Construction of Index of Quality Factor in Surface Ground for Analysis of Earthquake Ground Motions

日本国家地球科学与防灾研究所（NIED）在日本全国部署了数字强震仪（KiK-net），提供地面运动的垂直阵列记录。观察点总数接近700个。KiK-net观测到的数据可以用来识别地下地面的动态特性。然而，为了利用KiK-net数据识别地物性，需要进行传感器方向校正、波合成、绘制波、土壤数据准备等预处理。此外，还必须研究识别分析中使用的时间序列区间、阻尼模型和初始值的选择，因为它们会影响识别结果。提出了一种新的质量因子识别系统，在质量因子不敏感的频段内，将质量因子扫出可行范围。该系统免去了大部分麻烦的操作，使各种条件下的各种分析成为可能。利用该系统对土壤的动力特性进行了识别。本研究的主要结果如下：(1)质量因子随频率增加的变化趋势虽不大，但仍能识别出来。(2)在同一地点使用不同地震事件的质量因子估计值具有相似性。由于质量因子估计值的变化并不足够小，因此定量讨论其对频率的依赖关系还为时过早。为了建立频率相关的地下地面质量因子模型，需要积累分析结果，并发展本研究提出的方法。

National Research Institute for Earth Science and Disaster Prevention (NIED) deploys digital strong-motion seismographs (KiK-net) all over the country in Japan, which provide vertical array records of ground motions. The total number of observation sites is nearly 700. Data observed by KiK-net can be used to identify the dynamic properties of the subsurface ground.In order to identify the properties using KiK-net data, however, some pre-processes such as the correction of sensor direction, wave synthesis, drawing wave, preparation of soil data are needed. Furthermore, the selection of the interval of time series, damping model and initial values used in the analysis of identification must be investigated, because they influence on the results of identification.A new system is developed to identify the quality factor, in which quality factor is swept out of the feasible range in such the frequency band that the quality factor is not sensitive. The system is free from most of the troublesome operations, which makes the various analyses on various conditions possible.Using the system, the dynamic soil properties are identified. The major results in this study are as follows.(1) The trend of quality factor whose values increase according to frequencies can be recognized though the variations are not small enough.(2) The similarity of the estimated values of quality factor can be seen at the identical site using different events of earthquakes.Since the variations of estimated values of quality factor are not small enough, it is too early to discuss its dependency on frequencies quantitatively. Accumulation of analytical results of identification is necessary as well as the development of the method proposed in this study for the goal to model the frequency-dependent quality factor of subsurface ground.