TIME-VARIABLE RESPONSE CHARACTERISTICS OF GROUNDWATER RADON TO EARTHQUAKES

A clear coseismic anomaly of groundwater radon was observed for a magn itude 5.6 earthquake that occurred on May 11, 1992. The coseismic rado n anomaly was observed at a station which is located right on a major active fault in northeast Japan, and about 140 km away from the hypoce nter. This was the first time that an earthquake with M<6 had ever bee n accompanied by a clear radon anomaly at the station; although we had observed 12 similar coseismic radon anomalies at the station during t he observation period from 1984 to 1987, all of the earthquakes that w ere accompanied by radon anomalies in that period had been with magnit ude 6.0 and over. Surprisingly, the radon concentration has become mor e sensitive to show coseismic anomalies even for M<5 earthquakes since October 1992. This enhancement of sensitivity of the coseismic radon response may be attributed to the progress of micro-crack formation in the fracture zone of the active fault, which could be related to unus ual stress accumulation in the region.