Visualization of a high-strain shear zone based on a field survey for minor faults; A Case study of southern Kyushu, Japan

Goto, Akira; Shimada, Koji   ; Niwa, Masakazu   ; Terusawa, Shuji*

Owing to a nationwide GNSS network installed since mid-1990s, high-strain shear zone with a sinistral strike-slip was identified in southern Kyushu. There have had large earthquakes in this area such as the 1997 Northwestern Kagoshima Earthquake, however, any seismogenic faults associated with these earthquakes or active faults with strike-slip tectonic landforms had not been previously recognized at the surface. Mesoscale fault zones with clayey gouge and cataclasite are rare in the study area (northwestern Kagoshima (2216 km)). Instead, minor faults cutting bedding planes and deformation structures subparallel to the bedding, suggesting the formation clearly after the accretion of the Shimanto accretionary complex, are commonly observed. Slip directions can be examined from striations along the minor faults. Based on orientation data of these striations, we applied stress tensor inversion techniques to elucidate stress states. Our field survey revealed an occurrence of the E-W trending zone of 2 km wide showing a stress state consistent with the sinistral high-strain shear zone (NE-SW compression and NW-SE extension). This study can contribute to one of helpful approaches to examine potential future activities of faults (or shear zones) without clear tectonic landforms.