Low Vp and Vp/Vs zone beneath the northern Fossa Magna basin, central Japan, derived from a dense array observation

The northern Fossa Magna (NFM) basin is a Miocene rift system formed in the final stage of the opening of the Japan Sea. The northern part of the Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the northern Fossa Magna. In order to understand the active tectonics in these areas, it is essential to explain the seismic velocity structures, deep structures of active faults, and microseismicity near the active faults. In the autumn of 2002, we conducted a seismic array observation across the northern part of the TSTL and the NFM to obtain a structural image beneath the NFM. Arrival times of local earthquakes and explosive shots were used in a joint inversion for earthquake locations and 3-D Vp and Vp/Vs structures. P- and S-wave arrival time data were obtained from 73 events including 4 explosive shots, and 3809 P- and 2659 S-wave arrival times were used for the inversion analysis. We obtained a seismic velocity model revealing good correlations with the surface geology along the profile. In particular, we found thick low-velocity zones beneath the NFM and the Komoro basin and a high-velocity zone beneath the Central Uplift Zone. Beneath the NFM, a low-velocity zone with low-to-moderate Vp/Vs extends to a depth of approximately 10 km. The low-velocity suggests the existence of aqueous fluid-filled pores with high aspect ratios.