Effects of a soft surface layer on near-fault static and dynamic displacements

Abstract

While detailed crustal structure around a damaged area is surveyed for source parameters obtained by seismic waveform inversions (e.g. the ‘damage belt’ of the 1995 Hyogoken-nambu earthquake in Japan), the effects of a layered medium are often not considered in practical analyses of geodetic data. In this study, we calculate static as well as dynamic displacements with the discrete wavenumber method and compare them with analytical as well as numerical solutions for a half-space. The existence of a soft surface layer is found to affect both the amplitude and the spatial distribution of static displacement near a fault, in a different manner from those of dynamic displacement. The largest effect of a surface layer is shown in the case of a vertical strike-slip fault. While dynamic motions are amplified monotonically with the thickness of the surface layer, the amplification of static displacement is limited if the surface layer reaches the top of the fault because the moment release there is then reduced. Errors in the estimation of the fault depth and the amount of slip reaches as large as 30 per cent of the true values in the worst case. Our results strongly suggest that a strong bias may be introduced to the conventional estimation of fault parameters, assuming a medium to be a half-space, from static displacement or geodetic data by the existence of a low-velocity or low-rigidity surface layer.