[S-SS30_29PM2] New perspective of great earthquakes along subduction zones

[SSS30-34] Multiple damage zone system of an exhumed subduction zone megasplay fault

*Mari HAMAHASHI¹, Ryoji KAWASAKI¹, Rina FUKUCHI¹, Saneatsu SAITO², Yoshitaka HASHIMOTO³, Yohei HAMADA², Yujin KITAMURA⁴, Asuka YAMAGUCHI¹, Gaku KIMURA¹ (1.The University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology, 3.Kochi University, 4.Kagoshima University)

Keywords:subduction zone, megasplay fault, physical property of rock, damage zone, structural observation, rock deformation

More than 90% of the major earthquakes and tsunamis are known to occur at plate convergent margins, along plate boundary faults and megasplay faults. Investigating the mechanical properties and deformation patterns of these megathrusts are important to understand the generation of earthquakes and the dynamics on the subduction plate interface. Large displacement faults contribute to the reduction of steady-state strength at mid-crustal levels, and cause the frictional-viscous deformation at depth. As the candidate for such weak faults, foliated, phyllosilicate-rich fault rocks have been prevalently recognized in many tectonic settings. However, whether foliated fault rocks behave as weak structures in the longer terms and their roles in the strain localization and fault evolution, are poorly understood. Exhumed fault zones are helpful to constrain fault strength and deformation process of foliated cataclasites formed at upper-midcrustal depths over geological time. One of the well-studied exhumed major fault zones in subduction settings is the Nobeoka Thrust, a fossilized megasplay fault in Kyushu Shimanto Belt, southwest Japan, which exposes foliated fault rocks that were formed under the temperature range of ~180-350 ℃(Kondo et al., 2005). During the Nobeoka Thrust Drilling Project in 2011, core samples were retrieved containing both consolidated fault rocks and less consolidated, brecciated fault rocks, preserved from surface weathering and less likely to be drilling-induced. The core samples are expected to provide a different aspect on fault rock strength from previous geological studies on exposed, consolidated outcrops. In the current study, given the unique opportunity to determine the coexistence of cohesive and less cohesive fault rocks in a single fault system, we conduct macroscopic and microscopic structural observation and physical property measurements on the core samples, synthesizing with geophysical logs obtained from the drilling of the Nobeoka Thrust to characterize the damage zone architecture of the fault rocks formed in the frictional-viscous regime along the megasplay fault.The hanging wall consists of the shale-dominant intervals of dense development of phyllitic cleavages, the sandstone-dominant intervals of disturbed foliations, and the damage zone above the fault core characterized by cataclastically broken phyllite with thick abundant sandstone blocks. The observed density of brittle fractures, breccias, and mineral veins is increased at the sandstone-dominant intervals and near the fault core, whereas brecciated and less brittle/ductile structures are abundant within the shale dominant intervals. The brittle deformation near the fault core may have caused the wearing away of the shale-rich zones by abrasion, and as a result, the sandstone-rich zones that have relatively larger strength, remained and deformed cataclastically near the fault core. On the other hand, the footwall in the drilled range consists of six sets of fracture zones, all of which include a "brecciated zone" intensively broken in the center, sandwiched by a "surrounding damage zone" with abundant cohesive faults, mineral veins, and sandstone blocks. The surrounding damage zone is characterized by the increase in fault and fractures with distance from the fault core, and interestingly associate with the increase in resistivity, P-wave velocity, and density and decrease in porosity. The deformation in the surrounding damage zone is inferred to occur in a strain-hardening manner, strengthening with distance from the fault core. Shear localization may initiate more easily in the sandstone-rich area later forming the surrounding damage zone, and eventually develop an intensively deformed fault core in the center. These insights would enable to reinterpret the deformation processes and weakening mechanisms that occur in foliated fault rocks along the megathrust in subduction zones.