Estimation of three dimensional ground structure in Fukui Plain based on inversion analysis of observed ground motion

1948年福井地震期间，受灾严重的地区大多集中在覆盖着厚厚的第四纪沉积物的福井平原。这表明沉积构造可能是阐明损伤分布的最重要因素之一。首先，在假设垂直传播的SH波在水平分层系统中多次反射的情况下，提出了一种基于地震观测记录的地下结构反演方法。这一过程被描述为一个优化问题，以求求出每一层的厚度和Q值，以最小化计算和测量的傅里叶谱之间的差异。将这种反演方法应用于部署在福井平原中心的台站的实际强震记录，计算了平原下至第三系的二维S波速度结构。经证实，识别出的地下结构在监测和计算响应之间具有较好的一致性，所估计的结构与PS测井资料和P波反射测量结果相一致。其次，在微震观测的基础上，对福井平原周围的三维第四系构造进行了探索。覆盖福井痛的每一个小网格都进行了地面微震观测。从收集的傅立叶谱和H/V谱中确定了每个位置的优势周期和H/V比。在假定S波速结构的基础上，根据优势周期估算了场地第四系厚度。将该程序应用于地表条件资料较少的场地，可以较好地预测强震反应。

During the 1948 Fukui earthquake, most of the heavily damaged areas concentrated in the Fukui plain covered with thick quaternary deposits. This suggests that the sedimentary structure can be one of the most important factors to elucidate the damage distribution. Firstly, I propose an inversion procedure of underground structure based on the earthquake observation records, assuming a multi-reflection of vertically propagating SH-wave in a horizontally layering system. The procedure is formulated as an optimization problem to find the thickness and Q-values of each layer which minimize the different between calculated and measured Fourier spectra. Applying this inversion procedure to the actual strong earthquake records observed at the stations deploying in the center of Fukui plain, I calculated 2D S-wave velocity structure beneath the plain up to the tertiary. It was confirmed that the identified underground structure give a fairly good agreement between monitored and calculated responses, and that estimated structure is harmonized with PS-logging data and P-wave reflection survey. Secondary, 3-dimensional Quaternary structure around Fukui plain was explored based on the microtremor observation. The microtremor observations on the ground surface were carried out every small grid that covered the Fukui pain. Predominant periods and H/V ratios of each site were specified from collected Fourier and H/V spectra. By assuming S wave velocity structures, the thickness of Quaternary system of the sites were estimated based on the predominant periods. By applying the procedure to the sited having little ground condition data, one may be able predict the strong earthquake response with a good accuracy.