[S-CG55] 沈み込み帯へのインプット：海洋プレート誕生から沈み込み帯まで

[SCG55-09] Complicated fault geometry of outer-trench normal-faulting earthquakes based on the ocean bottom seismograph observations

*尾鼻 浩一郎¹、高橋 努¹、山本 揚二朗¹、中村 恭之¹、藤江 剛¹、三浦 誠一¹、小平 秀一¹ (1.海洋研究開発機構海域地震火山部門)

キーワード：日本海溝、アウターライズ

After the 2011 Mw 9.0 Tohoku-oki earthquake, intra-plate normal-faulting earthquakes, including several M7-class earthquakes, have occurred in the trench-outer rise region along the Japan Trench and concern of large outer-rise earthquakes and related tsunamis have been raised. Based on the aftershock distribution, earthquake ruptures consisting of multiple faults are suggested for some large outer rise earthquakes. For 1933 Showa-Sanriku earthquake, rupture along two roughly parallel faults dipping toward opposite direction (westward dipping and eastward dipping) are proposed by Uchida et al. (2016). In addition, aftershock distribution along two conjugate faults were observed for two M7-class earthquakes in 2005 and 2011 in the trench-outer rise region along the Japan Trench (Hino et al. 2009, Obana et al. ). The aftershock distribution used in these studies are based on the far-filed observations or observations conducted in a month to a few years later from the mainshock. During the OBS observations conducted in the trench-outer rise region of the central Japan Trench from September 2017 to July 2018, three M6-class outer-rise normal-faulting earthquakes (M6.3 on Sep. 21, 2017, M6.3 on Oct. 6, 2017, and M6.0 on Nov. 13, 2017) occurred around the OBS network. The results from the OBS observations succeeded to locate the rupture initiation of the mainshocks and aftershock activities including immediately after the mainshocks. The aftershocks of M6.3 earthquake on October 6 occurred along several inclined planes with different dipping angles and directions. Focal mechanisms indicate both normal-faulting and strike-slip aftershocks. In the trench-outer rise region, there are horst-and-graben structures formed by normal-faults cutting through the oceanic crust. The M6.3 earthquake on October 6 occurred at a joint of segments of the horst-graben-structures. Complicated distribution of the aftershocks with both normal-faulting and strike-slip suggests a possible compound rupture of multiple faults during the intra-plate normal-faulting earthquake. The other two M6-class earthquakes also show the complicated distribution of the aftershocks and suggest the relation to multiple faults both apparent and obscure from the bathymetry. Such complicated fault geometry should be considered as a possible fault of large intra-plate normal-faulting earthquakes and related tsunamis.