抄録
This is a continuation of the research on the observation of long-period microtremors for elucidating ground characteristics during an earthquake. Results of observation along a line of about 17km from Hachinohe to Misawa cities in Aomori Prefecture, Japan, are outlined in this paper. Essential efforts were made on the affirmation and extension of the conclusions in the previous paper—a systematic change of predominant periods of microtremors along a 5km-line from 0.7sec at the point close to the outcrop of the bed rock to 2.5sec at the strong motion accelerograph site, and the relation of the predominant periods to the deeper underground conditions. Instruments, observations, and data processings are almost the same to those in the first paper.
The results obtained are as follows. First, the reobservation of the microtremors along the same line revealed a similar and very systematic change of the predominant periods, though the spectrum of the input motions to the ground layers seemed significantly diferent. Second, the observation extended as far as Misawa city brought the longest predominant period of 3.5sec at the end point, and produced the conclusion that the changes of the period of microtremors were very responsible to the undulation of the bed rock formation. The layer thickness corresponding to 3.5sec in period was estimated as about 500m.
This investigation strongly suggests the importance of knowing the ground dynamic characteristics down to 500-1000m in depth for the aseismic construction of large-scale structures and thus the significance of observation of the long-period microtremors.
