[S-CG52] 海洋底地球科学

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

森本 尚里¹、*中東 和夫¹、古山 精史朗¹、山下 裕亮²、仲谷 幸浩³、平野 舟一郎³、八木原 寛³、宮町 宏樹³、松島 健⁴、山田 知朗⁵、篠原 雅尚⁵ (1.東京海洋大学、2.京都大学防災研究所、3.鹿児島大学、4.九州大学地震火山観測研究センター、5.東京大学地震研究所)

キーワード：沖縄トラフ

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