Investigation of local stress field in the north-central Kinki district by dense seismic observation of stress rotation near the active faults

Abstract Several active faults are distributed in the north-central Kinki district, making it one of the areas with their highest concentrations in Japan. The orientation of the maximum principal stress (σ1) in southwest Japan is generally in the east-west direction. In the north-central Kinki district, σ1 is also considered to be oriented uniformly in the east-west direction. However, the Arima-Takatsuki fault zone in this district is a vertical strike-slip fault with a strike that is close to east-west, and the angle that the surface trace of the fault makes with the north is 80°. The Mitoke fault zone is also a vertical strike-slip fault with an angle of 110°. The stress field is less likely to cause a strike-slip motion in these fault zones. Thus, in this study, we comprehensively examined whether the orientation of σ1 is uniform. We then investigated the relationship between the orientation of σ1 and the strike of fault. The mechanism solutions were obtained using a large amount of seismic data between November 17, 2008 and March 29, 2018. The stress fields were estimated by stress tensor inversion. At a depth of 10 km near the Arima-Takatsuki fault zone, the average optimum solution for the azimuth of σ1 was 104°, rotating clockwise from the east. The angle between this azimuth and the direction of the surface trace of the fault was 24°. In the Mitoke fault zone, the azimuth of σ1 was 81° and the angle was 29°. These results indicated that the stress field is not uniform and that σ1 rotates in an orientation that easily causes strike-slip motion for strike-slip faults. To elucidate the cause of the σ1 rotation, we assumed that the ductile fault extends into the lower crust as the deeper extension of the brittle fault zone and showed that the stress field in the upper crust rotating near these two fault zones can be explained by aseismic slip on the ductile faults. The findings indicated that the activity of the upper crust is closely related to that of the lower crust and more detailed analysis of the lower crust is essential.