LAUSR

We measured shear wave velocities in the shallow subsurface by applying seismic interferometry to earthquake records from eight vertical borehole arrays in eastern Hokkaido, Japan. We detected an increase of several percent in the seismic velocity during January to March due to seasonal frost dynamics. The velocity changes associated with seasonal frozen soil are affected by the frost depth and the extent of freezing, while the frost depth and the extent of freezing are mainly controlled by the cumulative temperature and the current temperature, respectively. Thus, a weighted cumulative freezing degree day is proposed to consider these two factors and used for stage division of the annual freeze‐thaw cycle. Based on the results of observation, we present an empirical model to relate the velocity changes with the weighted cumulative freezing degree days, which allows us to estimate the influence of seasonal frozen soil on near‐surface seismic velocity.