濃尾平野の地盤構造と強振動特性に関する研究

Abstract

It is important to know soil structure, especially S-wave velocity structure, in urban areas to study on seismic microzoning or to predict strong motion. However it is difficult to carry out a survey such as refraction or reflection because there is no space in urban areas for a survey. In these days, a survey technique to estimate underground velocity structure using microtremors instead of these method has been developed. The technique involves measurement of phase velocities of the fundamental-mode Rayleigh waves in microtremors and inversion of these phase velocity to geological structures. We carried out array observations at two sites (Nakagawa in Nagoya City, Kanie City) in Nobi plain, where we install strong motion station. Seven seismographs arranged in a suitable array observed the movement in a vertical direction of the ground surface. The data were processed mainly by the frequency-wavenumber (F-K) method. Phase velocities dispersion relations were obtained for the fundamental-mode of Rayleigh waves were obtained to be about 0.2 to 2.2km/s in frequency range from 0.2Hz to 4.0Hz at two observation sites. Then using an inversion technique, the S-wave velocity structures in depths of 2000 meters were obtained. The inverted models consist of seven or eight layers. Basement layer has an S-wave velocity of 3.1km/s. Theoretical site amplification factors were determined by assuming multi reflection of S-wave. They fairly agreed with those calculated on strong motion data. We may conclude that we can use the S-wave velocity structure models determined by array observations of microtremors for evaluating earthquake motions.