Integration of Site Effects into Probabilistic Seismic Hazard Assessment (PSHA): A Comparison between Two Fully Probabilistic Methods on the Euroseistest Site

Abstract

The integration of site effects into Probabilistic Seismic Hazard Assessment (PSHA) is still an open issue within the seismic hazard community. Several approaches have been proposed varying from deterministic to fully probabilistic, through hybrid (probabilistic-deterministic) approaches. The present study compares the hazard curves that have been obtained for a thick, soft non-linear site with two different fully probabilistic, site-specific seismic hazard methods: (1) The analytical approximation of the full convolution method (AM) proposed by Bazzurro and Cornell 2004a,b and (2) what we call the Full Probabilistic Stochastic Method (SM). The AM computes the site-specific hazard curve on soil, HC(Sas(f)), by convolving for each oscillator frequency the bedrock hazard curve, HC(Sar(f)), with a simplified representation of the probability distribution of the amplification function, AF(f), at the considered site The SM hazard curve is built from stochastic time histories on soil or rock corresponding to a representative, long enough synthetic catalog of seismic events. This comparison is performed for the example case of the Euroseistest site near Thessaloniki (Greece). For this purpose, we generate a long synthetic earthquake catalog, we calculate synthetic time histories on rock with the stochastic point source approach, and then scale them using an adhoc frequency-dependent correction factor to fit the specific rock target hazard. We then propagate the rock stochastic time histories, from depth to surface using two different one-dimensional (1D) numerical site response analyses, while using an equivalent-linear (EL) and a non-linear (NL) code to account for code-to-code variability. Lastly, we compute the probability distribution of the non-linear site amplification function, AF(f), for both site response analyses, and derive the site-specific hazard curve with both AM and SM methods, to account for method-to-method variability. The code-to-code variability (EL and NL) is found to be significant, providing a much larger contribution to the uncertainty in hazard estimates, than the method-to-method variability: AM and SM results are found comparable whenever simultaneously applicable. However, the AM method is also shown to exhibit severe limitations in the case of strong non-linearity, leading to ground motion “saturation”, so that finally the SM method is to be preferred, despite its much higher computational price. Finally, we encourage the use of ground-motion simulations to integrate site effects into PSHA, since models with different levels of complexity can be included (e.g., point source, extended source, 1D, two-dimensional (2D), and three-dimensional (3D) site response analysis, kappa effect, hard rock …), and the corresponding variability of the site response can be quantified.

The influence of source- and ground-motion model choices on probabilistic seismic hazard levels at 6 sites in France

“…Such an approximation is used (e.g., in Bazzurro and Cornell 2004) to convolve the distribution of rock hazard with the distribution of site amplification in order to derive an analytic formula for site-specific PSHA (see, e.g., Aristizabal et al 2018). It is also used in earthquake regulations to derive the coefficients associated with various building importance classes in relation to the corresponding exceedance probability levels (i.e., to the corresponding return periods).…”

A model to estimate the lifetime of structures located in seismically active regions

“…Two main strategies for this purpose can be used: i) Spectral matching using real accelerograms; or ii) stochastic time histories generation using local site conditions. Even though both methods should be applied considering both local magnitude-distance disaggregation and magnitude-frequency distribution (better known as truncated Gutenberg-Richter model) at the site of interest, the later will also required extra information for the source, path and site synthetic models (e.g., stress drop distribution, S and P waves velocity profiles, high frequency attenuation factor (κ), among others, see Aristizábal et al [28] for further information).…”

“…The next step in the process is the definition of: i) the cumulated damage of a structure; ii) its probability of failure due to different series of seismic shocks and iii) its associated uncertainty related to the order of earthquake occurrence (the Stochastic Earthquake Catalogue, SEC), as proposed by Aristizábal et al [28] was used. The following stages were performed to obtain different series of seismic shocks:…”

“…• In order to generate stochastic ground motion time histories, the stochastic method proposed by Boore [42] was used. This procedure is explained in Aristizábal et al [28]. This method is implemented using the SMSIM code [43].…”

Are the Standard VS-Kappa Host-to-Target Adjustments the Only Way to Get Consistent Hard-Rock Ground Motion Prediction?

“…Such methods only return an average response from a large number of sites within the same class, but cannot capture the whole features of a specific site response and may be either over-or un-conservative. The most advanced, fully sitespecific, methods explicitly account for the local site response and are preferable for the design of critical facilities, even though they are more complex and may have to cope with additional sources of uncertainties (Bazzurro and Cornell 2004a, b;Aristizábal et al, 2018). They need two fundamental elements: (i) an accurate estimation of the local site response, and (ii) a reliable estimate of the ''reference'' ground motion, i.e.…”