Electrical surveys in Kitami city, Hokkaido, for evaluating seasonal variation of site amplification in cold region

seasonal variation

The Society of Exploration Geophysicists of Japan

BUTSURI-TANSA(Geophysical Exploration)

We studied an applicability of the electrical survey to model shallow subsurface properties used in evaluating site amplification characteristics in the freezing season. The electrical surveys were carried out repeatedly for 1 year in Kitami city, Hokkaido, to evaluate the effects of freezing soil on the electrical property of soil. Apparent resistivity values affected by the shallow subsurface increased as the freezing depth became deeper, and apparent resistivity curves consequently show the typical shapes at individual stages in the freezing process. The resistivity and the thickness of the surface layer also increased as the freezing depth became deeper. A good agreement of the seasonal variations in the electrical properties with the observed soil conditions, such as soil temperature and water moisture content, shows that the resistivity can be useful information to evaluate the influence of the freezing in constructing a soil model for the ground motion calculation. Furthermore, the depth to the lower-resistivity layer which is deeper than the freezing depth varied similarly with groundwater level. It indicates that the electrical survey is also useful to model a P wave velocity and a density of near-surface for an accurate estimation of an earthquake ground motion.