Abstract
We study the effects of surface geological conditions on long-period ground motion, that is, the site effects, at various sites in Japan by means of strong-motion seismograms from near-by, intermediate-depth earthquakes. These seismograms represent the site response to nearly vertical incidence of plane S waves; they are simply composed of the primary S wave on rock sites, while they show the long-period scattered waves after the primary S wave at alluvial sites. Then we consider that the strength of scattered wave excitation represents the site characteristics, and define an index of scattered wave excitation by dividing the area of the envelope shape for the scattered waves by that for the primary S wave. The indices are determined at various sites in Hokkaido, Tohoku, Kanto, and Kyushu districts and are compared with a simple geological map. They are low at rock sites, while they vary from low to high values at alluvial sites and are not always in proportion to the size of the alluvial plain. The index defined here has the advantage over the simple geological map to represent the site response of long-period ground motion. Finally we discuss the relation between the index and sedimentary basin structure at some sites.
