HR: 0800h
AN: S41B-0980    [Abstracts]
TI: Electrical Structure of the Shallow Part of the Atotsugawa Fault, Central Japan: Detecting en Echelon Structure in the Fault Zone
AU: * Yamashita, F
EM: yamafuto@bosai.go.jp
AF: NIED, 3-1, Tennodai, Tsukuba, 305-0006 Japan
AU: Kubo, A
EM: akubo@cc.kochi-u.ac.jp
AF: Kochi Univ., 2-5-1, Akebono-cho, Kochi, 780-8520 Japan
AU: Yamada, R
EM: ryamada@bosai.go.jp
AF: NIED, 3-1, Tennodai, Tsukuba, 305-0006 Japan
AU: Omura, K
EM: omura@bosai.go.jp
AF: NIED, 3-1, Tennodai, Tsukuba, 305-0006 Japan
AB: Dense VLF-MT and TDEM surveys were carried out to image the electrical structure of a region interpreted as a creeping segment of the Atotsugawa Fault, central Japan. The Atotsugawa Fault is an active fault with a length of 60-70 km and a strike of approximately N60°E. The fault type is a right-lateral strike-slip. The most significant characteristic of this fault is a possible existence of creeping segment. In the central region, the stable slip with a rate of 1.5 mm/year was found by the observation of baseline change (Geographical Survey Institute, 1997). However, such slip has not been found at the southwestern region. Therefore, the central region is considered to be a creeping segment. In the creeping segment, many fault outcrops were found on the right bank of the Atotsu-gawa River that runs along the fault. Strikes of shear planes in outcrops were observed to be N30°-47°E, which is apparently different from that of the Atotsugawa fault. This observation suggested the existence of en echelon structure, which is the cluster of small shear zones oblique to main fault. Investigation of the nature of the en echelon structure will help us to understand the growth history of the Atotsugawa fault and the mechanisms of creeping phenomenon. Because a fracture zone usually includes much water, we can detect it as a low resistivity zone. In order to image the detailed structure of echelon, we carried out the electromagnetic surveys; VLF-MT and TDEM survey as a preliminary and main investigation, respectively. The results of VLF-MT survey has been reported by Yamashita et al. (2005), and therefore we don_ft refer to the results here. We acquired data at 10000 points with airborne TDEM survey, and over 4000 data were selectively used for modeling the subsurface structure. Apparent resistivity at each point was modeled assuming 1-D structure that consists of 30 and 70 m thick layers on a semi-infinite basement (three layers in total). Because over 4000 survey points were distributed densely, we could approximate the 3-D image of resistivity structure based on 1-D models. The results of TDEM survey showed that the modeled resistivity ranged widely from 10 to 1000 Ωm, which was consistent with the results of VLF-MT survey. A low-resistivity zone with a width of about 200 m was identified in planer projection of the 3-D resistivity structure, whereas its location was significantly different from the geologically determined trace of the Atotsugawa Fault. Outstanding short wavelength component of spatial variation in resistivity was also recognized, indicating the existence of the smaller subsurface structure than the main fault. We extracted the lineaments from the planar distribution of modeled resistivity using the Hough transformation. Although the strikes of the extracted lineaments were widely distributed, the mode of them was consistent with the strikes of shear planes observed in outcrops.
DE: 5109 Magnetic and electrical properties (0925)
DE: 7299 General or miscellaneous
DE: 8010 Fractures and faults
DE: 8038 Regional crustal structure
DE: 8163 Rheology and friction of fault zones (8034)
SC: Seismology [S]
MN: Fall Meeting 2005