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Abstract

Large modifications of seismic waves are produced by variations of material properties near the Earth's surface and by both surface and buried topography. These modifications, usually referred to as "site response", in general lead to larger motions on soil sites than on rock-like sites. Because the soil amplifications can be as large as a factor of ten, they are important in engineering applications that require the quantitative specification of ground motions. This has been recognised for years by both seismologists and engineers, and it is hard to open an earthquake journal these days without finding an article on site response. What is often missing in these studies, however, are discussions of the uncertainty of the predicted response. A number of purely observational studies demonstrate that ground motions have large site-to-site variability for a single earthquake and large earthquake-location-dependent variability for a single site. This variability makes site-specific, earthquake-specific predictions of site response quite uncertain, even if detailed geotechnical and geological information is available near the site. Predictions of site response for average classes of sites exposed to the motions from many earthquakes can be made with much greater certainty if sufficient empirical observations are available.

Earthquake hazard and risk analysis for natural and induced seismicity: towards objective assessments in the face of uncertainty

“…Consider, for example, a model that predicts ground accelerations as a function of earthquake size (magnitude) and the distance of the recording site from the source of the earthquake. The residuals of the recorded accelerations relative to the predictions define the aleatory variability in the predictions, but this variability will be appreciably reduced if the nature of the surface geology at the recording sites is taken into account, even if this is just a simple distinction between rock and soil sites (Boore 2004). In effect, such a modification to the model isolates an epistemic uncertainty-the nature of the recording site and its influence on the ground acceleration-and thus removes it from the apparent randomness; this, in turn, creates the necessity, when applying the model, to obtain additional information, namely the nature of the surface geology at the target site.…”

Hybrid-Empirical Ground Motion Estimations for Georgia

“…Concerning aleatory uncertainty, it is difficult to reduce it even for regions with dense seismic arrays and large databases of strongmotion records (Douglas 2003). In recent studies, it is observed that the effect of various parameters such as site classes Vs30, fault type on aleatory uncertainty is very low (e.g., Boore 2004, Bommer et al 2003. The large aleatory variability is accounted for using simple models in GMPEs for defining complex phenomena (Bommer and Abrahamson 2007).…”