Soil Amplification Based on Array Observation in Chiba, Japan

The amplification characteristics of earthquake ground motion were investigated based on the Chiba Array observation records. The amplification of peak acceleration occurred mostly at the top soft layer and is similar for the three ground motion components. The ensemble Fourier spectrum ratio and coherence function were calculated and the amplification can be clearly seen in them. The drop of the coherence function at natural frequencies was discussed. The Fourier spectrum ratio and coherence function were also obtained using the smoothing procedure and were compared with those of the ensemble. Microtremor was measured at 11 points corresponding to the location of the surface accelerometers. The power spectrum of microtremor was compared with that of the earthquake ground motion for different points and for different events. Emphasis has been placed on knowing the property of the peaks appearing in the power spectra

Use of Microtremors for the Estimation of Ground Vibration Characteristics

Short-period micro tremor array observation is conducted at five sites in the Tokyo Metropolitan area of Japan. The Fourier spectra of horizontal and vertical components show variations in time, but their ratio is stable for different time instants. The characteristics of the amplitude ratio are similar to that of Rayleigh wave and the period correspond to the peak ratio correspond to the predominant period of the sites for shear wave propagation. A parametric study for two-layered models reveals that the peaks for Rayleigh wave and shear wave are close for the ground having large impedance ratio. The F-K spectrum analysis using vertical components obtained the phase velocity close to the dispersion curve for the Rayleigh wave

特集2 : 研究解説 : Natural Disaster Risk Management : A Need for a Global Perspective

Although considerable progress has been made in managing natural disaster risk in developed countries, global risk to natural hazards is in fact increasing. By looking at natural disasters as a global challenge, general strategies are discussed to mitigate losses due to them.小特集 国際災害軽減工学研究センター(INCEDE