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A clear coseismic anomaly of groundwater radon was observed for a magnitude 5.6 earthquake that occurred on May 11, 1992. The coseismic radon 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 hypocenter. This was the first time that an earthquake with M<6 had ever been accompanied by a clear radon anomaly at the station; although we had observed 12 similar coseismic radon anomalies at the station during the observation period from 1984 to 1987, all of the earthquakes that were accompanied by radon anomalies in that period had been with magnitude 6.0 and over. Surprisingly, the radon concentration has become more 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 unusual stress accumulation in the region.
Igarashi et al. (Fri,) studied this question.