Microseismic constraints on the mechanical state of the Anninghe-Daliangshan fault zone, southeastern Tibetan Plateau

The mechanical state (i.e., creeping or locked) of fault zone systems implies future seismic hazards. The Anninghe-Daliangshan fault zone in southeastern Tibet serves as the central curved segment of the most seismically active fault system in mainland China, which has produced a remarkable sequence of historical large earthquakes. To better understand its deformation modes and seismic-aseismic slip partitioning, we built a high-resolution earthquake catalog along the Anninghe fault (ANHF) zone and the Daliangshan fault (DLSF) zone utilizing a dense seismic array deployed between 2017 and 2022. The machine-learning based workflow produces ∼16,000 events with various behaviors and patterns located within the two fault zones. We then systematically evaluated and compared the spatiotemporal seismic patterns, seismic slip rate, statistical properties (b-value, C_V value, and nearest neighbor distance distribution), as well as geodetic measurements. Our results infer that the lower seismogenic crust (16–30 km; brittle) of the northern DLSF zone is creeping and releasing elastic strain with abundant microearthquakes; and the northern ANHF behaves similarly. Moving toward the south, the ANHF transits to a locked state, characterized by sparse seismicity, significantly low seismic energy release and a low b-value. Separated around Mianning, the locked segment is prone to generate two ∼M7.3 earthquakes, posing significant seismic hazard to approximately one million residents nearby. Overall, our updated analysis on the creeping northern DLSF and locked southern ANHF could deepen the understanding of seismic behavior along in-land fault system and guide future seismic hazard assessment in densely populated Southwest China.