[S-CG45] Science of slow-to-fast earthquakes

[SCG45-P36] Long-term trends in very low-frequency earthquakes activity in the south Ryukyu

*中村 衛¹ (1.琉球大学理学部)

キーワード：超低周波地震、琉球海溝、地震活動

Very low-frequency earthquakes (VLFEs) occurred regularly along the Ryukyu Trench. The activity was concentrated around the Yaeyama Islands, Okinawa Island, and Amami Island. In each area, 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 each region. I used the catalog of VLFEs obtained by the same method as Nakamura and Sunagawa (2015). I used F-net, BATS, and temporary broadband seismic stations (since 2016) for the analysis. First, the maximum amplitude of VLFEs was picked from the vertical component waveforms after applying a 0.02-0.05 HZ band-pass filter. Then, the arrival time of the maximum amplitude at each station was used to determine the epicenter. VLFEs of M3.5 or greater were used in the analysis. The period was from 2002 to 2022.
As a result, the VLFE activity in the south Ryukyu decreased gradually from 2002 to 2010. In 2002, a series of MJ 7.0 earthquakes (March 26, 2002) and MJ 6.9 (March 31, 2002) interplate earthquakes in the Yaeyama took place, and the seismic activity above M5.0 increased temporarily. The afterslip associated with the March 26, 2002 earthquake occurred near Yonaguni Island (Nakamura, 2009). These activities would have caused the stress changes in the plate interface, which would have changed the VLFE activity.