Effects of Local Inhomogeneities on Tidal Strain Measurements

Abstract

Using continuous records obtained from four extensometers with laser interferometer systems at Amagase, Kyoto Prefecture, we analyzed tidal strains by the least squares method. The result shows that there exist remarkable discrepancies between observed values and theoretically predicted ones for a laterally homogeneous earth model. Amplitude enhancements of tidal strains observed with the horizontal and vertical components in normal directions to the axis of the tunnel are well explained by the cavity effects calculated by two-dimensional finite element techniques for the actual cross-section of the tunnel. After eliminating the cavity effects, observed values were compared with theoretically predicted ones containing the topographic corrections calculated by three-dimensional finite element techniques. As a result, differences of phases between observed and theoretically predicted values are within 5° in any case. Amplitudes of observed tidal strains in the direction along the tunnel are about 15% smaller than predicted values but these differences are comparable to errors inherent in calculations. On the contrary, amplitudes of observed values in the direction across the tunnel are 33～52% larger than the predicted ones. As the most possible source of these differences, we consider the hydrological perturbation caused by fluctuations of the ground-water level around the observation tunnel.