Analysis on constituents of amplification and propagation of strong ground motion based on precise underground structure data

本研究的目的是通过利用强震记录和在观测点附近进行多次岩土工程测量，清楚地了解局部地质和/或地下结构模型对估计地震地面运动的影响。在1993年北海道南胜町发生的最大余震中，在北海道西南海岸的大藤町观测到了1.6g的极高加速度。为了找出加速度很高的原因，我们利用浅井进行了弱运动台阵观测、小比例尺折射测量和P-S测井。结合这些数据，我们可以理解为：1)覆盖在很坚硬岩石上的浅层沉积物对岩石场地的地面运动在共振频率上的放大作用高达40倍；2)地形影响可以忽略不计。3)即使在强烈的震动中，大藤部沉积物的非线性也很小，另一方面，高频显著减少，这意味着深部沉积物场地(卡米诺·库尼)的非线性效应。地震过程中土体的复杂性需要用强震记录来研究，长周期强地震动的传播依赖于二维或三维的地下结构。我们利用1990年伊豆-大岛近海地震获得的关东平原及其周围的强震记录和对横向非均匀介质中面波传播的数值模拟，研究了这种影响。

The aim of this research is to clearly understand the effects of local geology and/or underground structure model for estimating the earthquake ground motion by using the strong motion records and by carrying out several geotechnical measurements near the observation site.A very high acceleration of 1.6g was observed during the largest aftershock of 1993 Hokkaido Nansei-Oki earthquake at Otobe-cho, south-west coast of Hokkaido. In order to find the reasons of the very high acceleration, we carried out weak-motion array observation, small scale refraction survey and P-S logging using shallow borehole. Combining these data, we could understand as following :1) Shallow sediments overlaying very hard rock contribute to amplification of ground motion as much as 40 times at resonance frequency respect to rock site.2) Topographical effects are negligible.3) Nonlinearity of sediments at Otobe was very slight even in severe shaking, on the other hand, significant decrease of high frequency which implies nonlinear effects at deep sediment site (Kamino kuni) was found. The complexity of soil during an earthquake should be investigated using strong motion records.Propagation of long period strong ground motion depends on 2 or 3 dimensional underground structure. We examined this effects using strong motion records obtained in and around Kanto plain from the 1990 Izu-Oshima Kinkai earthquake and by numerical simulation for surface wave propagation in laterally heterogeneous media.