Changes in Groundwater Level and Flow Rate Associated with the 2004 Off Kii-Peninsula Earthquakes

Observations of groundwater level and flow rate at 56 wells are continued by Geological Survey of Japan and Shizuoka Prefectural Government for monitoring seismic and volcanic activities. The 2004 Off Kii-Peninsula earthquakes occurred southeast off the Kii Peninsula, Japan on September 5, 2004. We detected 47 and 49 changes in groundwater level or flow rate in relation to the earthquakes of the foreshock (Mw7.1, 19:07JST) and main shock (Mw7.4, 23:57JST), respectively. The changes were classified into the three types; oscillation, increase and decrease. On the other hand, most of the observation wells were located in the area where coseismic contraction strain was expected by the fault models of the both earthquakes. Decreases in the groundwater level or flow rate that were qualitatively consistent with the coseismic strains were observed in the 21 and 23 wells at the foreshock and main shock, respectively. The amounts of the coseismic contraction strains of the both earthquakes were over 10-8 at the many wells. On the other hand, the amounts of step-like coseismic decreases of the groundwater level in the five wells were in the range from -7.9 to -28.9 mm at the foreshock and from -10.2 to -63.9 mm at the main shock. According to the strain sensitivities of the groundwater level by tidal analysis (5.2 - 21.8 mm/10-8), the values of the coseismic groundwater-level decreases in the five wells were consistent with those of the coseismic strain. Therefore the cause of the coseismic groundwater-level decreases in the five wells at least are inferred to be the coseismic strains.