| 講演要旨(和文) | | 筆者らは，2014年11月22日に長野県北部を震源とした地震（Mw 6.2）に伴い神城断層沿いに出現した地表地震断層の地下数10 mにおける断層の形状および地質構造を高解像度でイメージングすることを目的として，可搬型スィープ震源Elvis Seismic Vibrator（ELVIS）を用いた極浅層S波反射法地震探査を実施した．受振には96チャンネルのランドストリーマーを使用した．また，探鉱器にはGeoMetrix社製のGEODEを使用した．一般的な共通反射点重合法による解析の結果を表層地質や近傍のボーリングデータと対比した結果，地表近傍から最大で深度100 mまでの断層の規模・形状および地質構造を高解像度にイメージングすることができた．また，これまで確認されておらず，2014年の地震時にも活動していない伏在逆断層の存在も明らかにすることができた． |
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| | 講演要旨(英文) | | Coseismic net fault slip based on very shallow fault trajectories offers insight into understanding the properties of source faults for assessing seismic risk from crustal earthquakes in the overriding plate. To illuminate fault geometries of < 100 m depth, we carried out S-wave very shallow seismic reflection profiling across coseismic surface ruptures emerged during the 2014 Nagano-ken-hokubu earthquake (Mw = 6.2) along the Kamishiro fault, Itoigawa-Shizuoka tectonic line (ISTL) active fault system. We collected seismic data along six seismic lines in 2017 and 2018, using an Electrodynamic Vibrator System (ELVIS: GEOSYM) as an SH seismic source, a land streamer composed of 96 channels, and a GEODE (GeoMetrics) recording system. Through correlation of processed depth sections based on standard CMP stacking with surface geology and drill-hole data, the seismic sections up to a depth of approximately 100 m resolve thrust geometries extending from the 2014 coseismic surface ruptures. Our seismic sections also successfully illuminate previously unrecognized blind thrust faults deforming Quaternary sediments which were not activated during the 2014 event. Our results show that shear-wave seismic reflection surveys using Electrodynamic Vibrator System and a land streamer are effective for obtaining detailed subsurface images of active faults and unrecognized blind thrust structures. |
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