CHARACTERISTICS OF BOREHOLE VOLUME STRAINMETER AND ITS APPLICATION TO SEISMOLOGY

Abstract

The response of a borehole volume strainmeter to various kinds of disturbances is investigated. The effects of atmospheric pressure, rainfall, changes in the water table, seismic waves and earth tides are analyzed. The secular strain changes and the strain steps including co-seismic steps which may contribute to earthquake prediction and earthquake source studies are also discussed. Rock properties around the strainmeter's sites are taken into consideration. Two groups of the sites are discernible, depending on the sensitivities to atmospheric pressure changes and seismic waves. The difference in the site characteristics may be partly due to the various mechanical conditions of the ground and partly due to its coupling with the strainmeter. The absolute sensitivity of the strainmeter is determined for each site independently by observing both the Rayleigh waves from a distant earthquake and the P waves from a relatively nearby earthquake. In determining the sensitivity, seismograms are utilized to estimate the in situ strain. This sensitivity can be applied to a range of periods comprising those of seismic waves and earth tides. Comparison of the secular strain change observed by the Omaezaki strainmeter with those obtained from geodetic measurements shows that the sensitivity to secular strain change can be different from those of shorter-period disturbances and that the difference in the nature of the data should be kept in mind. The sensitivity for the co-seismic step is also different from the other ones. However, its magnitude recorded at selected sites can be used for estimating the source moment.