[S-CG52] Ocean Floor Geoscience

[SCG52-P16] Seismic activity in the northernmost Okinawa trough revealed by ocean bottom seismic observations

Naori Morimoto¹, *Kazuo Nakahigashi¹, Seishiro Furuyama¹, Yusuke Yamashita², Yukihiro Nakatani³, Shuichiro Hirano³, Hiroshi Yakiwara³, Hiroki Miyamachi³, Takeshi Matsushima⁴, Tomoaki Yamada⁵, Masanao Shinohara⁵ (1.Tokyo University of Marine Science and Technology, 2.Disaster Prevention Research Institute, Kyoto University, 3.Kagoshima University, 4.Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, 5.Earthquake Research Institute, University of Tokyo)

Keywords:Okinawa Trough

The Okinawa Trough is a back-arc basin that is forming on the back-arc side of the Ryukyu Arc where the Philippine Sea Plate is subducting. It is thought that the northernmost part of the Okinawa Trough is in the incipient stage of a back-arc rifting. In 2015, a magnitude 7.1 earthquake occurred west off Satsuma Peninsula, which was the largest earthquake ever recorded in the northern Okinawa Trough. Active aftershock activity has continued after that earthquake. Therefore, in order to clarify detailed aftershock activity, seismic observations using five ocean bottom seismographs were conducted around the ocean basin from August 1, 2019 to August 1, 2020. We determined the hypocenters of the aftershocks listed in the Japan Meteorological Agency catalog using both offshore and onshore seismic data. The relocated hypocenter distribution shows that most earthquakes occur within the upper crust, but that earthquakes also occur in the lower crust. Moreover, the obtained aftershock activity distribution shows that aftershocks can be divided into several clusters. The cluster in the northern part is located in the northeast-southwest direction, the same direction as the main shock. On the other hand, the cluster in the southern part of the observation area was found to be oriented in an east-west direction. This result indicates that the northernmost part of the Okinawa Trough has a complex crustal structure.
This work was supported by JAPS KAKENHI Grant Number 18H01306