Preseismic changes in groundwater level and volumetric strain associated with earthquake swarms off the east coast of the Izu Peninsula, Japan

We have observed the groundwater level at Omuroyama-kita (OMR) well on the Izu Peninsula, Japan since October 1994. During the observation period ther e were four earthquake swarms. We detected one or two preseismic groundwate r level changes together with preseismic crustal deformation recorded at th e other nearby stations. Therefore those preseismic groundwater level chang es are considered to be caused by crustal deformation or volumetric strain changes. The crustal: deformations are inferred to be caused by the magmati c like intrusion, which is a possible source of the earthquake swarms off t he east coast of the Izu Peninsula.