Abstract
At the time when we evaluate dynamic properties of soil deposits, it is important to clarify whether a predominant period of microseisms represents the natural characteristics of soil deposits or the source effect of vibration. Therefore, microseismic observation around the Nohbi plain was carried out for about one week since October 19 in 1981. Observed sites are roughly classified into two groups. Namely, one is mobile observation from a site on outcrop of bed rock to a site on very thick soil deposits on the bed rock and the other is fixed observation on outcrop of bed rock. The purpose of this paper is to discuss the characteristics of predominant period around the Nohbi plain. The results by these observation are summarized as follows. (1) Spectrum on the outcrop of the bed rock has one predominant period. Its value (about 5 seconds) seems to related to the source effect of vibration due to sea waves. (2) Spectrum on soil deposits up to 100-500 meters in depths has two predominant periods. One of both represents the source effect of vibration and the other corresponds to the layer thickness of soil deposits on the bed rock. (3) Spectrum on soil deposits of about 2 kilometers in depths has one predominant period in the same tendency as the spectrum on the outcrop of the bed rock. But it is assumed that the spectrum depending on the soil deposits and the spectrum depending on the source effect of vibration appears one on top of another in the predominant period (5 seconds). (4) Predominant period of horizontal component and vertical component on the outcrop of the bed rock has a very similar value respectively.
