Geological Survey of Japan made a groundwater observation network composed of 21 stations mainly along active faults in and around the Kinki district, which is a range of about 34-35.5°N and 134.5-136.7°E, for earthquake prediction research. These wells were newly made by Geological Survey of Japan after the 1995 Hyogo-ken Nanbu Earthquake (the Kobe Earthquake). Continuous observation at all the wells started in 1998. The 2001 Geiyo Earthquake (M6.7), occurred west off the network on Mar. 24, 2001. The depth of the earthquake is 51 km. The epicentral distance to the nearest observation well is about 170 km and that to the farthest about 380 km. There was no clear preseismic changes in groundwater levels and crustral strains observed at our wells, although coseismic and/or postseismic changes were observed at many of the wells. The analysis of the observational results show that these coseismic and/or postseismic changes were well explained by coseismic volumetric strain changes estimated from the fault model. Usually coseismic and/or postseismic groundwater level changes are caused by two factors. One is coseismic strain changes and the other is strong ground motion, which generates cracks in the aquifer and sometimes causes liquefaction. As the earthquake was relatively deep and far enough from the network, ground motion within the network should have been small. Therefore strain-induced coseismic and/or postseismic groundwater level changes are inferred to be larger than those induced by the other factor.
