JpGU-AGU Joint Meeting 2020

Presentation information

[E] Poster

S (Solid Earth Sciences ) » S-CG Complex & General

[S-CG58] Science of slow earthquakes: Toward unified understandings of whole earthquake process

convener:Satoshi Ide(Department of Earth an Planetary Science, University of Tokyo), Hitoshi Hirose(Research Center for Urban Safety and Security, Kobe University), Kohtaro Ujiie(Graduate School of Life and Environmental Sciences, University of Tsukuba), Takahiro Hatano(Department of Earth and Space Science, Osaka University)

[SCG58-P21] Offshore surveys of seismic structures and seismicity of regular and low-frequency earthquakes around Amami islands

*Tsutomu Takahashi1, Ayako Nakanishi1, Yojiro Yamamoto1, Yasushi Ishihara1, Aki Ito1, Ryuta Arai1, Seiichi Miura1, Shuichi Kodaira1, Yoshiyuki Kaneda2 (1. Japan Agency for Marine-Earth Science and Technology, 2.Kagawa Univ.)

Ryukyu arc, southwestern Japan, shows high activities of various slow earthquakes such as slow slip events (e.g., Heki & Kataoka 2008; Nishimura 2014) and shallow low frequency tremors (e.g.,Yamashita et al. 2015). We conducted passive and active source surveys around Amami islands to elucidate detailed activities of regular and slow earthquakes and seismic structures. For the passive source survey, we deployed 30 short-period ocean bottom seismographs (OBSs) and 5 temporal onshore stations at 25~40 km interval. Deployed OBS recorded ground motion from Mar. 2019 to July or Aug. 2019. As a part of active source surveys, we conducted wide-angle OBS and multichannel seismic (MCS) reflection surveys along a survey line between Amami-Oshima and Tokunoshima islands by the R/V Kaimei of Japan Agency for Marine-Earth Science and Technology (JAMSTEC). This survey line is almost perpendicular to the Ryukyu trench. We laid out 30 OBSs at 6km interval along this survey line for the refraction survey.

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.