Source model of the 2015 M_w 6.4 Pishan earthquake constrained by interferometric synthetic aperture radar and GPS: Insight into blind rupture in the western Kunlun Shan

Abstract

The Pishan, Xinjiang, earthquake on 3 July 2015 is the one of largest events (Mw 6–7) that has occurred along the western Kunlun Shan, northwestern edge of the Tibetan Plateau in recent time. It involved blind thrusting at a shallow depth beneath the range front, providing a rare chance to gain insights into the interaction between the Tarim Basin and the Tibetan Plateau. Here we present coseismic ground displacements acquired by high‐resolution ALOS‐2 SAR imagery and derived from GPS resurveys on several near‐field geodetic markers after the event. We observed a maximum displacement exceeding 10 cm in the epicentral region. Analysis of the data based on a finite fault model indicates that coseismic slip occurred on a subsurface plane of 22 km × 8 km in size with a dip of about 27° to the north and a strike of 114°, representing partial break of one ramp fault buried in Paleozoic strata at 8–16 km depths beneath the foothill of the western Kunlun Shan. This blind rupture is characterized largely by a compact thrusting patch with a peak slip of 0.63 m, resulting in a stress drop of 2.3 MPa. The source model yields a geodetic moment of 5.05 × 1018 N · m, corresponding to Mw 6.4. The Pishan earthquake suggests a northward migration of deformation front of the Tibetan Plateau onto the Tarim Basin. Our finding highlights slip along ramp‐décollement faults to build up the western Kunlun Shan as the Tarim slab is subducting beneath western Tibet.

Blind Thrusting, Surface Folding, and the Development of Geological Structure in the Mw 6.3 2015 Pishan (China) Earthquake

“…Our interferograms have good coverage above the surface projection of the fault planes, and we can be confident in our conclusion that the earthquake was blind. Our results are consistent with field observations that found no evidence of surface ruptures (Lu et al, ) and are also consistent with our interpretation of local geomorphology (section 10), with seismic reflection profiles (Li et al, ) and the slip distributions of Zhang et al () and Sun et al () but contradict earlier InSAR‐derived slip distributions by Wen et al () and He et al (). The latter slip distributions each show ∼20 cm of slip in the shallow portion of the fault plane, including at the surface.…”

“…The latter slip distributions each show ∼20 cm of slip in the shallow portion of the fault plane, including at the surface. Our interferograms show that there is no discontinuity at the surface projection of the fault, and we suggest that the shallow slip suggested by other studies (He et al, ; Wen et al, ) is an artifact as a consequence of their incomplete data coverage in the Taklamakan Desert (Figure ) above the surface projection of the fault plane—the area most sensitive to shallow slip.…”

Kinematics of Active Deformation Across the Western Kunlun Mountain Range (Xinjiang, China) and Potential Seismic Hazards Within the Southern Tarim Basin

“…The hypocentral depth seems less well resolved, with values mostly of 19–20 km (GEOSCOPE and USGS), but as shallow as 10–15 km for some of the proposed solutions (USGS, CMT), in agreement with some local solutions (T. Li et al, ). Refined source parameters of this earthquake have been obtained from the inversion of surface coseismic deformation measurements by InSAR (He et al, ; Wen et al, ). These inversions suggest that the earthquake ruptured a 24–27°S dipping blind ramp, with coseismic slip concentrated at a shallow depth of 8–16 km.…”

“…Their chronology is here defined according to seismic profiles and well data, in particular within the close‐up view of the Yecheng–Pishan fold in Figure 6. Bold rectangles by the Yecheng–Pishan fold show the possible locations and depths of the M w 6.4 Pishan earthquake, from the various geodetic (He et al, ; Wen et al, ) or global solutions (USGS, CMT, and CEA) in black and red, respectively. For consistency with structural constraints, the focal mechanism is projected on the Yecheng–Pishan blind ramp, within the close‐up view of Figure 6.…”

“…Despite geological evidence for recent tectonic deformation, this region remains almost aseismic, except for earthquakes on the Altyn Tagh–Karakax strike‐slip fault to the south of the Western Kunlun range and for three earthquakes ( M w 5–6; ISC catalog) along the topographic mountain front: two in the 1950s and the recent M w 6.4 2015 Pishan earthquake (He et al, ; Wen et al, ; Figure ). Moreover, existing GPS stations record little to no deformation (within measurement uncertainties) across the range (Figure ).…”