一関–石越撓曲線の変動地形と地下構造

Abstract

We should understand the earthquake potential in and around Quaternary fault zones, in view of recent destructive inland earthquakes at previously unknown active fault zones in Japan. The Senpoku Plain and its surrounding areas are characterized by high seismic activity in northeast Japan, highlighted by four destructive earthquakes, M 6.8 in 2008, M 6.4 in 2003, M 6.5 in 1962, and M 7.0 in 1900, which occurred during the past 100 years, although few geomorphic features indicate active faulting. A comprehensive survey was conducted on the tectonic geomorphology in the area to understand the structural and geomorphic expression of the Ichinoseki–Ishikoshi Flexure Line (IIFL), which suggests Quaternary activity. Geological and geomorphical mapping shows that the IIFL is located between the Kitakami Lowland Fault Zone and the Senpoku Plain. The IIFL extends about 30 km from Isawa to Ishikoshi with a slightly sinuous trace. A high-resolution seismic reflection profile and a gravity profile define the subsurface geometry of the IIFL. The IIFL is interpreted to be a steeply west-dipping reverse fault. The Pliocene Kazawa and Yushima Formations typically dip 40° to 20°E along the IIFL, and are overlain by the Pleistocene Mataki Formation, which becomes thinner toward the fold axis of the IIFL, and their dips decrease progressively upward. This suggests that the Mataki Formation was deposited concurrently with fault activity of the IIFL. Fission-track dating of a tuff layer within the uppermost section of the Kazawa Formation indicates that active reverse faulting of the IIFL began at about 2 Ma. At least 280 m of the tectonic uplift is consumed by active faulting and the average uplift rates are estimated to be 0.14–0.08 mm/yr. Vertical separations of Hh surface are about 15 to 40 m. Heights of fold scarps on L1 surface are about 2 m. Their ages are determined to be 0.4–0.5 Ma for Hh and 24–12 ka for L1, respectively. Therefore, the Quaternary average uplift rates of the IIFL are estimated to be 0.03–0.17 mm/yr. Quaternary activity of the IIFL is weak, but there are differences in the magnitude of dissection in the Iwai Hills between the hanging-wall and the footwall of the IIFL.