Revision of attenuation formula considering the effect of fault size to evaluate strong motion spectra in near field

Abstract

We propose a new distance measure, Equivalent Hypocentral Distance (EHD), which is the distance from a virtual point source that provides the same energy to the site as does a finite-size fault. As EHD includes the effects of fault size, fault geometry, inhomogeneous slip distribution and station location, we can use a far-source attenuation formula to estimate near-source spectra with EHD. We investigated the 1979 Imperial Valley and the 1984 Morgan Hill earthquakes and we found that the attenuation of Fourier spectra in the range from far to near source can be interpreted by the attenuation of body waves predicted by the point-source theory with EHD. We also confirmed that EHD can be applied to the estimation of the maximum accelerations. For near-source data, it is worth emphasizing that EHD does not require any new regression coefficients except those used in far-source attenuation formula. Therefore, if we use EHD, we can estimate the near-source spectrum by using regression coefficients obtained from the far-source data.