Seismicity of the northern part of the Itoigawa-Shizuoka Tectonic Line

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The relationship between recent seismicity and the active fault system was investigated using the precise hypocentral distribution by taking into consideration the heterogeneity of the velocity structure around the northern part of the Itoigawa-Shizuoka Tectonic Line (ISTL), the western boundary of the northern Fossa Magna basin. Since a velocity structure suitable for the ISTL is necessary for precise hypocentral determination, deep seismic reflection and refraction profiling were undertaken in 2002 across the northern part of the ISTL and a reliable velocity model was obtained. The hypocenters were located using the station corrections derived from the velocity model. The obtained hypocenters were found to be distributed in a straight line in an almost north-tosouth direction. The hypocenters, whose lower limit was approximately 15 km, were distributed around a vertical plane. The focal mechanism solutions were primarily strike-slip and the direction of their P-axis was NW-SE to WNW-ESE. The fault planes expected from these mechanisms are consistent with the distribution of the hypocenters. However, they are not consistent with the thrust fault planes of previous large earthquakes, which were inferred from the near-surface feature of the active fault system. This suggests that the recent seismicity cannot be attributed to aftershocks of the previous large earthquakes and is not induced by the movement of the active thrust fault.