Water Level Changes in Oi Well in Central Japan Associated with Large Earthquakes

The Hot Springs Research Institute of Kanagawa Prefecture (HSRI), Japan, monitors groundwater level in 6 wells of 150 to 500 m depth in and around Hakone volcano to investigate hydro-geological precursors to earthquakes. These observations have detected groundwater level changes associated with large earthquakes. The present study focuses on the observations by Oi well with 300 m depth, one of the HSRI wells. We used the1 Hz sampling digital data recorded in the period from 2010 to 2015. In this period distinctive water level changes have been observed at 9 earthquakes, whose magnitudes are ranging from 5.4 to 9.0, including the 2011 great Tohoku Earthquake. The focal depths are from 18 to 670 km. The epicenter distances form Oi well range from 20 to 850 km. The groundwater level in Oi well decreased 25cm following the Tohoku Earthquake. The groundwater level rose following the other earthquakes. The increases range from 5.5 to 10 cm. The water level changes lasted more than 1 hour. We tried to approximate the variation of water level increase during the 8 earthquakes by simple equations. By applying several equations including error function and decay function, we found the decay function provides the best curve for the approximation. The good approximation by the decay function may give us important insights for the modeling of the mechanism of water level change triggered by earthquakes. The decrease of groundwater level at the Tohoku Earthquake was probably induced by the large crustal deformation.