Underdamped responses of a well to nearby swarm earthquakes off the coast of Ito City, central Japan, 1995

We observed coseismic and noncoseismic water level changes of an underdamped well-aquifer system at epicentral distances of similar to 5 km during the 1995 earthquake swarm activity off the coast of Ito City, Shizuoka Prefecture, central Japan. Each water level change consists of a static offset with a linear trend component followed by a damped harmonic oscillation. The characteristic frequency and decay parameter of these oscillations are 18.3 s and 0.024 s(-1), respectively. We detected 14 such events within 8 days during the activity. The offset values are mostly 2 mm or less, but there were two exceptionally large offsets of 4 and 6 mm. The observed waveforms are consistent with an analytical expression for the theoretical well level response to step-like pressure head change in an underdamped well-aquifer system. This consistency allowed us to estimate the parameters of the well-aquifer system and the offset component of pressure head changes from the observed hydrographs. The seismic records at the nearest station indicate that among the 14 events only seven, including the two exceptionally large events, were apparently synchronous with swarm earthquakes. Among these seven events the largest two are quantitatively explained as the consequence of the coseismic volumetric strain changes. The other five are interpreted as either the effect of coseismic strain change near a node of the volumetric strain field or some other coseismic effects. The seven well events not synchronous with earthquakes indicate the occurrence of abrupt local pressure disturbances in the aquifer within a distance of a few tens of meters from the well.