Active tectonics of the Senya Hills and evolution of the Senya Active Fault, Eastern margin of the Yokote Basin Fault Zone, Northeast Japan

Kagohara, Kyoko*; Imaizumi, Toshifumi*; Miyauchi, Takahiro*; Sato, Hiroshi*; Uchida, Takuma*; Echigo, Tomoo*; Ishiyama, Tatsuya*; Matsuta, Nobuhisa*; Okada, Shinsuke*; Ikeda, Yasutaka*; Toda, Shigeru*; Koshiya, Shin*; Noda, Masaru*; Kato, Hajime*; Noda, Katsuya*; Miwa, Atsushi*; Kurosawa, Hideki*; Kosaka, Hideki*; Nohara, Tsuyoshi   

The eastern marginal fault zone of the Yokote Basin is one of seismogenic reverse faults developed in Northeast Japan, generating the 1896 Riku-u Earthquake (M7.2). We discussed the relationship among fault traces, geomorphic displacements and fault geometries on the Senya fault, based on a data from high-resolution seismic reflection profiling, investigations in tectonic geomorphology and structural geology, with the help of the balanced cross section method. By the restoring the balanced cross sections, the horizontal shortening amount is estimated to be totally 3 km through the thrust system, and the thrusting is retroactive to 2.4 Ma. Depending on the strike of fault traces and the morphotectonic features, the Senya fault is subdivided into three, the northern, central and southern portion. The initiation of thrust front migration is ca.1.6 Ma at the central portion and 0.6 Ma at the northern portion. This means that the central portion preceded the northern portion as an emergent fault, and suggests that the initial propagated fault extends from the fault end to the boundary fault.