Crustal deformation around the Kamishiro fault, northern Itoigawa-Shizuoka Tectonic Line and its relation to the 2014 Northern Nagano earthquake (Mw6.3)

Abstract:

The Itoigawa-Shizuoka Tectonic Line (ISTL) is a major geologic boundary intersecting the Japanese mainland into the northeastern and the southwestern parts. It is also an active fault system that is supposed to have a high seismic potential. We have conducted dense GPS observation and identified a highly localized E-W contraction around the Kamishiro fault at the northern ISTL. Kinematic modeling of this deformation pattern suggests that the fault is shallowly dipping to the east and accommodating the E-W contraction by aseismic faulting below the depth of 2-4 km. This aseismic fault is consistent with the base of the Neogene basin fill, which has accommodated E-W shortening over 10km. On November 22, 2014, a Mw 6.3 earthquake occurred at the Kamishiro fault. The hypocenter is located at the 5km depth and a 9km long surface rupture appeared along the fault trace. GPS observation and InSAR analysis with ALOS-2 data revealed northwestward displacement and uplift (max. 90cm) on the east, and southeastward displacement with subsidence (max. 30cm) on the west, indicating a rupture of the Kamishiro fault. The coseismic crustal deformation pattern is modeled by a faulting on a high-angle reverse fault from the surface to 7km depth, extending ~20km along the fault trace. A large fault slip is estimated at the shallowest (depth<2km) part corresponding to the surface rupture. The geodetic fault model is also consistent with the aftershock distribution. On the other hand, the source fault implies a rupture of the pre-Neogene basement below the basin fill, not the shallow-dipping fault estimated from interseismic deformation. Thus the relationship between the interseismic aseismic faulting and the coseismic fault is not totally clear. The large interseismic contraction mainly reflects inelastic process and only a small portion, if any, contributes to the stress accumulation of the main shock fault. This example demonstrates complexity of the earthquake cycle at a thrust fault system.