G. Igarashi et al., TIME-VARIABLE RESPONSE CHARACTERISTICS OF GROUNDWATER RADON TO EARTHQUAKES, Geophysical research letters, 20(17), 1993, pp. 1807-1810
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.