Very broadband analysis of a swarm of very low frequency earthquakes and tremors beneath Kii Peninsula, SW Japan

Abstract

We have conducted a temporal broadband seismic observation in Kii Peninsula, southwest Japan, and detected a swarm of 110 very low frequency earthquakes (VLFs) and deep low frequency tremors. During three days of the swarm activity, VLFs and tremors occur concurrently in two localized regions separated by ∼10 km. Stacking analyses are also employed to detect VLF signals of a period longer than 50 s, whose focal mechanisms are determined for the first time from data and shown to be consistent with the subducting plate motion. Evaluation of the VLF seismic moment implies that a substantial portion of the SSE seismic moment is released as a VLF swarm.