Tidal Response of Seismic Wave Velocity at Shallow Crust in Japan

Microcracks in geomaterials cause variations in the elastic moduli under applied strain, thereby creating seismic wave velocity variations. These are crucial for understanding the dynamic processes of the crust, such as fault-zone damage, healing, and volcanic activities. Solid earth tides have been used to detect seismic velocity changes responding to crustal-scale deformations. However, no prior research has explored the characteristics of the seismic velocity variations caused by large-scale tidal deformation. To systematically evaluate the tidal response to velocity variations, we developed a new method that utilized the flexibility of a state-space model. The tidal response was derived from hourly stacked noise autocorrelations using a seismic interferometry method throughout Japan. In particular, large tide-induced seismic velocity changes were observed in the low S-wave velocity region of the shallow crust. Overall, the tidal responses of velocity variations can provide new insights into the response mechanisms of the shallow crust to applied strain.