糸魚川－静岡構造線活断層系中部・富士見地域における反射法地震探査

Abstract

The Itoigawa-Shizuoka tectonic line (ISTL) active fault system displays one of the largest slip rates in the Japanese islands. In the Fujimi area, the southern part of the ISTL active fault system, there are two faults, trending NW-SE, parallel to each other: the Aoyagi fault to the east and the Wakamiya fault to the west. The distance between the two faults is 1-2km. The Aoyagi fault is a west-dipping reverse fault and the Wakamiya fault is east-dipping sinistral fault. To clarify the subsurface geometry and their connectivity, we carried out high-resolution shallow seismic reflection profiling across the two faults. The length of the seismic line is 3.6km, and the receiver and shot point intervals are 10m. Common mid-point seismic reflection data were acquired using a 144-channel recording system and a mini-vibrator. Based on the obtained seismic section, we interpreted that the deeper extension of the Aoyagi fault shows a west-dipping fault surface at a moderate angle, and that the Walkamiya fault merges with the Aoyagi fault. In other words, the oblique slip on the ISTL is partitioned into the dip slip on the Aoyagi fault and the left lateral slip on the Wakamiya fault in shallow fluvial deposits