Paleostress analysis and fault property of the Median Tectonic Line in the central Shikoku, southwest Japan

Fault strength is a large factor to control deformation processes and mechanisms during earthquakes. For knowledge of fault strength, it is essential to understand stress field in and around fault damage zones. In this study, we tried to evaluate fault property of the Median Tectonic Line (MTL) in Japan based on paleostress data. The MTL extending over 1000 km-long is a geological boundary fault that divides southwest Japan into Inner (mainly the Late Cretaceous Izumi Group and Ryoke granitic to low-P/T metamorphic rocks) and Outer (mainly the Cretaceous Sanbagawa high-P/T metamorphic rocks) Zone. The movement history are complex because the MTL is a long-lived fault system from the Middle Cretaceous. While the MTL from Shikoku to western Kii Peninsula is dextral active faults revealed by geomorphological and paleo-seismological surveys (e.g., Ikeda et al., 2017). Famin et al (2014) reported the long-term weakness of the MTL in the eastern Shikoku by fault slip inversion based on field survey. However, there is few information about stress condition of the MTL from central to west Shikoku. To reconstruct paleostress fields from central to west Shikoku, we conducted a structural analysis of a geological core sample (125 m-long) penetrating the inactive parts of the MTL in the central Shikoku. The structural analysis was performed in three sections; Izumi Group, MTL damage zone and Sanbagawa metamorphic rocks. As the results, we categorized the stress fields in and around the MTL into five faulting regime; (1) normal-faulting regime with NW-SE σ₃ axis, (2) normal-faulting regime with NE-SW σ₃, (3) strike-slip to normal-faulting regime with NNE-SSW σ₃ axis, (4) strike-slip-faulting regime with ENE-WSW σ₁ axis and WNW-ESE σ₃ axis, and (5) strike-slip-faulting regime with NNE-SSW σ₁ axis and WNW-ESE σ₃ axis. Moreover, we conducted slip-tendency (Ts) analysis (Morris et al., 1996) based on the stress fields obtained from the structural analysis for estimate the fault property of the MTL. The results show that the MTL has moderate-high Ts in the case of normal-faulting regimes and low Ts in strike-slip faulting regimes. The low Ts values of strike-slip faulting regimes suggest that the MTL behaved as weak faults when it activated as strike-slip faults. Whereas the MTL didn't have to be weak faults when it activated as normal faults.