ESTIMATION OF AN ACCELERATION FOURIER AMPLITUDE SPECTRUM AT AN ARBITRARY POINT USING SPATIAL DISTRIBUTION OF TRANSFER FUNCTION OF SURFACE STRATA BY MODIFIED KRIGING METHOD

Earthquake ground motion intensities (GMI’s) such as PGV, instrumental seismic intensity and spectrum intensity (SI) have been estimated for each mesh with side length of such as 250 m using hypothetic ground models created on the basis of boring investigation data. However, to employ effective measures for seismic damage mitigation and contribute to a resilient society, the GMI’s should be estimated in greater detail, such as at each construction site, and the seismic risk should be assessed accordingly. In order to do so, Sugai et.al proposed a modified Kriging method, which can take into account the non-negligible errors in estimations of the GMI’s of the boring points on ground surface. The method has already been applied to the estimations of seismic hazards and risks using earthquake scenarios in Owari-asahi City, Aichi prefecture, Japan. Developing the proposed method, it is thought that it becomes possible to estimate GMI’s at an arbitrary point by using a limited number of observation records obtained when an earthquake occurs. It is practically impossible to estimate the spatial distribution of GMI’s directly from the observation records by seismometers installed on the ground surface because it requires very dense distribution of seismometers. For example, in Owari-asahi City, approximately one seismometer in every 100 square meters is necessary considering the auto-correlation distance of GMI’s such as PGA, PGV and instrumental seismic intensity being about 150 to 700 m on the ground surface. On the contrary, the auto-correlation distance of GMI’s on engineering bedrock is much longer, about 1.5 to 2 km in the city. Accordingly, the spatial distribution of GMI’s on the engineering bedrock can be estimated with smaller number of observation records, approximately one seismometer in every 1 square kilo-meter. GMI’s on the engineering bedrock at the location of seismometers can be estimated by seismic response analysis of surface strata using the observation records (inverse analysis). One could estimate GMI on the ground surface at an arbitrary point by multiplying an amplification factor to the GMI’s on the engineering bedrock. However, an amplification factor of GMI depends on not only the characteristics of the surface strata but also the frequency characteristics of an earthquake ground motion. The aim of the present paper is to propose a method for estimating an acceleration Fourier amplitude spectrum (FS) at an arbitrary point using the lim