonic stress, and gravity to simulate the evolutionary stress field when using the observed Global Positioning System (GPS) vectors as the boundary conditions. In addition, we calculate the strong earthquake probability based on the calculated stress and a Poisson probability distribution. The stress evolution result shows the Xianshuihe fault (XSHF), the Huya fault (HYF) and the southern segment except for the Yaan County of Longmenshan fault (LMSF) are located in the stress enhancement area indicating high seismic potential. However, the Minjiang fault (MJF) is located in the stress shadow area. After the Lushan and Jiuzhaigou earthquakes, there is still a high possibility of strong earthquakes in the MJF, HYF, and LMSF zone, according to the strong earthquake probability result.

Coseismic Deformation and Fault Inversion of the 2017 Jiuzhaigou Ms 7.0 Earthquake: Constraints from Steerable Pyramid and InSAR Observations

The 8 August 2017 Ms 7.0 Jiuzhaigou earthquake was generated in the transition zone between the Tazang fault, Huya fault, and Minjiang fault, all being part of the East Kunlun fault system. In this study, two pairs of SAR (synthetic aperture radar) data from Sentinel-1 satellite were used to derive the surface displacement observations along the satellite line-of-sight (LOS) directions using the differential interferometric SAR (D-InSAR) method. A steerable pyramid filtering method (i.e., a method for a linear multiscale, multidirectional decomposition and filtering technology) was proposed to optimize and enhance the geological features from interferometric image and coseismic deformation field. The 3D deformation was derived under the constraint of the combined D-InSAR and MAI method. The small baseline subset InSAR (SBAS-InSAR) time series method was used to obtain the cumulative deformation across the fault system. Fault slip inversion results from interferogram of InSAR indicate that the 2017 Jiuzhaigou earthquake was dominated by left-lateral slip, the surface movement was dominated by horizontal deformation, the vertical deformation was small, and the coseismic deformation variable in the east–west direction was the largest, with a maximum deformation of 0.2 m to the east and 0.14 m to the west. The maximum slip is about 77 cm, which is located at a depth of 9 km. The moment magnitude obtained by inversion is Mw 6.6, and the seismic fault is the Huya fault.

Three-dimensional electrical structure of the Huya fault and adjacent areas of the eastern margin of the Tibetan Plateau and its geological significance

In this study, we present new two- and three-dimensional resistivity models of the Huya fault region in the east margin of the Tibetan Plateau, imaged with magnetotelluric (MT) method. According to those models, a high-angle low-resistivity anomaly beneath the Huya fault is revealed, which is interpreted as a crustal thrust fault. The Minshan Uplift is imaged as a two-layered resistivity structure. The Bikou Block exhibits relatively uniform high-resistivity characteristics in our models, with its basement connected to a high-resistivity body beneath the Longmenshan fault belt. Thus, we infer that the Bikou Block belongs to the Yangtze Block. The Longmenshan fault belt exhibits strips of alternating high- and low-resistivity anomalies in the upper crust, with its middle and lower crust being highly resistive. The middle and lower crust of the western study region shows low resistivity, with its geometry changed greatly as it extends southeastward and upward. This is interpreted as an eastward channel of escaping material of the Tibetan Plateau. However, this channel is imaged to be blocked horizontally by the Bikou Block near the Huya fault, causing part of mid-to-lower crustal material to be upwelling, resulting in the Minshan Uplift. Another part of the low resistivity anomaly extended southeastward and was blocked by the lithospheric high-resistivity body beneath the Longmenshan fault belt, and then extended to the northeast and the southwest.

Coseismic fault model of the 2017 MW 6.5 Jiuzhaigou earthquake and implications for the regional fault slip pattern

On August 8, 2017, an MW 6.5 earthquake occurred in Jiuzhaigou County, Sichuan Province, China, on the eastern margin of the Qinghai–Tibet Plateau. This study investigates the coseismic deformation field and fault model with ascending and descending Sentinel-1 synthetic aperture radar (SAR) images, aftershock distribution, and elastic half-space dislocation model. The regional fault slip pattern is then quantitatively examined using the boundary element method. The results show that the ascending and descending interferometric synthetic aperture radar (InSAR) coseismic deformation fields display an overall NNW–SSE trend, with more significant deformation on the southwest side of the fault. The coseismic fault geometry is divided into NW and SE sub-faults with strikes of 162.1° and 149.3°, respectively. The coseismic fault slip is dominated by a left-lateral strike-slip movement with an average rake of −2.31°, mainly occurring at a depth of 0–13.04 km with a shape of an approximately inverted triangle. The fault slip features two peak slip zones, with a maximum of 1.39 m. The total seismic moment is 6.34 × 1018 N·m (MW 6.47). The boundary element calculation quantitatively indicates that the regional fault slip pattern may be mainly attributable to the changing strike and dip. The strike changes from NNW–SSE to nearly NS direction, and the dip gradually decreases from the Jiuzhaigou earthquake fault in the north to the Huya fault in the south. With these characteristics, the Huya and the Jiuzhaigou earthquake faults form the eastern boundary of the Minshan uplift zone and accommodate the accumulated deformation.

Aftershock sequence of the 2017 Mw 6.5 Jiuzhaigou, China earthquake monitored by an AsA network and its implication to fault structures and strength

SUMMARY We deployed a seismic network near the source region of the 2017 Mw 6.5 Jiuzhaigou earthquake to monitor aftershock activity and to investigate the local fault structure. An aftershock deployment of Array of small Arrays (AsA) and a Geometric Mean Envelop (GME) algorithm are adopted to enhance detection performance. We also adopt a set of association, relocation and matched-filter techniques to obtain a detailed regional catalogue. 16 742 events are detected and relocated, including 1279 aftershocks following the Mw 4.8 aftershock. We develop a joint inversion algorithm utilizing locations of event clusters and focal mechanisms to determine the geometry of planar faults. Six segments were finally determined, in which three segments are related to the Huya fault reflecting a change in fault dip direction near the main shock hypocentre, while the other segments reflect branches showing orthogonal and conjugate geometries with the Huya fault. Aftershocks were active on branching faults between the Huya and Minjiang faults indicating that the main shock may have ruptured both major faults. We also resolve a fault portion with ‘weak strength’ near the main shock hypocentre, which is characterized by limited coseismic slips, concentrated afterslip, low aftershock activities, high b-value and high sensitivity to stress changes. These phenomena can be explained by fault frictional properties at conditional stable sliding status, which may be related to the localized high pore-fluid pressure produced by the fluid intrusion.

Contemporary kinematics in the eastern Tibetan Plateau: Insights from 3D geomechanical modeling

The slip rates on the faults in the eastern Tibetan Plateau play an important role in understanding the plateau's evolution and assessing its seismic hazard. In order to obtain a quantitative picture of the continuous slip rate along and slip partitioning between the faults, we establish a comprehensive 3D geomechanical-numerical model of the eastern Tibetan Plateau. Our results show that the NW-SE-trending faults usually slip faster than the faults striking in NE-SW direction. A substantial variation of the slip rate gradient along the Xianshuihe fault is revealed. The southeastern segment (Moxi fault) has a higher slip rate gradient than the northwestern segment, which is inferred to be caused by a clockwise rotation of the strike of the Xianshuihe fault from NW-SE to NNW-SSE near Kangding. This rotation combined with the accommodated southeastward movement of the Chuan-Dian Block further promotes normal faulting on the southwestern-most section of the Longmen Shan fault. According to the relatively higher slip rate of the Huya fault and the geometric distortion of the Yingxiu-Beichuan fault, we suggest that the Huya fault may have penetrated into the Longmen Shan fault zone and partially transmitted deformation of the Minshan Uplift into the sedimentary cover of the Sichuan basin. Based on the analysis of contemporary kinematics in the eastern Tibetan Plateau, we suggest that the Moxi fault has a high seismic hazard with the potential of an Ms7.4 earthquake, while the northeastern segment and the southwestern-most section of the Longmen Shan fault zone tend to produce more frequent but lower magnitude earthquakes.

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

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

Probing the Fault Complexity of the 2017 Ms 7.0 Jiuzhaigou Earthquake Based on the InSAR Data

On 8 August 2017, a surface wave magnitude (Ms) 7.0 earthquake occurred at the buried faults extending to the north of the Huya fault. Based on the coseismic deformation field obtained from interferometric synthetic aperture radar (InSAR) data and a series of finite fault model tests, we propose a brand-new two-fault model composed of a main fault and a secondary fault as the optimal model for the Jiuzhaigou earthquake, in which the secondary fault is at a wide obtuse angle to the northern end of the main fault plane. Results show that the dislocation distribution is dominated by sinistral slip, with a significant shallow slip deficit. The main fault consists of two asperities bounded by an aftershock gap, which may represent a barrier. In addition, most aftershocks are located in stress shadows and appear a complementary pattern with the coseismic high-slip regions. We propose that the aftershocks are attributable to the background tectonic stress, which may be related to the velocity-strengthening zones.

Confirmation and Characterization of the Rupture Model of the 2017 Ms 7.0 Jiuzhaigou, China, Earthquake

AbstractOn 8 August 2017, an Ms 7.0 earthquake struck the Jiuzhaigou town, Sichuan Province, China, rupturing an unmapped fault, which is adjacent to the Maqu seismic gap in the Min Shan uplift zone in the easternmost part of the Bayan Har block. Having summarized the previous studies on the source of this earthquake, we confirmed the rupture model by jointly inverting the teleseismic P-wave and SH-wave data, Interferometric Synthetic Aperture Radar line-of-sight displacement data, and the near-field seismic and strong-motion data, a most complete dataset until now. The confirmation showed that a scalar seismic moment of 6.6×1018 N·m was released (corresponding to a moment magnitude of Mw 6.5), and 95% of the release occurred in the first 10 s. The slip area was composed of two asperities, with a horizontal extension of ∼20 km and a depth range of ∼2–15 km. A bilateral extending occurred at shallow depths, but the rupturing upward from deep depth dominated in the early time. The rupture process was found generally featuring the slip-pulse mode, which was related to the weak prestress condition. The aftershocks almost took place in gaps of the mainshock slip because of the coulomb stress change. Combining the aftershock relocations, aftershock focal mechanism solutions, and our confirmed rupture model, we suggest that the seismogenic fault was a northward extension of the mapped Huya fault. The occurrence of this earthquake made the Maqu seismic gap at a higher level of seismic risk, in addition to the moderate to high strain accumulation on the easternmost tip of the Kunlun fault system and the weak lower crust below.

A hybrid source mechanism of the 2017 Mw 6.5 Jiuzhaigou earthquake revealed by the joint inversion of strong-motion, teleseismic and InSAR data

As occurred during the tourist season, the 2017 Mw 6.5 Jiuzhaigou (China) earthquake led to destructive damages. The seismogenic fault of this event was merely speculated to be the northern extension of the Huya fault, while no apparent surface ruptures were discovered in the field investigation. Previous studies and released moment tensor solutions indicated the Jiuzhaigou earthquake was dominated by left-lateral strike slip with partial normal-slip component. It seems unintelligible that the normal slip occurred in this event as the epicenter was located near the boundary of convergent blocks. Hence, a reasonable and elaborate source rupture model is necessary to investigate such a blind fault and the role it plays in the complex fault system of this region. Combining space-based geodetic, teleseismic or regional seismic observations can provide detailed information about earthquake ruptures. We first attempted to determine the fault geometry using the Bayesian approach with synthetic aperture radar interferograms (InSAR). And then the fault geometry was refined based on the relocated aftershock distribution, and a two-segment fault model was constructed. Based on the two-segment model, we resolved the source rupture process of the 2017 Jiuzhaigou earthquake through the joint inversion of strong-motion, teleseismic body-wave and InSAR data. The inversion results reveal a hybrid source mechanism, in which normal and thrust slips coexist besides the strike-slip component. We suggest that the eastward motion with the extrusion of the lower crustal flow in the northeastern margin of the Bayan Har block is responsible for such a faulting behavior. The co-seismic Coulomb stress changes show a significant stress loading in the western segment of the Tazang fault, increasing its seismic hazard. Due to the lack of aftershocks to the southeast of the seismogenic fault, the probably enhanced seismicity in the northern segment of the Huya fault is also worth further attention.

3‐D Magnetotelluric Imaging of the Easternmost Kunlun Fault: Insights Into Strain Partitioning and the Seismotectonics of the Jiuzhaigou Ms7.0 Earthquake

AbstractWe present a three‐dimensional electrical resistivity model of the crust and upper mantle beneath the easternmost Kunlun fault (EKLf), obtained by three‐dimensional inversion of magnetotelluric (MT) data. The crust of the Songpan‐Ganzi block is characterized by high resistivity from the surface to a depth of around 20 km, and by low resistivity in the mid‐lower crust in the depth range 20–40 km. The eastern edge of the high conductivity layer is coincident with the EKLf and the Huya fault. The electrical resistivity structure provides new insights into both (1) the generation of recent M > 6 earthquakes and (2) strain partitioning on this segment of the EKLf. Our model reveals that the Huya fault is the main branch of the EKLf in the region. Together with the EKLf, the Huya fault defines the boundary between the Songpan‐Ganzi and Bikou blocks. The mid‐lower crust of the Songpan‐Ganzi block in this region has a low resistivity that likely represents a mechanically weak layer. The 2017 Jiuzhaigou Ms7.0 earthquake and other recent M > 6 earthquakes may have been controlled by the change in viscosity in the mid‐lower crust that occurs across this boundary. The high conductivity may be acting as either (1) a channel of lower crustal flow, or (2) as a weak layer that decouples the upper and lower crust. The fact that the high conductivity layer does not extend along the north side of the Sichuan Basin questions the idea that crustal flow occurs in this area.

Dynamic rupture simulations with heterogeneous initial stresses inversed from a given slip distribution: A case study of the 2017 Mw 6.5 Jiuzhaigou earthquake

Heterogeneous initial stresses are extremely important in understanding the physics of earthquake rupture processes. We propose a method to estimate heterogeneous initial stresses by comparing the slip from dynamic rupture simulations and the input joint inversion result and apply it to the 2017 Mw 6.5 Jiuzhaigou earthquake, which occurred in the intersection area of the Minjiang, Tazang and Huya faults, in Sichuan Province, China, with the slip distributed on two distinct left-lateral strike-slip faults as shown in previous geodetic and seismic joint inversion. We use the seismic efficiency empirical relation to infer the average stress, and the average stress drop is estimated using the static stress drop. With the angle of the maximum principal stress and the nucleation fault determined from the relocated aftershocks, different principal stresses are calculated with different combinations of average stress and average stress drop. The initial normal and residual stresses can be determined using the projection of the principal stresses. The initial shear stresses are generated by adding the residual stress and the heterogeneous stress drop, which is generated by a set of Gaussian random numbers with a given mean and the variance, and mapped one-by-one with the given slip on the fault so that the area with a large slip has a large stress drop. By comparing the slip from dynamic rupture simulations and the input joint inversion result, we show that the case with a 36 MPa average stress and 2.1 MPa average stress drop provides the best-fitting slip snapshots. The dynamic Coulomb stress change shows that the 2017 Mw 6.5 Jiuzhaigou earthquake could bring the southeast of the Tazang fault and the middle of the Minjiang fault closer to failure. This work provides a new insight to understand the initial stresses of nature earthquakes.

Rupture processes and Coulomb stress changes of the 2017 Mw 6.5 Jiuzhaigou and 2013 Mw 6.6 Lushan earthquakes

Since the 2008 great Mw 7.9 Wenchuan earthquake, two destructive earthquakes, the 2013 Mw 6.6 Lushan earthquake and the 2017 Mw 6.5 Jiuzhaigou earthquake, struck the eastern margin of the Tibetan Plateau, causing many casualties and significant property damage. The rupture processes and Coulomb stress change of the Lushan and Jiuzhaigou earthquakes are investigated in this study. The general patterns of the slip models of the two events are similar, where the slip is concentrated around the hypocenter and the primary ruptured zone extends about 20 km along strike. The rupture zone of the 2017 Jiuzhaigou earthquake spans a depth range of 4–16 km with a peak slip of ~ 115 cm, whereas the rupture zone of the 2013 Lushan earthquake is concentrated at 8 to 20 km depth with a peak slip of 125 cm. The coseismic static Coulomb stress changes induced by the two events are computed with the obtained slip models. The Tazang fault and the northern extremities of the Minjiang and Huya faults were strongly loaded by the Jiuzhaigou earthquake, whereas the Lushan earthquake mainly affected its surrounding faults. Therefore, we infer that the seismic hazard potential in these regions has probably been increased further, and the Lushan earthquake did not contribute significantly the occurrence of the Jiuzhaigou earthquake. Additionally, we also compute the stress changes imparted by the 2008 Wenchuan earthquake. The computed stress changes in both events’ hypocenters exceed the trigger threshold (0.1 bar), which suggests that the Wenchuan earthquake played a pivotal role in the occurrence of these two earthquakes.

Electrical structure of the Kunlun–Qinling fault system, northeastern Tibetan Plateau, inferred from 3-D inversion of magnetotelluric data

Located in the northeastern Tibetan Plateau, the East Kunlun fault is a west–east left-lateral strike-slip fault that branches out into a horsetail-shaped structure known as the Kunlun–Qinling Fault system (KQFS) at its eastern terminus. The 2017 Jiuzhaigou Ms 7.0 earthquake occurred in this horsetail-shaped structure. The electrical structures of four profiles that cross different sections of the KQFS and Jiuzhaigou earthquake region are obtained in this study. Our results reveal key differences among these sections. The East Kunlun fault is the only electrical boundary in the Maqu section, whereas the Tazang, Bailongjiang, and Guanggaishan–Dieshan faults in the Tazang section form an imbricate fan structure that extends from south to north and merges into a high conductivity layer (HCL) in the mid–lower crust. Alternating high and low resistivity bodies below the KQFS revealed by the magnetotelluric method (MT) may be related to the cause of slip rate segmentation from the East Kunlun fault into the KQFS. The hypocenter of the 2017 Ms 7.0 Jiuzhaigou earthquake lies between the high and low resistivity bodies at the northeastern, shallowing edge of the HCL. The northern section of the Huya fault corresponds to a deep crustal electrical boundary. The Huya and Tazang faults form a one-sided imbricate fan structure that merges at depth into the HCL. The HCL is deep in the southwestern part of our study area but shallows to the northeast. This observation suggests that northeast movement of the Tibetan Plateau causes tectonic uplift and the frequent occurrence of strong earthquakes in this region.

Source Characteristics of the 2017Ms7.0 Jiuzhaigou, China, Earthquake and Implications for Recent Seismicity in Eastern Tibet

AbstractOn 8 August 2017, a strong earthquake occurred on an unmapped extension of the Huya fault, Sichuan, China, causing severe damage to local communities. This event highlights the northward migration of major earthquakes along a single strike‐slip fault. We analyze the source characteristics with Global Positioning System (GPS)/ Interferometric Synthetic Aperture Radar static displacements and high‐rate GPS/teleseismic dynamic motions. Our analyses show a complex slip pattern characterized by two patches with a mean slip of 0.6 m and predominantly strike‐slip components. Slip propagates at a rupture speed of approximately 3.2 km/s for about 30 km in length and 20 km in width. Given a slip rate of 0.5 mm/year derived from GPS observations, the mean slip of 0.6 m inferred from the slip model suggests a recurrence interval of ~1,000 years for this earthquake and alike on the Huya fault. The Coulomb stress increase of 0.82 bar caused by preceding earthquakes promoted the failure of the northern Huya fault in 2017. In particular, the nucleation of this event was partially facilitated by the 2008 Wenchuan earthquake and postseismic deformation. Long‐term slip deficits on most parts of the Huya fault have been balanced largely by a series of strong earthquakes since the early 1970s. As yet, a few of seismic gaps remain hazardous during the final stage of one seismic cycle. In particular, the ongoing postseismic relaxation will hasten further the 20‐ to 25‐km long segment in the Jiuzhaigou National Natural Reserve closer to failure in a moderate size earthquake.

Temporal and Spatial Evolution of Load/Unload Response Ratio Before the M7.0 Jiuzhaigou Earthquake of Aug. 8, 2017 in Sichuan Province

The load/unload response ratio (LURR) method has been proposed to evaluate earthquake potential. This method can be used to investigate the regional stress–strain field change and fault instability associated with the preparation of a large earthquake. In this study, we explore the temporal and spatial evolution of the LURR anomalies before the M7.0 Jiuzhaigou earthquake that occurred on Aug. 8, 2017 in Sichuan Province. We calculate the LURR values by using Benioff strain of the small earthquakes within 100, 200, 300 and 500 km from the epicenter and 12 years prior to the main shock. The loading and unloading phases are determined by calculating the change of Coulomb failure stress caused by earth tide in the tectonically preferred slip direction on the fault surface of the major event. Results show that the LURR values remain at a low level for years until 2014 when the LURR values increase to an anomalously high level. Meanwhile, the magnitude of the anomalies decrease with the distance from the epicenter, and can hardly be observed within the 500-km radius. Combining with the spatial distribution of the LURR, we find that the development of anomalies before the mainshock can be divided into two stages: during the periods of 2014–2015, the significant anomalies were observed in the epicenter and its northwest region; and then in 2016, the anomalies were detected in the southeast area of the earthquake, which expanded around the Huya fault and finally reached the epicenter months before the mainshock. The earthquake is located at the intersection area of LURR anomalies of the two time periods, suggesting the generation of the Jiuzhaigou earthquake.

Crustal Deformation Prior to the 2017 Jiuzhaigou, Northeastern Tibetan Plateau (China), Ms 7.0 Earthquake Derived from GPS Observations

The 2017 Jiuzhaigou Ms 7.0 earthquake occurred on the northeastern margin of the Tibetan Plateau, with no noticeable rupture surface recognized. We characterized the pre-seismic deformation of the earthquake from GPS (Global Positioning System) data at eight continuous and 73 campaign sites acquired over the 2009–2017 period. With respect to the Eurasian plate, the velocity field showed a noticeable decrease, from west of the epicenter of the Jiuzhaigou earthquake to the western edge of the Longmenshan fault, in the southeast direction. The total northwest west–southeast east shortening rate in the vicinity of the epicentral area was in the range of 1.5 mm/y to 3.1 mm/y. With a GPS velocity transect across the Huya fault (HYF), where the epicenter was located, we estimated the activity of the HYF, showing a dominant left-lateral slip rate of 3.3 ± 0.2 mm/y. We calculated strain rates using a spherical wavelet-based multiscale approach that solved for the surface GPS velocity according to multiscale wavelet basis functions while accounting for spatially variable spacing of observations. Multiscale components of the two-dimensional strain rate tensor showed a complex crustal deformation pattern. Our estimates of strain rate components at the scale of seven and eight revealed extensional strain rate on the northern extension of the HYF. The Jiuzhaigou earthquake occurred at the buffer zone between extensional and compressional deformation, and with significant maximum shear rates being 100–140 nanostrain/y. In addition, a maximum shear strain rate of 60–120 nanostrain/y appeared around the epicenter of the 2013 Ms 6.6 Minxian–Zhangxian earthquake. These findings imply that inherent multiscale strain rates could be separated to identify strain accumulation related to medium- and large-sized earthquakes.