Seismic Amplification by Soil Deposits Inferred from Vibrational Characteristics of Microseisms

Abstract

Vibrational characteristics of microseisms as well as of seismic surface waves from near earthquakes were studied to estimate amplification factor due to soil deposits in the Kyoto basin. Around Kyoto microseisms are considered to be surface waves composed mainly of Rayleigh type coming from some specific regions in the Japan sea and the Pacific. The excitation of microseisms depends on the height of sea waves around Japan, in particular on that of the Japan sea coast in winter, and changes with time according to meteorological conditions. Therefore it is indispensable to monitor the wave characteristics of incident microseisms for the sake of seismic microzonation of the basin. From this standpoint observations of microseisms generated by various weathers such as typhoons, cold fronts and monsoons were carried out at several ground sites in the basin with routine observation at the Sumiyama Seismic Station located on an outcrop of bedrock. The following results are obtained through spectral analyses with automated analogue band pass filtering technique and the Fast Fourier Transform: Although the spectral characteristics of microseisms such as peak frequency and peak amplitude at the ground site as well as at the rock site change remarkably according to meteorological conditions, the averaged spectral ratio of microseisms at the ground site to that at the rock site shows a frequency dependent constant shape, which is considered to be an amplification factor of microseisms due to soil deposits. The reliability of the estimated amplification can be checked with a standard deviation as long as the observations with a recording length of longer than 4 minutes are repeated at least 6 times. Amplification in the horizontal component begins to increase at lower frequencies than that in the vertical one. The horizontal component has larger peak amplification than the vertical one. The frequency of peak amplification and critical frequency at which amplification begins to increase become lower as an observational ground site moves from the environs to the center of the basin. The variation of amplification with component and site location is interpreted to be attributable to the mode conversion of incident surface waves at the boundary of the basin and to the resultant interference between modes in the basin. Amplification of seismic surface waves derived from spectral ratios of near earthquakes resembles that of microseisms very well. Therefore microseismic study is useful for direct measurement of seismic amplification due to soil deposits in the frequency range of lower than about 1 Hz.