[S-CG55] Dynamics in mobile belts

[SCG55-10] A High-angle Strike-slip Model for the Northern and Central Itoigawa-Shizuoka Tectonic Line Fault System, Central Japan

*Takeshi Sagiya¹ (1.Disaster Mitigation Research Center, Nagoya University)

Keywords:Itoigawa Shizuoka Tectonic Line Fault System, strike-slip fault, crustal deformation, source fault model

The Itoigawa-Shizuoka Tectonic Line (ISTL) in central Japan is one of the major active faults in Japan. While the geological slip rate of ISTL is 5-9 mm/year and the average repeat time is as short as 700-1700 years, no earthquakes over magnitude 7.0 have occurred after the 9th century, suggesting a high probability of a large earthquake. Thus, strong motion prediction for large earthquakes at ISTL is essential to disaster mitigation. However, the earthquake source model for ISTL is highly uncertain. The current model assumes an eastward dipping reverse fault, a high-angle strike-slip fault, and a westward dipping reverse fault for the northern, central, and southern parts of ISTL, respectively. In the current governmental evaluation of strong motion for ISTL, because of changes in the faulting type and the geometry, the northern and central segments of ISTL are analyzed separately, which may underestimate the earthquake size and shaking. On 22 November 2014, an M_w6.3 earthquake occurred at the Kamishiro fault in the northern ISTL, and surface ruptures were observed. The hypocenter was located at a depth of 5 km, about 4 km east of the surface fault trace, and the source mechanism based on the initial motion showed a left-lateral slip on a steeply dipping fault. Despite its thrust-type rupture at the surface, the main faulting started as an N-S trending deeper strike-slip fault. This earthquake demonstrated a problem in the current source model. The surface manifestation of active faults and the underground source fault do not have to be the same. From a seismological perspective, the CMT data inversion showed that the regional stress in the central-northern ISTL is consistent with the strike-slip faulting under NW-SE compression. Also, there exists a linear alignment of earthquakes in the N-W direction, which may continue to the source region of the 2014 earthquake though the actual structure should be complex. From a geodetic point of view, in the northern ISTL, the regional NW-SE contraction rate (~15 mm/year) is much larger than the geological slip rate (~3 mm/year). This discrepancy can be interpreted if the relative motion is accommodated at depth and the shallow part deforms mostly inelastically. With such a model, the strike-slip fault can be about 100 km long, consistent with the large unit coseismic slip estimated from trench surveys.