Outcrop of the Active Structure at Construction Site of Tohoku-Chuo Expressway, Murayama City, Yamagata Prefecture, in North East Japan

Abstract

A new outcrop of active structure was observed at the construction site of Tohoku-Chuo Expressway through the Murayama city, Yamagata prefecture. Active structure running across the newly discovered outcrop is one of traces of the western marginal fault zone of Yamagata Basin, but it had not been described on active fault maps. We observed some outcrops which exposed folding or monoclinal flexure of middle to late Pleistocene deposits. From its continuation and cumulative deformation, we recognized active concealed thrust fault at the eastern margin of the Kawashimayama hill and newly called this fault “Ukinuma Fault”. Ukinuma Fault is concealed one without surface fault, but it has folding and monoclonal flexure. Ukinuma Fault's slip rate is estimated 0.45～0.55m/ky based on age of the terraces and vertical displacements of the fault inferred from hangingwall side's geomorphic profiles and footwall side's geological cross sections. It is important to research not only surface fault scarp but also a distribution and an array of geomorphic features and subsurface geological structure at concealed fault for disaster prevention.