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

[SCG46-06] Structural characteristics of Japan Trench plate boundary and splay fault zones in the region of shallow coseismic slip

*氏家 恒太郎¹、Kirkpatrick James²、Conin Marianne³、Fulton Patrick⁴、小平 秀一⁵、Regalla Christine⁶、江口 暢久⁵、前田 玲奈⁵、奥津 なつみ⁵、Toczko Sean⁵、Expedition 405 Scientists (1.University of Tsukuba、2.University of Nevada, Reno、3.University of Lorraine、4.Cornell University、5.JAMSTEC、6.Northern Arizona University)

キーワード：IODP、JTRACK、clay-rich fault zone、principal slip zone

International Ocean Discovery Program (IODP) Expedition 405 (JTRACK) successfully drilled into the plate boundary fault zone and a splay fault zone in the region of shallow coseismic slip during the 2011 Tohoku-Oki earthquake. Cores and logging-while-drilling (LWD) data from this expedition (Holes C0019H, J, K), combined with those from the previous IODP Expedition 343 (JFAST) (Holes C0019B and E), provide new insights into the structures of the fault zones that host shallow coseismic slip. The plate boundary fault zone sharply separates mudstone in the frontal prism above from pelagic clays below and corresponds to an increase in gamma-ray associated with an increase in clay content. The thickness of the fault zone and the interval of increased gamma-ray varies from ~1 m to 15 m or more, even over the tens of meters of lateral separation between the holes. The fault zone is clay-rich and is characterized by the development of a scaly fabric that surrounds or bounds the less deformed fractured mudstone and ~10-12 cm thick shear zones composed of multicolored clays. ~10 mm thick dark gouges are observed near the upper boundary of the fault zone, and one of the gouges shows an injection structure. A splay fault zone is identified at ~610 meters below seafloor (mbsf) at Hole C0019J where there is an age reversal (Late Miocene brownish mudstone over Pleistocene dark gray mudstone). This fault zone is ~1 m thick and shows asymmetric deformation across a 5 mm thick dark layer, anastomosing to planar scaly foliated clays and fractured/brecciated mudstone above and relatively less fractured/brecciated mudstone below. Another fault zone is also inferred from an age reversal and change in physical properties at ~207 mbsf, but the structural characteristics of this fault zone remain unclear due to poor core recovery and quality in this interval. Overall, both the plate boundary fault zone and the splay fault zone are localized along clay-rich layers and contain ~5-10 mm thick dark gouge/layer interpreted to represent principal slip zones. Given the static and dynamic frictional properties of clay-rich materials, the structural characteristics inferred from JTRACK and JFAST cores and data appear to be consistent with long-term slip localization along the clay-rich layers and low resistance to earthquake rupture propagation, which can explain shallow coseismic slip, including during the Tohoku-Oki earthquake. Post-expedition studies of fault rock samples and laboratory experiments will seek to confirm these preliminary interpretations.