やや長周期の微動観測と地震工学への適用 (8)

Abstract

A series of experimental studies of long-period microtremors has been continued with the intention of elucidating dynamic characteristics of deep ground soil during an earthquake and the stepwise results have been reported in the previous papers. Through those observations, we pointed out that the temporal variation of microtremors amplitude obviously correlates with that of sea waves and therefore further understanding of microtremors will be advanced by an investigation of sea wave characteristics.In this paper, using the date of simultaneous observation with sea waves, their relationship is discussed, and the following results are obtained. The temporal variations of both amplitudes of microtremors and sea waves are in clear correspondence. From the comparison of these spectra, it is concluded that the predominant period of microtremors is about a half of that of the sea waves. This fact enables us to insist that the Longuet-Higgins' theory is also applicable to the generation mechanism of the microtremors under consideration. Empirical equations connecting both of the spectral amplitudes of microtremors and of sea waves are newly derived.Generalizing these results, a revision of the standardized observation scheme proposed in the previous paper was made for the long-period microtremors.