9:15 AM - 9:30 AM
[SSS10-02] The Wakayama earthquake swarm: Seismic activity and geofluids
An earthquake swarm in the Wakayama prefecture, Japan, is known as the most active and persistent swarm. However, no systematic studies have highlighted the source of this intriguing non-volcanic earthquake swarm to date. We systematically investigate the temporal and spatial evolution of the Wakayama earthquake swarm and estimates the seismic velocity structure around the Kii peninsula. Our results show that seismicity associated with the Wakayama earthquake swarm occurs almost evenly in both time and space and the majority of the earthquakes occur along well-defined planes. We also reveal that a northwestward-dipping low-velocity zone exists beneath the Wakayama swarm and the low-velocity zone is sandwiched by high-velocity anomalies in the continental crust interpreted as impermeable and rigid materials on both sides in the subduction direction. This unique tectonic setting controls a pathway of the upward migration of slab-derived fluids to the surface, with the high fluid concentration in the dipping low-velocity zone. We further propose that the valley-shaped geometry of the PHS slab beneath the Kii peninsula is caused by the rigid materials in the continental crust.