Abstract
In this study, to understand the seasonal variation of the result of surface wave and microtremor array explorations in areas where freezing occurs in the surface soil during the winter, we conducted both explorations throughout the year at the same site at Kitami Institute of Technology, located in Kitami City, Hokkaido, a cold region in northern Japan. The explorations were conducted at intervals of about 1 to 2 months from October 2020, when the ground is not frozen, through the severe winter season to October 2021 of the following year. First, the S-wave velocity structure was estimated using both phase velocities in October, the non-freezing period. The estimated structure shows a soft layer of less than 100 m/s from the ground surface to 1 m. The phase velocity of Rayleigh waves during the freezing period was higher than that during the non-freezing period in the frequency range of 10 to 50 Hz. The difference was especially clear above 15 Hz. The results of the small-radius microtremor array explorations conducted at the same time also showed that the phase velocity changed at over 10 Hz. On the other hand, there is almost no seasonal change below 10 Hz, and their standard error increases during the freezing period. The phase velocities from the microtremor array explorations can be connected with the results of the surface wave ones. Seasonal variations in phase velocities for each frequency corresponded with other observations.
