Conductivity structure around the upper boundary of a seismogenic layr of inland type earthquakes.

1996 Fiscal Year Final Research Report Summary

• Project/Area Number: 07640556
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: 固体地球物理学
• Research Institution: KOBE UNIVERSITY
• Principal Investigator: YAMAGUCHI Satoru Faculty of Science, KOBE UNIVERSITY Lecturer, 理学部, 講師 (70191228)
• Co-Investigator(Kenkyū-buntansha): OSHIMAN Naoto Kyoto University, Disaster Prevention Research Institute, Associate Professor, 防災研究所, 助教授 (70185255)
• Project Period (FY): 1995 – 1996
• Keywords: subsurface conductivity structure / seismogenic layr / Low resistive layr / crustal structure / shiga prefecture

Research Abstract

One of the most dense observation of micro earthquakes covers the north area of Kyoto prefecture and northwestern area of Shiga prefecture. This area is suitable for the study of the structure and condition of seismogenic layr. Eighteen points were selected as candidates of observation points in thisarea. Remote reference method is necessary for this study because artificial electro-magnetic noise should exist. Therefor the selection of remote site is on the most important item of this study. We carried out MT observation at Mineyama site which is the our first candidate as the reference site and also carried out preliminary observation at Mizunomi site in Wachi town. It was founded that our study area is widely contaminated by artificial noise and also found that Mineyama station is affected by unusual telluric noise current. Therefore we have to change the observation method from the broad-band MT method only to the combination method of the broad-band MT method and the Network MT me thod, which make us to enable to get long period data in noisy area. This result lead us to move our study area from the north area of Kyoto prefecture to the northwestern area of Shiga prefecture which is just adjacent to our first study area and we also carried out the Network MT observation.
The broad-band MT measurements was made at threes sites in Kutsuki village, northwestern area of the Shiga prefecture. We also set the remote site at the Nosegawa village, north area of the Nara prefecture. It is about 150Km from the Kutsuki village. We analyze data using the remote referencemethod. Phase value of all of three sites increase from 45 degree to 60 degerr and apparent resistivity decrease gradually as frequency of geomagnetic variation decrease. One dimensional model analysis was also carried out. The most suitable models show that highly conductive layr exist below 1000(]SY.+-。[)300m in depth and the bottom of the layr may coincide with the top layr of the seismogenic layr. This result is meaningful because two independent observation show the consistent results that highly conductive layr cap the seismogenic layr.