[S-SS12] Active faults and paleoseismology

[SSS12-01] Active blind thrusts in Quaternary sedimentary basins in Japanese islands: Overview

*Tatsuya Ishiyama¹, Naoko Kato², Hiroshi Sato³ (1.Earthquake Research Institute, the University of Tokyo, 2.College of Humanities and Sciences, Nihon University, 3.Center for Integrated Research and Education of Natural Hazards, Shizuoka University)

Keywords:Active blind fault, Quaternary, Tectonic geomorphology, Seismic reflection profiling

We collected high-resolution seismic reflection data across Quaternary sedimentary basins in Japanese islands to clarify their structural characteristics. In many cases, we identified previously unrecognized blind thrusts that deform Quaternary basin sediments indicative of recent fault activities. Structural characters of these active blind thrusts are (1) along-strike extensions of basin-bounding active thrusts plunging into alluvial plains, or (2) those completely lacking structural linkages with previously recognized active thrusts. These blind thrusts are characterized by weak or no geomorphic expression in rapidly sinking sedimentary basins so that prior identification of similar structures has their own difficulties. In some cases, very subtle Holocene geomorphology of alluvial plains and stream networks appear consistent with underlying blind thrust-related folds and may be usable for their good indicators. In such settings, super high-resolution seismic reflection profiling and its combinations with geomorphic and geophysical observations is the most effective tool to determine geometries and activities of active blind thrusts. In this presentation, we will summarize our results on structural characters of active blind thrusts, and discuss on their geomorphic, geologic and geophysical signals, impacts on seismic hazards, and strategy to resolve their locations and fault activities.