1996 Volume 30 Issue 1 Pages 1-16
Some characteristics of groundwater behavior related to earthquakes are discussed with examples recently observed by continuous monitoring of well water level and radon concentration in groundwater. By calibrating strain sensitivity using tidal responses, well water level can be used as a sensitive strain meter. It should be noted, however, that local inhomogeneity near an aquifer system affects the behavior of well water level at shallow wells significantly. Not only crustal activities directly related to earthquakes, various aseismic crustal activities are also likely to cause well water level changes. An exmple is water level changes observed at three wells about one day before an M 5.9 earthquake that occurred deep beneath the Tokyo Bay on February 2, 1992, which may be attributed to a large slow slip that released strain energy even larger than the main shock of the M 5.9 earthquake. Groundwater radon concentration observed at a "suitable" site shows sensitive responses to earthquakes. More than ten of coseismic changes were observed at a radon monitoring station located in the eastern part of Fukushima Prefecture from 1984 to 1987. However, it has turned out that, over longer time scale, the response characteristics of groundwater radon to earthquakes is time variable. It is likely that the nature of groundwater responses to tectonic events is largely affected by tectonic events themselves such as occurrences of nearby large earthquakes. At a well located in the region where people suffered a miserable disaster from the southern Hyogo Prefecture earthquake on January 17, 1995, remarkable increases in groundwater radon concentration began to be observed several months before the earthquake. These precursory radon changes suggest that groundwater movement and pore presure changes in aquifer systems play important roles in the generation process of inland large earthquakes.
