Co- and postseismic deformation of the 2014 Mw 6.9 Yutian earthquake and its implications for regional tectonics

Abstract:

The ~2000 km long, ENE-striking Altyn-Tagh fault (ATF) is the most prominent active strike-slip fault system in northern Tibet. Between 2008 and 2020, four M_W > 6 earthquakes occurred on faults branching off the southwestern ATF system. Here we present the first co- and postseismic ground deformation fields related to the largest strike-slip earthquake of these events, the 2014 M_W 6.9 Yutian earthquake, generated from both TanDEM-X radar (1.3 m) and SPOT-6 optical satellite images (1.5 m) that fully cover the fault rupture. These data show that the seismogenic fault consists of two main fault segments with a 4.5 km step over in between them, close to the Xor Kol Lake. We delineated the surface trace of the seismogenic fault from computed image-offset measurements and used the result to constrain the surface location of the modeled fault. We then inverted the coseismic data for optimal fault dip and spatially variable slip. We find that the majority of fault slip occurred in the upper 10 km with peak slip of ~2.5 m at ~6 km depth. The estimated shallow slip of the slip model is consistent with field observations of the surface rupture. Studying Sentinel-1 postseismic time-series data acquired between 2014 and 2016, we find that the earthquake was followed by shallow afterslip concentrating at the southwestern end of coseismic rupture, in an area of many aftershocks and positive Coulomb failure stress change. Finally, we explore the influence of the 2014 event on its neighboring faults and find a Coulomb failure stress increase of 0.01 MPa on the fault segment of the last of the four large events in the area, which occurred in 2020, suggesting a possible time advancement of that event.