Seismic velocity changes caused by the Earth tide: Ambient noise correlation analyses of small-array data

We examine seismic velocity changes due to the Earth tide by conducting cross-correlation function (CCF) analyses of ambient seismic noise recorded at a small array composed of seven seismometers in northeastern Japan. We calculate CCFs for the dilatational and contractional episodes that are predicted from theoretical tidal volumetric strains. CCFs of the two episodes are highly correlated, but tiny differences are found in their phases. The phase differences are explained by seismic velocity changes of -0.19± 0.06% at 1-2 Hz, which are interpreted to be caused by opening/closure of cracks or pores in the shallow subsurface due to the tidal strain. Strain sensitivities of the seismic velocity changes are estimated to be 6.9× 10₄ strain_-1, which are almost consistent with those reported in previous studies using artificial sources.