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Abstract:
Very low-frequency earthquakes (VLFEs) occurred regularly along the south Ryukyu Trench. The activity was concentrated at the slab depth of 10-20 km along the trench axis. The VLFEs occurred every two to three months for a few days in a seismic swarm. Long-term variations in VLFE activity reflect temporal changes in the degree of strain accumulation and release in the plate interface. Therefore, I investigated the long-term changes in VLFE activity in the south Ryukyu Trench. First, I analyzed continuous waveforms by NIED F-net and Taiwan BATS broadband seismic stations from 2003 to 2019. Second, I detected the VLFEs using the cross-correlation technique by Asano et al. (2014). As a result, I detected VLFE candidates with similar waveforms to those known as regular earthquakes as template events. The VLFE activity in south Ryukyu Trench decreased gradually from 2003 to 2009. The degree of VLFEs decrease was greater west of the center of the clusters and more minor to the northeast. In 2002, a series of MJ 7.0 earthquakes (March 26, 2002) and MJ 6.9 (March 31, 2002) interplate earthquakes occurred in the west and south of the VLFE clusters. The afterslip associated with the March 26, 2002 earthquake occurred in the north of western part of the VLFE clusters (Nakamura, 2009). These would have caused stress changes in the plate interface in the central and western parts of the VLFE clusters and would have activated the VLFEs. After then, the VLFE activity would have decreased gradually, accompanying the stress release.
