ALBERT

Description: Using records from co-located broadband and digital strong motion (SM) instruments, it is first shown that the displacement waveforms obtained by double integration of the accelerogram need not be free of unrealistic baseline drift to yield reliable spectral ordinates up to at least 10 s. Secondly, to provide objective criteria for selecting reliable digital SM records for ground motion predictions at long periods, a set of synthetic accelerograms contaminated by random long-period noise has been used, and the difference between the original accelerograms and the spurious ones in terms of response spectra has been quaintified, by introducing a noise index that can be easily calculated based on the velocity waveform of the record. The results Qf this study suggest that high-pass filtering the digital acceleration record from a cutoff period selected to suppress baseline drifts on the displacement waveform appears to be in most cases too conservative and unduly depletes reliable information on long-period spectral ordinates. Copyright © 2007 John Wiley & Sons, Ltd.

Description: Site‐specific ground‐motion hazard, as assessed by a probabilistic seismic‐hazard analysis one‐step approach that handles a single‐site sigma and its uncertainties and uses a simple logic tree, is compared with a two‐step approach that includes bedrock motion evaluation and wave propagation through a local soil profile with consideration of the main epistemic uncertainties. The one‐step analysis relies on accelerometer data from the Po Plain, a sedimentary basin in northern Italy where an earthquake sequence with two Mw∼6.0 events was extensively recorded in 2012. Uniform hazard spectra (UHS) on soil and exposed bedrock are evaluated at three deep‐soil accelerometer sites (MRN, NVL, and T0821), using residual measures available from other studies, by which uncertainties in site terms (δS2S) and single‐site sigma (σss,s) are estimated. Despite similarity in geologic conditions, at least one out of three sites in the one‐step analysis displays substantial differences in mean level (T0821) or in the (84–16) percentile spread (NVL), depending on differences in site terms and single‐site sigma, possibly caused by source‐to‐site propagation effects.The two‐step approach was applied to the remaining site (MRN) using carefully selected and broadband‐matched acceleration signals and linear, equivalent‐linear, and nonlinear approaches as excitation in propagation analyses. We found that assumptions on soil degradation curves dominate the variability of results. The linear approach provided the best results, based on (1) the similarity of the one‐step nonergodic UHS with the two‐step result based on the linear approach; (2) the comparison with observed records at MRN during the 2012 sequence mainshocks, showing peak ground acceleration and short‐period spectral levels well beyond those predicted by different nonlinear assumptions; and (3) similar evidence from a set of 21 stations at deep soil sites of the Japanese KiK‐net.

Description: In this paper the site categorization criteria and the corresponding site amplification factors proposed in the 2021 draft of Part 1 of Eurocode 8 (2021-draft, CEN/TC250/SC8 Working Draft N1017) are first introduced and compared with the current version of Eurocode 8, as well as with site amplification factors from recent empirical ground motion prediction equations. Afterwards, these values are checked by two approaches. First, a wide dataset of strong motion records is built, where recording stations are classified according to 2021-draft, and the spectral amplifications are empirically estimated computing the site-to-site residuals from regional and global ground motion models for reference rock conditions. Second, a comprehensive parametric numerical study of one-dimensional (1D) site amplification is carried out, based on randomly generated shear-wave velocity profiles, classified according to the new criteria. A reasonably good agreement is found by both approaches. The most relevant discrepancies occur for the shallow soft soil conditions (soil category E) that, owing to the complex interaction of shear wave velocity, soil deposit thickness and frequency range of the excitation, show the largest scatter both in terms of records and of 1D numerical simulations. Furthermore, 1D numerical simulations for soft soil conditions tend to provide lower site amplification factors than 2021-draft, as well as lower than the corresponding site-to-site residuals from records, because of higher impact of non-linear (NL) site effects in the simulations. A site-specific study on NL effects at three KiK-net stations with a significantly large amount of high-intensity recorded ground motions gives support to the 2021-draft NL reduction factors, although the very limited number of recording stations allowing such analysis prevents deriving more general implications. In the presence of such controversial arguments, it is reasonable that a standard should adopt a prudent solution, with a limited reduction of the site amplification factors to account for NL soil response, while leaving the possibility to carry out site-specific estimations of such factors when sufficient information is available to model the ground strain dependency of local soil properties.

Description: In this paper the site categorization criteria and the corresponding site amplification factors proposed in the 2021 draft of Part 1 of Eurocode 8 (2021-draft, CEN/TC250/SC8 Working Draft N1017) are first introduced and compared with the current version of Eurocode 8, as well as with site amplification factors from recent empirical ground motion prediction equations. Afterwards, these values are checked by two approaches. First, a wide dataset of strong motion records is built, where recording stations are classified according to 2021-draft, and the spectral amplifications are empirically estimated computing the site-to-site residuals from regional and global ground motion models for reference rock conditions. Second, a comprehensive parametric numerical study of one-dimensional (1D) site amplification is carried out, based on randomly generated shear-wave velocity profiles, classified according to the new criteria. A reasonably good agreement is found by both approaches. The most relevant discrepancies occur for the shallow soft soil conditions (soil category E) that, owing to the complex interaction of shear wave velocity, soil deposit thickness and frequency range of the excitation, show the largest scatter both in terms of records and of 1D numerical simulations. Furthermore, 1D numerical simulations for soft soil conditions tend to provide lower site amplification factors than 2021-draft, as well as lower than the corresponding site-to-site residuals from records, because of higher impact of non-linear (NL) site effects in the simulations. A site-specific study on NL effects at three KiK-net stations with a significantly large amount of high-intensity recorded ground motions gives support to the 2021-draft NL reduction factors, although the very limited number of recording stations allowing such analysis prevents deriving more general implications. In the presence of such controversial arguments, it is reasonable that a standard should adopt a prudent solution, with a limited reduction of the site amplification factors to account for NL soil response, while leaving the possibility to carry out site-specific estimations of such factors when sufficient information is available to model the ground strain dependency of local soil properties.

Description: Published

Description: 4199–4234

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal

Description: Paper describes the extensive work done in the SIGMA project, aimed at improving knowledge on data, methods and tools to better quantify uncertainties in seismic hazard assessment (SHA). The authors cooperated in the study of potential faults and geological structures, earthquake catalogues, selection of ground motion prediction equations, and methods for site effect evaluation suitable for SHA. All the contributions merged into a probabilistic seismic hazard study conducted for three representative sites of the Po Plainin Northern Italy. Po Plain is a low-to-moderate seismicity region, characterized by some critical features, such as blind faulting and deep alluvium sediments, and by scarcity of strong motion data; these sources of uncertainties in seismic hazard estimation are common to other low seismicity areas around the world. Within SIGMA, special care was devoted to: (a) the use of the single station sigma approach inside the probabilistic SHA, (b) the comparative use of generalized attenuation functions to evaluate the hazard contribution of composite fault systems, and (c) the study of the epistemic uncertainties at play when different modelling approaches to site effects are used.

Description: Published

Description: 2529–2564

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal