The Importance of Converted Waves in Comparing H/V and RSM Site Response Estimates

YingRu Wang

Abstract

Surface geology has long been recognized to affect earthquake ground motion significantly. Unconsolidated sediments overlying bedrock amplify seismic motion in specific frequency bands. It is common accept that the horizontal-to-vertical spectral ratio (HVSR) can lead to significant differences compared to the reference site-method (REM). In particular, even if the estimates of the resonance frequency from the two methods are fairly consistent, the levels of amplification are not. This report used numerical modeling to isolate the contribution of pure and converted waves to the site response, and shows that the conversion of body waves at the sediment-bedrock interface leads to difference site response obtained by the HVSR and REM. Such differences are consistent with observations in the field. The lower level of amplification obtained by HVSR at frequencies higher than the fundamental one is due to a transfer of energy of the vertical component caused by S- to P-wave conversion. Applying the RSM technique to the vertical component of the P-wave window could obtain stable estimates of the fundamental resonance frequency that are consistent with 1D resonance. The amplification from the P-wave window of the horizontal component might be considered as a lower boundary.

References

Parolai, S., and S. M. Richwalski (2004). The importance of converted waves in comparing H/V and RSM site responses, Bull. Seism. Soc. Am. 94, 304-313.

(Abstract) (Full text)

Parolai, S., D. Bindi, M. Baumbach, H. Grosser, C. Milkereit, S. Karakisa, and S. Zunbul (2004). Comparison of different site response estimation techniques using aftershocks of the 1999 Izmit earthquake, Bull. Seism. Soc. Am. 94, 1096-1108.

(Abstract) (Full text)