Evaluation of ground-motion characteristics for seismic hazard prediction

在横滨的镰仓和北坂地区进行了强震和微震观测，以评价该地区的地面运动特征。在镰仓，四个加速度计安装在软岩和全新世低地上。虽然观测到的记录受到建筑物振动的污染，但在每个站点都包含站点放大特性。结果表明，利用微震的H/V谱比可以检测到微震的位点放大特性。通过引入水平运动和垂直运动的相干性，对H/V谱方法进行了改进。将该方法应用于镰仓地区的场址放大评价。结果与1923年关东地震时镰仓地区的震害分布一致。北上坂地区在壤土高原和全新世低地分别安装了3台加速度计。强运动观测表明，高原站点比低地站点具有更大的站点放大特性，其峰值水平地面速度约为1.5倍，傅里叶振幅谱的频谱峰值幅度约为2倍。放大是由于软岩上积聚的地下土壤。利用微震的H/V谱比可以检测强震部位的位点放大特性。在该地区进行的现场微震观测表明，高原的共振周期与海拔有关；高站点放大区分布在低地和下须吉高原的河流沿线。强震动与微震动的频谱特征比较表明，微震动在地震灾害预报中评价地震动特征的适用性。

Strong-motion and microtremor observations are carried out in Kamakura and Kitahassaku area, Yokohama, in order to evaluate ground-motion characteristics in the areas. In Kamakura, four accelerographs are installed on soft rocks and Holocene lowland. The observed records contain siteamplification characteristics at each site although the records are contaminated by building vibrations. It is shown that the site-amplification characteristics can be detected by H/V spectral ratios of microtremors. The H/V spectral method is modified by introducing coherence between horizontal and vertical motions. The new method is applied to site-amplification evaluation in Kamakura. The results are consistent with the damage distribution in Kamakura area during the 1923 Kanto earthquake. In Kitahassaku area, three accelerographs are installed on loam plateau and Holocene lowland. Strong-motion observation shows that a plateau site has larger site-amplification characteristics in comparison to lowland sites by a factor of about 1.5 in peak horizontal ground velocity and about 2 in spectral peak amplitudes of Fourier amplitude-spectra. The amplification is due to subsurface soils which are accumulated on soft rocks. The site-amplification characteristics at the strong-motion sites can be detected by H/V spectral ratios of microtremors. Spot microtremor observations conducted in the area demonstrate that resonance periods on plateaus are related to elevation ; high site amplification areas distribute along rivers in the lowlands and the Shimosueyoshi plateau. The comparisons of spectral characteristics of strong motions and microtremors show the applicability of microtremors to evaluation of ground-motion characteristics for seismic hazard prediction.