The July 2020 Mw 6.3 Nima Earthquake, Central Tibet: A Shallow Normal-Faulting Event Rupturing in a Stepover Zone

Abstract

Abstract On 22 July 2020, an Mw 6.3 earthquake with a predominantly normal-faulting mechanism struck the Yibug Caka fault zone, central Tibet, where the overall tectonic environment is characterized by left-lateral strike-slip motion. This event offers a chance to gain insight into the tectonic deformation and the cause of shallow normal-faulting earthquakes in this little studied region. Here, we use Sentinel-1A/B Interferometric Synthetic Aperture Radar data to investigate the coseismic and postseismic deformation related to this earthquake. The earthquake ruptured a previously mapped West Yibug Caka fault and is dominated by normal slip with a peak value of 1.9 m at depth of 6.9 km. Postseismic deformation analysis indicates that the observed subsidence signals of up to ∼4.7 cm are a consequence of afterslip. Most of the afterslip is confined at depths between 0.8 and 8.4 km, peaking at 0.27 m at depth of 6.1 km. The significant coseismic slip and afterslip involved in the earthquake highlights a complex interaction between the major normal fault and the secondary synthetic fault. By an integrated analysis of the inversions, regional geology geomorphology, fault kinematics, and seismicity background, we propose a tectonic model that attributes the occurrence of this normal-faulting event to the release of extensional stress in a stepover zone controlled by the northeast-striking sinistral strike-slip Riganpei Co fault and Bu Zang Ai fault. Compared with that the structural stepover often acts as a barrier to affect the propagation of earthquake rupture, our study demonstrates that the failure of a stepover may potentially induce the occurrence of earthquake along the bounding strike-slip faults.