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 underdampe d well-aquifer system at epicentral distances of similar to 5 km during the 1995 earthquake swarm activity off the coast of Ito City, Shizuoka Prefect ure, central Japan. Each water level change consists of a static offset wit h a linear trend component followed by a damped harmonic oscillation. The c haracteristic 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 d uring the activity. The offset values are mostly 2 mm or less, but there we re two exceptionally large offsets of 4 and 6 mm. The observed waveforms ar e consistent with an analytical expression for the theoretical well level r esponse to step-like pressure head change in an underdamped well-aquifer sy stem. This consistency allowed us to estimate the parameters of the well-aq uifer system and the offset component of pressure head changes from the obs erved hydrographs. The seismic records at the nearest station indicate that among the 14 events only seven, including the two exceptionally large even ts, were apparently synchronous with swarm earthquakes. Among these seven e vents the largest two are quantitatively explained as the consequence of th e coseismic volumetric strain changes. The other five are interpreted as ei ther the effect of coseismic strain change near a node of the volumetric st rain field or some other coseismic effects. The seven well events not synch ronous with earthquakes indicate the occurrence of abrupt local pressure di sturbances in the aquifer within a distance of a few tens of meters from th e well.