Abstract
Characteristics of secular variation, response to precipitation, and change of atmospheric pressure coefficient for strain observed by borehole volume strainmeter at Omaezaki are investigated. Generally, strain contracts by precipitation and the more rain falls, the more it contracts, but there seems to be a ceiling to this effect. Atmospheric pressure coefficients show seasonal variation which is considered to be related to change of groundwater level. Sudden contraction of strain that has been observed several times are not a net increase of contraction but seems to exhibit a recovery to a stationary state. By carrying out similar investigations for strain data of other observation sites and comparing them, we will be able to discriminate phenomena due to regional stress change from various noises of local origin such as caused by change of the coupling state of the very site and change of ground water level, etc., which is the most important for catching significant signals related to the focal processes before earthquake occurrence.
