Comparison of soil nonlinearity (in situ stress-strain relation and G/Gmax reduction) observed in strong-motion databases and modeled in ground-motion prediction equations

Guéguen, Philippe and Bonilla, Fabian and Douglas, John (2019) Comparison of soil nonlinearity (in situ stress-strain relation and G/Gmax reduction) observed in strong-motion databases and modeled in ground-motion prediction equations. Bulletin of the Seismological Society of America, 109 (1). pp. 178-186. ISSN 0037-1106

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    Abstract

    Earthquake ground motions are strongly affected by the upper tens of meters of the Earth’s crust and consequently local site effects need to be included in any ground-motion prediction. It is increasingly common in ground motion prediction equations (GMPEs) to account for possible non-linear behavior of near-surface materials (soil). These non-linear site terms adjust observations made on soft soil sites to the ground motion expected on bedrock and hence allow these abundant soil records to be used within the regression analysis for the derivation of empirical GMPEs. These nonlinear site terms also allow rapid predictions of the expected ground motions on soil rather than requiring a site response analysis to be conducted. In this study we compare the signature on observed peak ground acceleration as a function of a strain proxy of non-linear soil behavior within four large strong-motion databases to the predicted signature from four recent GMPEs, three of which explicitly include non-linear site terms. We find that observed non-linearity in the databases, interpreted in terms of strain-stress relationships and reduction of shear modulus, is limited but that even this limited effect is underestimated by the non-linear site terms of the considered GMPEs, which suggests that predictions from these GMPEs could be biased for soft soil sites but also on bedrock. Some of this mismatch could be explained by the use of the average shear-wave velocity in the top 30m (Vs30) to characterize sites as well as errors in these values.