Groundwater level and flow rate at 44 wells are continuously observed by the Geological Survey of Japan and the Shizuoka and Gifu Prefectural Governments for monitoring seismic and volcanic activities. The 2003 Tokachi-oki earthquake (M8.0) occurred off the south coast of Hokkaido Island, Japan on September 26, 2003. The epicentral distance to the nearest observation well is about 250 km and that to the farthest is about 1200 km. At the 22 wells, we detected changes in groundwater level or flow rate in relation to the earthquake. Most of the changes are coseismic step-like changes and/or short-period oscillations. In the nearest two observation wells, long-period oscillations with the periods of 39 and 53 minutes were also observed for several days after the earthquake, which is likely due to tsunami. In comparison between distributions of changes in groundwater level and theoretical coseismic strain by the fault model, it is clear that step-like increases were found in the contraction area of the coseismic strain. The relationship between amounts of the observed step-like groundwater-level changes and theoretical ones, calculated by the fault model using strain sensitivities of groundwater level indicates that the groundwater levels in the several wells responded to the coseismic strain.

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 motior 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.

In order to understand the alteration process in subsurface granitic rocks, drilled core (100 meters deep) taken from Mesozoic granitic rock distributed in the Chugoku area of Japan has been investigated. The rock is characterized by the coarse size of rock-forming minerals such as quartz, plagioclase, orthoclase and biotite. The borehole logging and core observations show that there are several fractured zones, and almost all these structural features are associated with alteration zones that have been formed by water-rock interaction. The studied samples were collected from the altered zone at the location of 40 to 50 meters below the ground surface. Detailed analysis of the pore geometry, mineralogical observation and geochemical analysis were carried out to reveal the changes of textual and geochemical characteristics due to water-rock interaction. Alteration feature of rock-forming minerals such as plagioclase and biotite shows that the granitic body has two stages of thermal and water-rock interactions. One is the high-temperature hydrothermal alteration after the magma solidification, and the other is relatively low-temperature alteration probably due to groundwater circulation after the being exposed at the surface. This kind of methodology used for rock forming minerals of subsurface granitic rock can be applied to understand the alteration history due to the water-rock interactions during the formation of granitic body.

No abstract