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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.

Using initial P-wave records at 298 seismic stations from the Kiban-Kyoshin network (KiK-net), the P-wave seismograms method is employed to estimate the near-surface shear wave velocity in Japan. The applicability of this method is validated by comparisons between the measured and estimated time-averaged shear wave velocity to depth Z( V SZ, 5< Z < 300 m). Using a second-order polynomial regression relating log V SZ and log V S30, the estimated V S30 values agree well with the measured values. We also find that V S30 is directly related with the ratio of radial to vertical components of the initial P-wave velocity time series ( R P). Compared with the theoretical method, the empirical relationship between R P and V S30 has an improvement in the accuracy for V S30 estimation, is basically region-independent for Japan and Central and Eastern North America (CENA), does not need any other parameters, and is potentially useful for other regions of the world.

We apply seismic interferometry to earthquake records to investigate temporal change of near-surface shear wave velocity associated with rainfall in Northeast Honshu, Japan. We observe there exists a threshold (around 100 mm of 24 h rainfall total) of the temporal influence of rainfall on near-surface shear wave velocity. The velocity does not change significantly under low-intensity rainfall, while it is reduced by 1-3% with one exception of 10% when the rainfall intensity exceeds the threshold. Compared with the travel time of no rain, the travel time increment of lowintensity rainfall is no more than 3 × 10 −4 s, while that of high-intensity rainfall ranges between 10 −3 and 10 −2 s. Based on the results of observation, we propose a one-dimensional model to compute the travel time increment, which can give a preliminary estimation of the temporal influence of rainfall on near-surface shear wave velocity.

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