Disaster mitigation

[DM-10] Seasonal variation on the results of surface wave analysis by multi-channel active source at the cold region in Japan

*Nobuo Takai¹, Akari Tsukamoto¹, Naofumi Nakagawa¹, Michiko Shigefuji², Shingo Nomoto³, Jun Horita³, Tetsunori Kishikawa³, Shunzo Kawajiri⁴, Seiji Tsuno⁵, Hiroaki Yamanaka⁶ (1. Hokkaido University (Japan), 2. Kyushu University (Japan), 3. Geo-Technical Research Co., Ltd. (Japan), 4. Kitami Institute of Technology (Japan), 5. Railway Technical Research Institute (Japan), 6. Tokyo Institute of Technology (Japan))

In the cold region, the exploration seasons change the results of underground structures because the physical properties of the shallow underground are affected by seasonal temperature changes. These instabilities make it complex for the evaluation of the site amplification characteristics in the shallow surface. Therefore it is important to know how the soft seismic velocities of surface soil change in the cold winter season. There are few studies to know the seasonal velocities change on the field during continuous exploration. We selected two sites in Kitami cities, the eastern area in Hokkaido, Japan. In these sites, it is small snowfall and low temperatures in winter. We carried out the multi-channel surface wave method with the active source to explore the shallow underground structures. We can see that the recorded waves are changed by several seasons at all sites. We analyzed F-K spectra from these waveforms to get phase velocities of the Rayleigh waves and grasped those variations between the warm and cold seasons. The phase velocities of the cold season are higher than those of the other seasons. We compare the varieties of these results and underground temperature, etc. These basic data will upgrade the resolution of the underground exploration method for the cold region and reduce the instabilities of the evaluation of the site amplification characteristics.