12:00 PM - 12:15 PM
[SSS08-17] Shear-wave seismic reflection surveys across the Kamishiro fault, Itoigawa-Shizuoka Tectonic Line active fault system, central Japan
Keywords:seismic reflection survey, Kamishiro Fault, Itoigawa-Shizuoka Tectonic Line, active fault, subsurface structure
The shear-wave seismic reflection surveys were performed in Hakuba Village from November 3 to 10 in 2017 along the three seismic lines, named Shiojima, Oide and Wing 21. All of these seismic lines were trending east-west, located to the west of the Hime River, and crossing the surface rupture trending NNE-SSW. This area is covered with the fluvial terrace deposits.
We performed shear-wave seismic reflection surveys using a seismic acquisition system combined with Electrodynamic Vibrator System and a land streamer. The seismic source ELVIS (Electrodynamic Vibrator System) manufactured by GEOSYM, was used in Shear-wave mode to generate horizontally polarized shear waves (SH-waves). The sweep length was 10 seconds, and its frequency was 20-80 Hz. Seismic signals were recorded by a land streamer equipped with 96 SH geophones. The source intervals were 1 m (2 m only in the Wing 21 seismic line), and the receiver intervals were 0.5 m. We selected the recording system GEODE (GeoMetrics), and its sampling rate was 1 msec. The Shiojima seismic line to the north of the Matsu River had 60 shot points, 168 receiver points, and a length of 83.5 m. The Oide seismic line to the north of the Onara River had 239 shot points, 476 receiver points, and a length of 237.5 m. The Wing 21 seismic line to the south of the Onara River had 108 shot points, 236 receiver points, and a length of 117.5 m. In this seismic line, we performed hammer-impacts at intervals of 2 m to compare the performance with ELVIS.
In the Oide seismic line, CMP stacking after NMO corrections using 0.3 km/s for S-wave velocity reveals the geometry of a fault. The obtained seismic section demonstrates the eastward dipping seismic event, which corresponds to the subsurface extension of surface rupture up to a depth of approximately 50 m. Shear-wave seismic reflection surveys using Electrodynamic Vibrator System and a land streamer were very effective to obtain great detailed subsurface images of active faults.