Inversion of phase velocity of long-period microtremors to the S-wave-velocity structure down to the basement in urbanized areas.

Abstract

Engineering seismology now requires a convenient and easy survey method for S-wave-velocity structures which enables exploration down to the basement even in urbanized areas. We have attempted an application of long-period (0.5Hz to 3.0Hz) microtremors to answer this demand. The method consists of three steps: (1) microtremors are observed using an array of seismometers; (2) their phase velocities are determined by the frequency-wavenumber-spectral analysis of array data; and (3) the S-wave-velocity structure is determined from the obtained phase velocities by the generalized inversion method. As an exploration method, this procedure has several advantages: (1) microtremors can be observed at any time and location; (2) observation is much easier than with other exploration methods; (3) it causes no environmental problems; and (4) geological conditions down to a depth of more than 100m can be inverted, as far as microtremors of required frequency range are observed. The method was applied at two sites located in and near urban areas, and the whole S-velocity structure above the basement was determined. This method proves to be a useful and practical tool for determining S-wave-velocity structures especially in urbanized areas.