Depth‐dependent activity of non‐volcanic tremor in southwest Japan

Abstract

With a new location method, we obtained a high‐resolution map of non‐volcanic tremor along the subducting Philippine Sea plate in southwest Japan and clarified the depth‐dependent behavior of the tremor activity. A bimodal distribution of tremor along‐dip is apparent in regions where short‐term slow slip events (SSEs) and very‐low‐frequency earthquakes are frequently detected. The separation of bimodal peaks is 5 to 10 km in depth. The updip tremor activity occurs episodically, coincident with major bursts that often accompany geodetically detectable SSEs. The downdip tremor activity is, however, rather stable with frequent minor bursts. This depth‐dependent tremor activity likely reflects variation of interplate slip properties, specifically weakening with increasing depth and temperature. In westernmost Shikoku, anomalous tremor activity was detected in only the updip section in late 2003. This is interpreted as being triggered by a long‐term SSE in the shallower edge of the tremor zone.