ESTIMATION OF UNDERGROUND STRUCTURE AROUND THE SABAE FAULT BASED ON MICROTREMOR OBSERVATION

IR

Abstract

Reliable Quaternary structure is indispensable in order to accurately predict the seismic damage of the region. We executed microtremor observations along the several exploration lines orthogonal to the Sabae Fault. The phase velocities of surface waves were calculated from the array observations by applying the spatial autocorrelation analysis. The dispersion curves were inverted to a vertical S-wave profile using a genetic algorithm. To interpolate the array observations, the single-point 3-components observations were made. It was confirmed that there were two predominant periods in the H/V spectrum and each predominant period were caused by the Alluvium and the Diluvium respectively. We estimated the Quaternary configuration from these predominant periods using the estimated S-wave velocities based on the quarter wavelength rule. The 3D S-wave structure model was proposed by using the vertical structure of the microtremor observation sites. Furthermore, we observed gravity anomaly and calculate the density structure. The validity of the estimated structure from the microtremor observation was confirmed by comparing with the density structure and with the existing deep well data. The estimated configuration showed that the Quaternary deposit has a folded mound near the Sabae fault and that the lower diluvium is shifting over 100m across the fault.

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