新潟県南部の浅発地震によるやや長周期地震動の解析

抄録

The southern part of Niigata prefecture is located on thick Tertiary sedimentary rock layers. On Dec. 7, 1990, a shallow earthquake (M=5.4, H=15km) occurred in this area and strong motion records were obtained at stations of a seismic array about 24km north of the epicenter. In this study, characteristics of the recorded motions in period range of 1 to 10 seconds are investigated using multiple band-pass filtering technique. The results are as follows:
(1) The displacement seismograms at ground level and at depth of about 300m are very similar. Correlation coefficients of band-pass filtered seismograms show that the ground motions with periods which are longer than natural period of overlaying layers are very similar.
(2) The long periods ground motion with periods about 3 to 8 seconds consists of surface waves. Both Rayleigh wave and Love wave are recognized and they are propagating mainly from the epicenter. So, the correlation coefficient of radial and transverse components show different frequency distribution.
(3) Because of short epicentral distance, the surface waves arrived soon after body waves' arrival. So, the principal parts of seismograms have longer duration than as usual and the principal parts of radial and transverse components have different periods.