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

Abstract

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.

Seismological and geological characterization of the crust in the southern part of northern Fossa Magna, central Japan

“…The location of deep reflectors is after Imai et al (2004). The resistivity structure along the Itoshizu 2002 (Ogawa et al, 2002), the relocated, hypocentral distribution (Sakai, 2004) and the reflectors observed on the shot gathers by explosive sources (Imai et al, 2004) are superposed on the geologic cross-section (Fig. 5).…”

“…As suggested by Ogawa et al (2002), the hypocentral distribution has tendency to concentrate between low and high resistivity area. The focal mechanisms of these earthquakes are strike-slip with WNW trending P-axis (Sakai, 2004). Including the northern Fossa Magna region, the most of the focal mechanisms in the back-arc side of the southern part of the northern Honshu shows the strike-slip type with WNW trending P-axis (Kosuga, 1999).…”

“…The observed micro-earthquakes in the footwall, forming the NNW trending zone (Sakai, 2004), are possibly produced by slip of preexisting NNW trending fault system and/or triggered by migration of fluid to less permeable (more resistive) crust (e.g. Ogawa et al, 2001).…”

“…From these observations, it is highly probable that the Moho-depth is located at about 40 km. with the resistivity structure (Ogawa et al, 2002), the hypocentral distribution (Sakai, 2004) and P-wave velocity structure obtained by seismic tomography . The location of deep reflectors is after Imai et al (2004).…”

“…Using the obtained velocity structure by refraction analysis, hypocentral distribution of micro-earthquakes were redetermined and projected on the Itoshizu 2002 seismic line (Sakai, 2004;Fig. 5).…”