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| | Abstract | | 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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