Along-strike variation in fault structural maturity and seismic moment deficits on the Yushu-Ganzi-Xianshuihe fault system revealed by strain accumulation and regional seismicity

Abstract

The 900-km-long Yushu-Ganzi-Xianshuihe fault system that separates the Bayan Har and Qiangtang/Chuandian blocks is a primary tectonic structure in central eastern Tibet. Being one of the world's most seismically active fault systems, it hosted 12 M>7 earthquakes since the 18th century, and its future seismic risk has been the subject of extensive research over the past two decades. Previous studies have suggested that the western segment of the fault system differs largely from its eastern segment in strain accumulation and release, but the origin of such difference remains unknown. Here we use Sentinel-1 data to produce a complete map of present-day deformation for the entire fault system, in order to explore the along-strike variation in fault slip behaviour. Based on the InSAR deformation map, we solve for fault kinematic parameters including slip rate, locking depth and dip angle using a Bayesian Markov Chain Monte Carlo inversion approach. Slip rate on the Dangjiang-Yushu-Ganzi fault is consistently ∼5–6 mm/yr, only half of that on the Xianshuihe fault. This rate difference originates from nonuniform deformation mechanism within the Bayan Har block, with the western part of the block deforming more rigidly than the eastern part. Both historical and modern earthquake catalogues show that the western fault segment has only recorded M>7 earthquakes whereas the eastern segment has seen many small-to-moderate ruptures in addition to large earthquakes. These observations imply that the western fault segment may be more structurally mature than the eastern segment, which agrees with the long-term fault propagation as indicated by geochronological data and fault geometrical complexities. We calculate the seismic moment deficits along the fault system and find that several segments, e.g. the Dangjiang fault and the Manigange segment of the Ganzi fault, have accumulated a large seismic moment (equates to an Mw>7 earthquake) since the last major event.