Detection and Characterization of Earthquake Swarms in Nankai and Its Association With Slow Slip Events

Abstract

AbstractVarious types of slow earthquakes occur in the Nankai subduction zone, among which the slow slip events (SSEs) seem to connect the shallow and deep part of the subduction plate and further trigger seismic or aseismic transients in surrounding areas. This study uses a version of space‐time epidemic‐type aftershock sequence (ETAS) model with non‐stationary background rate to detect earthquake swarms that may be related to SSEs in the Nankai region. We detect 999 swarm sequences including 13,387 earthquakes, accounting for about 18 percent of all M ≥ 1.0 earthquakes in the selected region along the Nankai trough. The detected swarms are mostly distributed in the Hyuga‐nada, Bungo Channel, Kii Channel, and Tokai regions, forming a complementary pattern to SSEs. Nearly 70 percent of swarm events are located in western Nankai, with the southern part characterized by a higher occurrence rate, which is consistent with the smaller recurrence interval of SSEs in this region. By comparing occurrence times of swarm events to those of SSEs, we find that some SSEs are accompanied by enhanced swarm activities during their occurrence periods. In addition, the swarm rates increase to higher levels in the southern part of western Nankai during the 2003 and 2010 Bungo SSEs, indicating the high sensitivity of earthquake swarms to the migrations of slow earthquakes in this region.