Probability of megathrust earthquake following slow slip and earthquakes: Southernmost Ryukyu subduction zone

Abstract:

Slow slip events (SSEs) are one of slow earthquakes with slowest rupture velocities and longest durations from days to months or years, which relieve tectonic stresses in subduction zones and are widely observed geodetically. Recent studies have observed that the timing of SSEs propagation is prior to the rupture of nearby large earthquake in several subduction zones. This mechanism is regarded as a triggering by transient static stress loading. However, most of approaches of probabilistic seismic hazard analysis for subduction earthquakes did not consider such effect caused by SSEs. We apply a simple SSE-based earthquake probability model in the Southernmost Ryukyu subduction zone, where a shallowest locked zone, Ryukyu Fault, could rupture as a size over M_w 8.0 and is surrounded by episodic SSEs. To know the time-dependent probability of Ryukyu Fault rupture caused by the SSEs and the other local mainshocks, we established a synthetic time-dependent shear stress catalog for the study area considering the earthquake stress drop, earthquake recurrence intervals, and background stressing rate, and then estimated those transient static stress loads on the Ryukyu Fault. Our results show that the annual probabilities of M_w 8.0 earthquake are less than 3% on the Ryukyu Fault. The probabilities do not strongly depend on the variations of earthquake stress drop and slip-deficit rate when the assumed earthquake size increases. Nevertheless, we notice that the rupture probabilities increase by at least 1.3 times relative to the background annual probabilities when the SSEs or mainshocks occurred. The scenarios illuminate a potential interaction between SSEs and large earthquakes in Southernmost Ryukyu subduction zone and imply the importance of SSEs monitoring in where the earthquake rupture zone is adjacent.