EXAMINATION OF ESTIMATED SURFACE LAYER PROFILES BASED ON SOIL DATA AND MICROTREMOR RECORDS USING OBSERVED SEISMIC GROUND MOTIONS

摘要

The soil profile is one of the most important factors in earthquake engineering and is closely related to the earthquake damage. A microtremor test is one of the powerful means for inferring the soil profile from the viewpoint of simplicity and cost. In our project, microtremor tests are carried out at the ground surface of 341 locations in Nagoya City, and the horizontal-to-vertical spectral ratios (H/V spectra) of microtremors have been calculated. H/V spectra are classified into seven categories from the viewpoint of the spectral shapes, and soils in Nagoya City are also classified into seven groups from the viewpoint of the topography and the geology based on existing soil data. The relationship between the shapes of H/V spectra and the soil profiles is examined. Then the acceleration Fourier spectra of the seismic ground motions at 48 locations in Nagoya City are also classified into seven groups from the viewpoint of the shapes and the predominant frequencies of Fourier spectra. It is clearly seen that the distribution shape of the acceleration Fourier spectra is very much similar to the shape of the H/V spectra of the same locations. The possibility of estimating the soil profile and predicting the seismic ground motion based on the microtremor observations is pointed out. The estimated depth of the top of the seismic bedrock based on the H/V spectra and the rough average of shear wave velocities has a good accuracy. The estimated depth of the top of the engineering bedrock based on the H/V spectra and the rough average of shear wave velocities is deeper than the depth of the top of the AMA-YATOMI layers. It can be said that the estimated depth corresponds to that of the third gravel layer within the AMA-YATOMI layers.