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

Various 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. Plain Language Summary Aseismic slips in subduction zones, such as slow slip events (SSEs), can cause changes in seismicity rate, sometimes reflected by the occurrences of earthquake swarms. Such phenomena have been detected worldwidely in subduction zones, including the Japan, Aleutian and Hikurangi trenches. However, the swarm activities related to slow slip events along the Nankai trough remains unclear. Since SSEs along the Nankai subduction zone recur frequently and may affect the generation of large earthquakes in the updip locked areas, it is crucial to detect earthquake swarms and reveal the spatiotemporal relationship between swarms and SSEs in the Nankai region. Using a statistical branching model, we can detect earthquake swarms in the Nankai subduction zone, which might correlate with SSEs in space and time. We find that the detected earthquake swarms are mostly located in western Nankai, and generally complement with slips of SSEs. The occurrence rate of swarms in southern Hyuga‐nada with low interplate coupling is relatively high. Furthermore, the migrations of slow earthquakes in the s