Coseismic changes in groundwater temperature of the Usu volcanic region

Abstract

Observations of groundwater temperature to an accuracy of 0.001 →C or better have been made at about 30 wells in various areas of impending earthquakes in Japan. The main aim of the observations was to monitor the crustal strain and/or the movement of groundwater which relate to earthquake occurrence. One thermometer was installed at a depth of 200 m in an active volcanic area, Usu, where an upward movement of the magma is occurring. We report here that there was a generally increasing temperature trend at this location: 0.3 →C in the half-year of observation. Decreases in the rate of increase preceded intense local earthquake activity. Temperature jumps were accompanied by large local earthquakes with magnitudes up to 4.2, and the polarity of the steps corresponded to the location of hypocentres. This is probably the first microtemperature measurement made in the proximity of seismic activity.