やや長周期の微動観測と地震工学への適用 (3)

Abstract

Continued to the previous papers observations of the long-period microtremors were carried out in Aomori and Miyako cities for a farther comparative research of their characteristics to the SMAC type accelerograph records in Tokachi-oki earthquake of 1968. The purpose we extended the observations from Hachinohe to these areas is following. The very large acceleration amplitudes in 1- to 5-sec in period were recorded more clearly in Aomori than in Hachinohe, but no significant indication was seen on the accelerograph record in Miyako. Still the differences were seemed too extreme to be explained by the epicentral distances. In Hachinohe we successfully gave an interpretation that the predominancy of the earthquake motions in the long period was considerably due to a large amplification effect by the deep soils. But we did not know if this was true in general.The observation and the analysis of the microtremors were performed in 1975 with a similar manner to those reported in the previous papers.The SMAC site in Aomori is situated on the ground of which depth to the bed rock is around 600m, and the predominant period of microtremors, obtained by a series of careful observations from bed rock to the SMAC site, was 3.0sec. This is nothing but a predominant period in the strong motions of the Tokachi-oki earthquake.On the contrary to this, the SMAC accelerograph in Miyako has been installed above the hard rock with a very thin superficial layer, and no predominant period in the long-period range was observed in the microtremors. This must be the same reason why there were no long-period amplitudes in the acceleration records in Miyako.Combining these results with that in Hachinohe we can conclude that the correspondency of the predominancy and the predominant period between the microtremors and the strong motion records is very remarkable and thus the observation of the long-period microtremors is a good alternative to estimate the long-period input motions to large-scale structures by future earthquakes.Finally a short recommendation on the observation schema of long-period microtremors was added. The problem left untouched is the wave-theoretical considerations of the microtremors, which will be scheduled before long.