[S-CG54] 東北地方太平洋沖地震から10年―固体地球科学の到達点

[SCG54-10] Earthquake faulting in the Japan Trench subduction zone: lessons learned from deep ocean drilling and the fault rocks in the exhumed accretionary complex

*氏家 恒太郎¹、伊東 慶祐²、長手 彩夏²、田畑 皓輝² (1.筑波大学生命環境系、2.筑波大学生命環境学群)

キーワード：遠洋性粘土、東北地方太平洋沖地震調査掘削、付加体、シュードタキライト、熱破壊

Drilling into the shallow coseismic slip zone, which produced > 40 m of slip during the 2011 Tohoku-Oki earthquake, revealed that the earthquake faulting was localized along the smectite-rich pelagic clay. High-velocity friction experiments using the fault zone material demonstrated that large slip at the shallow portions of the Japan Trench subduction zone resulted from coseismic weakening of the fault due to the abundance of smectite and thermal pressurization. However, the actual conditions of earthquake faulting in the deeper portions of the Japan Trench subduction zone are largely unknown. The fault rocks in the Jurassic chert–clastic complex in central Japan provide windows to observe seismogenic processes in the deeper portions of the cold subduction zone, where pelagic sedimentary rocks were subducted at a temperature of ~220 °C. On the basis of structural and Raman spectroscopic analyses, we recognize the pseudotachylyte in the thrust fault branched from the plate-boundary décollement, which is marked by the preferential melting of illite, thermal erosion, wall-rock fracturing, and increased carbonization at temperatures higher than 900–1100 °C. The pseudotachylyte is derived from the frictional melting of black carbonaceous mudstone flanked with pelagic chert. The thermal modeling, the measurement of the fracture density in wall rocks, and the quartz-dominated mineral composition of the chert suggest that wall-rock fracturing is caused by thermal fracturing associated with the alpha–beta transition of quartz in chert. Such thermal fracturing tends to occur along the coseismic slip zone, flanked by rocks with high thermal diffusivities, and, thus, may be a characteristic process during seismic slip along the black carbonaceous mudstone flanked with chert. The thermal fracturing of wall rocks could lead to a dilatancy-induced fluid pressure drop in the coseismic slip zone and reduction in the stiffness of wall rocks, potentially contributing to the generation of frictional melt and acceleration of seismic slip. The seismic slip processes recorded in the Jurassic chert–clastic complex may be applicable to the deeper portions of the Japan Trench subduction zone, where pelagic clays are subducted with pelagic chert.