Long‐term observation of in situ seismic velocity and attenuation

Abstract

We conducted 1‐year continuous measurements of in situ compressional wave velocity and attenuation across a distance of 12 m in a vault at the coast of Miura Bay, Japan, using a piezoelectric transducer as the ultrasonic wave source. We detected, for the first time, periodic variations in both velocity and attenuation, with amplitudes of 0.3%, and of 4%, respectively, corresponding to the diurnal and semidiurnal lunar and solar tides. These variations correlate well with the in situ areal strain change due to tidal ocean loading estimated from the strain records about 100 m away from the observational site. Relative minimum velocity and relative maximum attenuation occur at every time of relative maximum areal strain (expansion), suggesting that opening/closure of pores or cracks of in situ rocks is responsible for these periodic variations. The velocity variation shows a remarkable 14‐day periodicity corresponding to the spring and neap tides, arising from a nonlinear response of the velocity change to tidal strain change such that the velocity change depends on how slowly the tidal dilatation departs from its peak value.