The Osaka Basin, western Japan, is a typical sedimentary basin. It is composed of thick soft sediments with the maximum thickness of the sediments 3 km in the sea zone and 1.8 km in the plain zone. This geologic circumstance results in the prevalence of long-period (above 1sec) strong ground motions with long-lasting duration of several minutes in earthquake motions inside the basin. To mitigate earthquake damage to long natural-period structures due to the next M8 class interplate earthquakes, Nankai and Tonankai earthquake, we make the map of the predominant period of surface waves in the Osaka Plain by using the three-dimensional basin model. This map enables us to pick up high-rise buildings that may be resonant with the long-period strong ground motions, and to give a warning to reinforce them. We draw the conclusion that high-rise buildings in the height range between 150 and 300m are risky in the Osaka City, and the number of them was 16 in 256 buildings (about 6%).
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