Comparison of tidal strain changes observed at the borehole array observation system with in situ rock properties in the Tono region, central Japan

We have developed a digital multi-component borehole instrument for boreholes deeper than 1 km and with diameters smaller than 98 mm, and installed it at the bottom of seven deep boreholes in the Tono region, central Japan. So far, our borehole instruments have observed not only tidal signals, but also strain-steps associated with large earthquakes, and environmentally induced signals. In this study, we analyze the strain data from four observation sites using the tidal analysis program BAYTAP-G and compare the tidal amplitudes of strain with in situ rock properties. We observed an obvious difference in amplitude and phase of the M2 and O1 tidal strains from four observation sites that are located in the same Toki granite bedrock within a distance of 10 km. There are strong negative correlations between the semi-diurnal E–W, areal tidal strain and in situ rock hardness (shear modulus and Young's modulus), while N–S tidal strain is unrelated. In contrast, strain-steps associated with large earthquakes increase with hardness, as observed at two sites. We consider that the inconsistency of the behavior of the tidal amplitudes and strain-steps may be caused by the heterogeneity of rocks near the borehole strainmeters.