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tectonic stress or by permeability change due to micro-crack formation in rocks prior to the destructive earthquake.

The result indicates that groundwater observations can provide useful information on the earthquake formation process and clues to understanding the mechanism of inland earthquakes. The use of commercial bottled water will be a source of information to obtain preseismic data. Detailed descriptions of the study will be published elsewhere. Further study including stable isotope measurements will be helpful in strengthening the findings.

5. Conclusions

Detection of precursory phenomena is of primary importance for earthquake prediction purposes. Coseismic signals, similarly, are useful for clarifying the mechanisms of precursory phenomena. Appearance and nonappearance of coseismic drops in radon content are indicative of stress states in the region.

Our experience enables us to arrive at several conclusions. (i) It is clear that movements of fluids in the crust are associated with earthquakes, (ii) Earthquake-related changes are not observed at all observation wells but only at a limited number of wells. At sensitive sites, marked changes in groundwater movement that are significantly large and effective even at large distances are observed. (iii) Even at sensitive wells, the appearance of signals likely depends on the state of stress accumulation in the region. The sensitivity change is thought to be caused by changes in physical properties of rocks including opening and closure of microcracks.

A possible precursory change in groundwater chemistry at the time of the 1995 Kobe earthquake will substantiate the importance of geochemical and hydrological study.

Fluid movements in the crust play a vital role in earthquake occurrence. Besides its original purpose of predicting earthquakes, monitoring of groundwater movements will also be essential to supplement all kinds of observations on the ground surface and particularly to increase the accuracy of crustal deformation measurement including GPS measurement.

The author thanks his colleagues, M.Ohno, T.Mori, S. Xu, and U. Tsunogai, for help in drawing illustrations and chemical analysis.

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