Two-Layer Modeling of Ground Based on Microtremor Observations and Its Application to Areas Devastated by Recent Major Earthquakes in Japan

Abstract eng:
We developed an empirical method of modeling surficial soil deposit based on microtremor measurements observed on a ground surface. The method, replacing a layered soil with an equivalent two layer soil model based on horizontal-to-vertical spectral ratio of microtremors, enables evaluation of site amplification factors near/below the predominant frequency of the site. The method is then applied to areas devastated by two recent major earthquakes which occurred consecutively in the Hokuriku area, Japan in 2007. The first one occurred in March at northern Noto Peninsula. Severe damage to houses was concentrated at the Tohge area, Monzen. The second one occurred in Kashiwazaki, Niigata. Severe damage to wooden houses occurred in downtown areas developed on a sand dune. Microtremor measurements were made at approximately 20 locations for each area so as to cover the most severely damaged areas. Obvious relationships between damage to houses and estimated soil characteristics are recognized in Monzen. Contrary, we could not find a clear relationship between damage to houses and site amplification in Kashiwazaki.

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