[SCG58-P21] Offshore surveys of seismic structures and seismicity of regular and low-frequency earthquakes around Amami islands
Seismic data of the passive survey observed many regular earthquakes and several sequences of low frequency earthquakes (LFEs). We relocated a few hundred earthquakes (M>2) listed in the earthquake catalog by the Japan Meteorological Agency (JMA) by using OBS and onshore stations’ data. We also estimated epicenters of about 200 LFEs by envelope correlation method. LFEs distribute between Amami islands and Ryukyu trench, and distance from the Ryukyu trench is about 20-80km. One of the LFE sequences locates at off west of the Amami plateau. Source area of this sequence shows relatively high activity of regular earthquakes. While, the source area of another LFE sequence off the southeast of Amami-Oshima island shows quite low activity of regular earthquakes. Our seismic section of active source survey crosses the latter LFE area. MCS reflection data shows strong reflection with negative polarity from the plate boundary beneath this LFE area. This spatial coincidence of reflective plate boundary and LFEs is similar with the southern Ryukyu arc (Arai et al. 2016). Power spectra and duration times of these LFEs are also resemble with those observed at southern Ryukyu arc. Bathymetry of the Philippine sea plate implies different geometrical properties of subducting plate between these two areas. Similarities of LFEs in these areas would become an important constraint for source and structure models at Ryukyu subduction zone.
Acknowledgement: This study is supported by “Research Project for Compound Disaster Mitigation on the Great Earthquakes and Tsunamis around the Nankai Trough Region” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. We are grateful for Amami-city, Tokunoshima-town, Amagi-town, China-town and Toshima-village for their kind corporation for onshore seismic observations.