Estimation of site-dependent spectral decay parameter from earthquake ground motions in southern Korea

Abstract

For the simulation of ground motions, various seismic parameters are tobe estimated in the region where the motions are applicable. One of the most important parameters is the spectral decay parameter κ. The spectral decay parameter κ in conventional sense is for a regional area rather than for a local site. For the site-specific simulation of ground motions, however, a site-dependent spectral decay parameter should be estimated at an observation site. It is difficult to deduce the characteristics of each observation site using the conventional procedure. In order to obtain the site-dependent spectral decay parameter κ, we should estimate site-independent κ q and site-specific κ s successivel. A procedure is to use the coda normalization method for the computation of κ q value as the first step followed by the next step, the computation of κ s for each site using the given κ q evaluated at the first step. For the application of the method, we used a total of 754 accelerograms from small earthquakes occurring in southern Korea from January 1999 to April 2003. After inversion process for κ q , we obtained the κ q value of 0.0001382 sec/km in southern Korea. The κ s values are distributed from −0.15 to 0.35 second at 29 observation sites. The values of κ s are relatively large at the eastern and southern part of southern Korea. In addition, we also estimated the value of κ0, distance independent term of κ at regional area, as 0.007 second using all acceleration spectra in southern Korea.