A Study on the Long Period Ground Motions Observed in Osaka Bay Area

Abstract eng:
Through the 2011 off the Pacific coast of Tohoku earthquake, large long period ground motions were observed in Osaka bay area and high-rise building with natural period around 7 seconds suffered considerable damage. In the area, strong ground motions with accelerometers at ground surface and in bedrock at 2000m deep were observed. The spectral amplitude ratio between the two sensors at around 6 seconds was about 50 times. The same phenomena were observed at the site through the 2000 western Tottori earthquake and the 2004 off the Kii peninsula earthquake. The bedrock depth at the site is estimated as 1550 m from previously conducted geophysical surveys. Predominant period of ground motions in Osaka bay area has been assumed about 6 seconds due to the thick sedimentary structures. From minute analysis of the data, it is found that amplitude ratios are smaller in beginning portions of the record, while the ratios grow larger with elapsed times. Amplification ratios in the beginning are around ten times that can be explained from sedimentary responses of vertical incident body waves. However, ratios in the later part are around several tens and it might be caused by node and antinode of surface waves at observed points. The results suggest that the large amplitude long period waves might affect on other thick sediment area even by distant earthquakes.

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