HR: 0800h
AN: T41B-1298    [Abstracts]
TI: Subsurface structure along the eastern marginal fault zone of Yokote Basin by Seismic reflection profiling studies, Northeast Japan
AU: * Kagohara, K
EM: d0-0b-kago-@mail.tains.tohoku.ac.jp
AF: Graduate school of Tohoku University, 6-3, Aramaki Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Imaizumi, T
EM: imat@mail.tains.tohoku.ac.jp
AF: Tohoku University, 6-3, Aramaki Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Echigo, T
EM: techigo@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, 7-3-1, Hongo, Bnkyo-ku, Tokyo, 113-0033 Japan
AU: Miyauchi, T
EM: tmiya@faculty.chiba-u.jp
AF: Chiba Unniversity, 1-33, Yayoi-cho, Inage-ku, Chiba, 263-8522 Japan
AU: Sato, H
EM: satow@eri.u-tokyo.ac.jp
AF: E.R.I University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Typical reverse faults, which are known as Senya earthquake faults appeared along the western foot of the Mahiru Mountains, associated with The Rikuu Earthquake (Mj7.2) of 1896 in Northeast Japan. Eastern marginal fault zone of the Yokote Basin consist of four main surface ruptures, about 35 km long, Obonai fault, Shiraiwa fault, Ota fault and Senya fault, depending on their continuity and strike (Matsuda et al., 1980). We carried out the seismic reflection profiling across these faults (Kawaguchi03 Seismic line, Unjono04 Seismic line and Kotaki05 Seismic line) to clarify the subsurface structure of these reverse fault system based on the data of tectonic geomorphology and structural geology and furthermore, to discuss the timing of migration of the thrusting from the range front to the basin margin. The seismic source was mini-vibrator trucks, with 20seconds of 10-100Hz signals at 10m or 5m intervals. The sweep signals were recorded by a digital telemetry system (GDAPS-4a) with 10 Hz geophones. The obtained seismic reflection data were processed by conventional Common mid-point (CMP) methods, including post-stack migration and depth conversion. The resulting seismic reflection profile reveals a thrust structure beneath these areas. At the Center of Senya hills there are two thrusts and one high angle reverse fault (1997 Seismic Line). Senya fault is an active frontal emergent thrust with flat and ramp structure. Although, the high angle reverse fault, located along the foot of the range is a short-cut branching fault from the Senya fault in the central part of the Senya hill (Sato et al., 1998), in the Unjono04 seismic line, the depth of the flat and ramp structure gradually shallow in the north part of the Senya hill, where the flexure scarp accompanied with antithetic faults formed on the fluvial terraces. In the Kawaguchi03 seismic line, the concealed fault, 0.5km below the surface, branched from the master Ota fault, form a flexure scarplet on the alluvial fan in the basin. In the eastern margin of the Yokote basin, several levels fluvial terraces were developed along the rivers from the Mahiru Mountains, alluvial fans are in the basin side. The mean vertical slip rate is 0.5-0.8 mm/yr (net slip rate 1.2-1.6mm/yr) in range, in the Senya fault among the compositional faults, based on the distribution and displacement of fluvial terraces. According to the balanced cross section, a total 3.2km of horizontal shortening is obtained. Assuming a uniform net slip rate, the first motion of the Senya thrust system took place at 2.6-2.0 Ma (Sato et al., 1998). Furthermore, basin-ward migration of thrusting and uplifting of Senya hills has been beginning in 0.5-0.4 Ma. The higher to middle terraces distributed on the Senya hill, are clearly tilted to the eastward. In contrast, new thrusting migrated from the master Ota fault (Kawaguchi03 Seismic line) and Shiroiwa fault (Kotaki05 Seismic line) has been occurred in younger time than that of the Senya fault. These Differences of the subsurface structure and the timing of the migration of faulting are inferred to due to the difference of the geologic conditions in front of the master fault.
DE: 8015 Local crustal structure
DE: 8036 Paleoseismology (7221)
DE: 9320 Asia
SC: Tectonophysics [T]
MN: Fall Meeting 2005