地震に伴う地下水・地球化学現象

抄録

Hydro-geochemical anomalies associated with large earthquakes are reviewed and discussed from the viewpoints of (1) the response of confined groundwater to minute deformations of crustal rocks, (2) ground-water anomalies related to earthquake source mechanisms, (3) origins of groundwaters and dissolved gases in them, and (4) factors to cause groundwater gas anomalies.
Theoretical models for deformations of water-saturated porous or cracked rocks under the undrained condition indicate that compressive and extensive stress changes induce increasing and decreasing changes in the pore water pressure, respectively. The pore pressure change is to be the same order of the external stress change, but the maximum change of pore pressure cannot exceed the stress change. These are consistent with observed responses of confined groundwaters to the earth tides.
Coseismic groundwater anomalies accompanied by the 1946 Nankaido earthquake and the 1923 Great Kwanto earthquake are directly related to their residual fields of megathrusting faults. Preseismic groundwater anomalies of these great earthquakes commonly suggest that megathrusting fault motions began to start preceding main seismic events.
Chemical compositions of groundwater gases are controlled by admixing of the dissolved atmospheric air with the subsurface gases enriched in CH₄, He and others. Groundwater gas anomalies can be described by the change in mixing ratio of the two different gases and/or by the compositional change of the admixing subsurface gas itself. Such an admixing process could be affected by anomalous pore pressure distributions induced by the deformation of crustal rocks related to earthquakes. H₂ gas anomalies are distinctive among subsurface gas anomalies reported so far, but the origins are not fully understood. The characteristics of H₂ anomalies are discussed.