[S-CG46] Uncovering stress accumulation and fault strengthening of megathrust earthquakes

[SCG46-05] Subduction of Horst-and-Graben Structures and Their Impact on Plate Boundary Faults: Numerical and Drilling Perspectives

*宮川 歩夢¹、奥田 花也²、濱田 洋平²、中村 恭之³、高下 裕章¹、野田 篤¹、Regalla Christine⁴、Kirkpatrick James⁵、氏家 恒太郎⁶、Conin Marianne⁷、Fulton Patrick⁸、小平 秀一³、江口 暢久³、前田 玲奈³、奥津 なつみ³、Toczko Sean³、Expedition 405 Scientists (1.国立研究開発法人　産業技術総合研究所 地質調査総合センター、2.海洋研究開発機構 高知コア研究所、3.国立研究開発法人海洋研究開発機構、4.Northern Arizona University、5.McGill University、6.Faculty of Life and Environmental Sciences, University of Tsukuba、7.University of Lorraine、8.Cornell University)

キーワード：Geodynamic modeling、plate boundary fault、IODP、JTRACK

In subduction zones, the interface between the subducting and overriding plates is defined by a plate boundary fault, which is able to generate great plate boundary earthquakes. In the Japan Trench, the 2011 Tohoku-oki earthquake occurred along the plate boundary fault, highlighting such a importance of understanding its characteristics.

Geodynamic models with numerical simulations were conducted to model the development of the plate boundary fault in the Japan Trench. In these simulations, the characteristic structural features of the Japan Trench, specifically the horst-and-graben structures on the incoming oceanic plate, were implemented. Additionally, the sedimentary layer on the oceanic plate was modeled to be thinner than the height of the horst-and-graben structures to reflect observations from seismic data.

The numerical simulation demonstrated that the repeated subduction of horst-and-graben structures results in the formation of a complex evolution of plate boundary fault system within the frontal prism. On the horst, the plate boundary fault is initially located at the interface between the oceanic plate and its overlying sedimentary layer. During this stage, the sediments on the horst are scraped off by the plate boundary fault and accreted to the overriding plate. When the graben subducts, the plate boundary fault develops at the interface between the previously accreted sediments and the sediments in the graben. With ongoing subduction, the plate boundary fault steps down to the interface between the sediments in the graben and the oceanic plate. This stepping down process results in the continuous landward accretion of graben sediments, leaving the former plate boundary fault stranded landward. Each set of the subducting horst-and-graben leads to a transition of the location of plate boundary fault through time.

The geological structures developed during the simulated subduction of horst-and-graben structures can be compared with data obtained from ocean drilling in the Japan Trench (IODP Expedition 405: JTRACK) and also existing seismic surveys. Core samples and logging data from JTRACK indicate repetitive lithological sequences within the frontal prism of the Japan Trench. Furthermore, a major fault zone accompanied by distinct fault zone has been identified within the prism. These observations suggest that the accretion of sediments near the trench occurs in response to the subduction of horst-and-graben structures. Additionally, the major fault within the prism, characterized by fault zones, may correspond to an ancient plate boundary fault that formed or was reactivated due to the subduction of the horst-and-graben structures.