[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-P01] Compilation of bathymetry and seismic reflection data to characterize outer rise faults in the Kuril Trench

*小平 秀一¹、野 徹雄¹、中村 恭之¹、藤江 剛¹、尾鼻 浩一郎¹、三浦 誠一¹ (1.海洋研究開発機構　海域地震火山部門)

キーワード：千島海溝、アウターライズ地震、折れ曲がり断層

It has been reported that stress changes in the oceanic plate associated with an interplate megathrust earthquakes cause to activate seismic activity in the oceanic plate before subduction, which is sometimes accompanied with a large normal fault earthquake. For example, along the Kuril Trench, an extensional earthquake (Mw=8.1) occurred within the oceanic plate in 2007, following a shallow megathrust event (Mw=8.3) at the plate boundary in 2006. In the Japan Trench region, normal fault earthquakes in the outer-rise have been activated immediately after the 2011 Tohoku-oki earthquake. Based on results of our previous earthquake observations and seismic imaging, it has been concluded that, i) the outer rise normal fault earthquakes dominantly occur along the horst-and-graben structure formed by plate bending, ii) the horst-and-graben structure is mainly developed oblique to the fabric of the oceanic plate, and iii) dip of the normal fault is as high as 70° at the depth of the crust and uppermost mantle. On the other hand, in the Kuril Trench, the trench axis and oceanic fabric run almost parallel, and the development of bend-related faults along the oceanic fabric can be confirmed from bathymetry data. In order to clarify the differences in a formation process of outer-rise faults and fault geometry between the Japan Trench, where new bend-related faults are dominantly formed, and the Kuril Trench, where bend-related faults develop along the oceanic fabric, this study compiles high resolution seismic reflection data, conventional seismic reflection data, and detailed bathymetry data, which have been acquired in the Kuril Trench by JAMSTEC since 2016. The results indicate that the half-graben fault structure formed by seaward trending high-angle normal faults is dominant in the Kuril Trench, unlike the Japan Trench where horst-and-graben faults dominantly develop. The conventional seismic reflection data also shows oceanic Moho, but the current processing of the data does not allow us to identify if there is an offset of the Moho associated with normal faulting. Reflection events indicating normal faults in the oceanic crust are not imaged, but it is still unknow whether this is due to structure or processing. Further data processing focusing on fault images in the oceanic crust and Moho offsets is necessary. We will use these data to describe the fault geometry and discuss the formation process of the outer rise fault. As a final goal of the project, we will also create a fault map of the offshore Kuril Trench to contribute mitigation of tsunami hazards.