A Study on Earthquake Response Characteristics of Small Sedimentary Basin

The purpose of this research is set on the evaluation of site response characteristics for the small sedimentary basins such as Mihara and Kure city，Hiroshima prefecture，Japan。Based on the results of observations of microtremors and the compiled boring data，the precise underground structure models for these regions were constructed。The models were simplified and employed for the numerical simulation analyses.The parameters for the analyses were the dip angle of slant strata，soil property and so on。The concluding remarks of the study are summarized as follows：·Boring data collected in Mihara and Kure city were arranged and compiled into a database.·Distributions of predominant period of ground in the target area were estimated based on H/V spectral ratios of microtremors.·Empirical relation between the predominant period of ground and the depth down to the engineering bedrock was proposed。·Deep underground structures down to bedrock layer of Vs=2 km/sec were estimated by array observations of microtremors.·Response velocity calculated by the FEM for period ranges of 0.5-2seconds in the coastal area area 3-5times larger than that in the mountainous area.·Seismic motions calculated by the FEM is larger than those by 1-D analysis in area where underground structures rapidly shift from horizontally layered to slope.This tendency depends on the dip angle of the slant layer.·Increase of the dip angle of the slant layer brings expansion of the area where earthquake motions influenced by the slant layer are observed.·Seismic motions are amplified greatly in the area above the bottom of the gorge due to valley structure like character“V”.This tendency becomes remarkable with increase of the dip angle.

The purpose of this research is set on the evaluation of site response characteristics for the small sedimentary basins such as Mihara and Kure city, Hiroshima prefecture, Japan. Based on the results of observations of microtremors and the compiled boring data, the precise underground structure models for these regions were constructed. The models were simplified and employed for the numerical simulation analyses. The parameters for the analyses were the dip angle of slant strata, soil property and so on. The concluding remarks of the study are summarized as follows:・Boring data collected in Mihara and Kure city were arranged and compiled into a database.・Distributions of predominant period of ground in the target area were estimated based on H/V spectral ratios of microtremors.・Empirical relation between the predominant period of ground and the depth down to the engineering bedrock was proposed.・Deep underground structures down to bedrock layer of Vs= 2 km/sec were estimated by array observations of microtremors.・Response velocity calculated by the FEM for period ranges of 0.5-2 seconds in the coastal area are 3-5 times larger than that in the mountainous area.・Seismic motions calculated by the FEM is larger than those by 1-D analysis in area where underground structures rapidly shift from horizontally layered to slope. This tendency depends on the dip angle of the slant layer.・Increase of the dip angle of the slant layer brings expansion of the area where earthquake motions influenced by the slant layer are observed.・Seismic motions are amplified greatly in the area above the bottom of the gorge due to valley structure like character "V". This tendency becomes remarkable with increase of the dip angle.