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

The horizontal-to-vertical spectral ratio (HVSR) has been used by many rese archers to characterize local conditions in terms of the dynamic response o f the soil. One of its variants is that proposed by Nakamura (1989) in whic h records of microtremors are used. Usually, the analysis is aimed to obtai n the predominant period of the site under study. In this work we explore w hat can be achieved by using this method. We study the response of differen t 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 insi de the basin and changing only the properties of the bedrock. Then, we comp ute the seismic response using the horizontal sediment-to-bedrock spectral ratio (SBSR) at various locations on the free surface of the basins, and co mpare it with the one calculated by the HVSR at the same locations. The com parison shows that, in general, the predominant period computed with the HV SR is not the same as that obtained by the SBSR in all the locations. On th e other hand, the HVSR approximation can reasonably well predict the fundam ental local frequencies when the impedance contrast between the basin and t he bedrock is low. However, HVSR cannot be used in sedimentary basins havin g a high impedance contrast with respect to the bedrock below.