Possible mechanism of hydrological responses to the historical Nankai earthquakes at old hot springs

The large earthquakes along the Nankai trough, which are referred to as the Nankai, Tonankai and Tokai earthquakes, occurred nine times since 684. The Dogo and Yunomine hot springs, which have histories of more than 1000 years, are located in and around the source region of the Nankai and Tonankai earthquakes. Several ancient writings reported that the Dogo and Yunomine hot springs stopped or decreased discharging after four and five of the nine large earthquakes, respectively. At the Dogo hot spring, an 11.2 m decrease of well water level was observed after the 1946 Nankai earthquake (M 8.0). Moreover 1.9 m increase of well water level at the same well was observed 49 hours after the 2001 Geiyo earthquake (M6.7). We estimate volumetric strain changes at the Dogo hot spring associated with the 1946 Nankai and 2001 Geiyo earthquakes using the Okada's program and estimated fault models. As a result, changes of 5.3 - 6.0 x 10-6 extensional strain and 4 x 10^{-8} contractional strain are estimated at the Dogo hot spring associated with the 1946 Nankai and 2001 Geiyo earthquakes, respectively. We found that the observed coseismic changes in groundwater level are proportional to the estimated coseismic strain steps associated with these earthquakes at the Dogo hot spring. In order to estimate strain sensitivity of the groundwater level, we started groundwater level observation at the same well at the Dogo hot spring from June 2003. We estimated that the strain sensitivity of groundwater level is 1.72 mm/nstrain using tidal analysis. This strain sensitivity estimated by the tidal analysis is consistent with the relationship between the observed coseismic changes in groundwater level and the estimated coseismic strain steps associated with the 1946 Nankai and 2001 Geiyo earthquakes at the Dogo hot spring. We think that the reported stops or decreases of self-discharge of the hot water at the Dogo hot spring after the historical Nankai earthquakes were caused by extensional strain changes associated with the earthquakes.