抄録
Geological Survey of Japan made a groundwater observation network composed of 40 stations in and around the Kinki district and Tokai district for earthquake prediction research. In the Kinki district, which is a range of about 34-35.5N and 134.5-136.7E, continuous observation at those wells started in 1998. In the Tokai district, which is in a range of 34.5-35.5N and 137-139E, continuous observation started since 1978. The Western Tottori Earthquake 2000 (M7.3), occurred northwest of the network on Oct. 6, 2000. The epicentral distance to the nearest observation well is about 120km. There was no clear pre-seismic change in groundwater levels on our wells, although co-seismic and/or post-seismic changes were observed at several wells. The analysis of the observational results shows that these had co-seismic step-type changes and/or post-seismic slope-type changes. These co-seismic steps of groundwater level were explained by co-seismic strain changes, but post-seismic slope type changes were not so well. This difference is caused by frequency dependence of groundwater response for strain. Observed co-seismic strain changes were also not so well explained by co-seismic volumetric strain change estimated from the fault model. Our borehole type strain-meters are inferred to detect strain of very local region.
