Re-thinking site amplification in regional seismic risk assessment

Abstract

Probabilistic assessment of seismic hazard and risk over a geographical region presents the modeler with challenges in the characterization of the site amplification that are not present in site-specific assessment. Using site-to-site residuals from a ground motion model fit to observations from the Japanese KiK-net database, correlations between measured local amplifications and mappable proxies such as topographic slope and geology are explored. These are used subsequently to develop empirical models describing amplification as a direct function of slope, conditional upon geological period. These correlations also demonstrate the limitations of inferring 30-m shearwave velocity from slope and applying them directly into ground motion models. Instead, they illustrate the feasibility of deriving spectral acceleration amplification factors directly from sets of observed records, which are calibrated to parameters that can be mapped uniformly on a regional scale. The result is a geologically calibrated amplification model that can be incorporated into national and regional seismic hazard and risk assessment, ensuring that the corresponding total aleatory variability reflects the predictive capability of the mapped site proxy.

A regionally-adaptable ground-motion model for shallow crustal earthquakes in Europe

“…Site-to-site response variability is captured by the site-specific random-effects ΔS2S = N 0, 2 S2S . The potential of S2S s in site-specific GMMs is well-known, and are useful in studying regional differences in site-response scaling with V S30 (timeaveraged shear-wave velocity in 30 m top-soil) as in K16 or other site-response proxies (Kotha et al 2018;Weatherill et al 2020b) 5. The left-over residuals = N 0, 2 contain the unexplained natural variability of ground-motion observations, and thus represent the apparent aleatory variability of the model.…”

“…However, it is unlikely that every site has sufficient ground-motion data to estimate it's site-specific S2S s . In that case, alternative site-response proxies are sought to predict the S2S s , as in Kotha et al (2018);Weatherill et al (2020b). However, even in these studies, while the long period site-response could be partially explained using some geotechnical parameters, short-period site-response is much more variable-even among the so-called reference rock sites (Bard et al 2019;Pilz et al 2020).…”

“…We keep all sites in the dataset irrespective of whether their V S30 , measured from geotechnical investigations, is provided or not in ESM. This is to estimate the site-specific terms ( S2S s ) at as many sites as possible, and then explore various site-response proxies to characterize them (Kotha et al 2018;Weatherill et al 2020b) 4. Choice of distance metric is R JB where available, otherwise the epicentral distance R epibut only for events with M W ≤ 6.2 .…”

Empirical nonergodic shaking scenarios based on spatial correlation models: An application to central Italy

“…The site-specific terms (δS2S s ) are also zero-mean normally distributed ∼N(0, ϕ S2S,s ) random variables that define the systematic bias of ground motions recorded at each station and can be considered as a proxy of the empirical amplification of each station. 26,33 The δS2S s corrections are available in Electronic Supplement #2 (ESUPP2). In this case, we compute the site-specific terms with respect to the average GMM prediction provided by the fixed-effect model of Equation 1, which is calibrated on the reference stations (ie, rock sites unaffected by site-effects).…”

“…The site‐specific terms ( δS2S s ) are also zero‐mean normally distributed ∼ N (0, ϕ S2S,s ) random variables that define the systematic bias of ground motions recorded at each station and can be considered as a proxy of the empirical amplification of each station 26,33 . The δS2S s corrections are available in Electronic Supplement #2 (ESUPP2).…”