Structural controls on coseismic rupture revealed by the 2020Mw 6.0 Jiashi earthquake (Kepingtag belt, SW Tian Shan, China)

Abstract

SUMMARYThe Kepingtag (Kalpin) fold-and-thrust belt of the southern Chinese Tian Shan is characterized by active shortening and intense seismic activity. Geological cross-sections and seismic reflection profiles suggest thin-skinned, northward-dipping thrust sheets detached in an Upper Cambrian décollement. The 2020 January 19 Mw 6.0 Jiashi earthquake provides an opportunity to investigate how coseismic deformation is accommodated in this structural setting. Coseismic surface deformation resolved with Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) is centred on the back limb of the frontal Kepingtag anticline. Elastic dislocation modelling suggests that the causative fault is located at ∼7 km depth and dips ∼7° northward, consistent with the inferred position of the décollement. Our calibrated relocation of the main shock hypocentre is consistent with eastward, unilateral rupture of this fault. The narrow slip pattern (length ∼37 km but width only ∼9 km) implies that there is a strong structural or lithological control on the rupture extent, with updip slip propagation possibly halted by an abrupt change in dip angle where the Kepingtag thrust is inferred to branch off the décollement. A depth discrepancy between main shock slip constrained by InSAR and teleseismic waveform modelling (∼7 km) and well-relocated aftershocks (∼10–20 km) may suggest that faults within sediments above the décollement exhibit velocity-strengthening friction.