资源环境科技发展态势分析平台

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.

Plain Language Summary Over a half of the total land surface in the Northern Hemisphere freezes and thaws seasonally. For these areas, when the temperature drops below the freezing point in winter, the surface soil will become much harder as the water in the soil freezes, and when the temperature rises above the freezing point in spring, the surface soil will gradually soften as the ice in the soil melts. This seasonal change of surface soil has a tremendous impact on the infrastructures and ecosystem. We exploit the fact that the speed of seismic waves in the shallow subsurface is sensitive to minor environmental change. We measure near-surface seismic velocities using earthquake records from eight seismic stations in eastern Hokkaido, Japan. We detect an increase of seismic velocity in a depth range of a few hundred meters during January to March due to seasonal frost activity. We find that the seismic velocity change is closely related to the local atmospheric temperature in winter and spring months. We propose a temperature-dependent model to estimate the influence of seasonal frozen soil on near-surface seismic velocity. These results open the possibility of frost monitoring by using earthquake records.