On the horizontal-to-vertical spectral ratio in sedimentary basins

The horizontal-to-vertical spectral ratio (HVSR) has been used by many researchers to characterize local conditions in terms of the dynamic response of the soil. One of its variants is that proposed by Nakamura (1989) in which records of microtremors are used. Usually, the analysis is aimed to obtain the predominant period of the site under study. In this work we explore what can be achieved by using this method. We study the response of different configurations under incident waves coming from an explosive source using the indirect boundary element method (IBEM). We investigate two cases: low- and high-velocity contrast, holding constant the physical properties inside the basin and changing only the properties of the bedrock. Then, we compute the seismic response using the horizontal sediment-to-bedrock spectral ratio (SBSR) at various locations on the free surface of the basins, and compare it with the one calculated by the HVSR at the same locations. The comparison shows that, in general, the predominant period computed with the HVSR is not the same as that obtained by the SBSR in all the locations. On the other hand, the HVSR approximation can reasonably well predict the fundamental local frequencies when the impedance contrast between the basin and the bedrock is low. However, HVSR cannot be used in sedimentary basins having a high impedance contrast with respect to the bedrock below.