全国地形分類図による表層地盤特性のデータベース化, および, 面的な早期地震動推定への適用

Abstract

We developed digital maps of the geomorphological land classifications and the site amplification factors in Japan, and applied them to strong motion estimations immediately after earthquakes. For making the maps, we first digitized the geomorphological land classification maps of Japan (1:200, 000 or 1:100, 000 scale) and constructed 500m mesh data. Then, we made up the maps of site amplification factors using the empirical relation by Midorikawa and Matsuoka (1995). We evaluated their accuracies by comparing the estimated strong ground motions with the records of K-Net for recent earthquakes in the Kanto area. We estimated the strong motions using the site amplification factors and the two methods: the attenuation relation (Si and Midorikawa, 1999), and an interpolation method using the records. We found that the second method gave more reliable results than the first method, because the first method strongly depended on accuracies of the source and path effects. However, since we may not be able to obtain quickly strong motion records near highly damaged areas, it would be efficient to evaluate the strong motions using the first method immediately after an earthquake, then to replace them by the second method after getting the records. Finally, we compared the site amplification factors using the following two methods. The first is based on the average of shear wave velocities from the free surface to the 30m depths using boring data. The second is the above-mentioned method based on the land classifications. The comparisons showed that the first method was more accurate and reliable. Therefore, it is necessary to replace the digital maps of the site amplifications by those using the first method, when we obtain boring data.