The thermal measurements taken at hot springs in volcanic areas.
Most of the observations were taken within 50 km of the epicenters of the coming
earthquakes, although the greatest reported epicentral distance for an
anomaly was 470 km. In all cases an increase in ground temperature
was reported, with the largest change being 6 °C and most of the
changes being b1 °C. Five of the temperature changes in groundwater
were reported to have been coseismic, i.e., having occurred at the time
of the earthquake, while 5 were reported to take place within the
10 days prior to the earthquake. The rest of the observations did not
report the time at which the temperature change was reported.
All of these reported changes in temperature associated with
earthquakes were from Greece and Japan. Both are areas of active
plate subduction with active volcanoes and numerous geothermal
features. It is not known if there might be temperature changes in the
groundwater of non-geothermal areas prior to earthquakes, as there
although frictional heating on fault surfaces could contribute to
ground temperature changes. Because rocks have a relatively low
thermal conductivity, any such temperature-related changes that may
occur at depth in the earth would take a long time to reach the surface.
Therefore such a temperature anomaly is expected to be relatively
small.