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Erratum: Peak-over-threshold: quantifying ground motion beyond design ( Earthq Eng Struct Dyn, (2020), 49, 5, (458-478), 10.1002/eqe.3248)
No full textThere is an error in Equation (12) and in the subsequent description. In fact, Equation (12) should read as:1 (Formula presented.) The description of the equation terms should read as follows: (Formula presented.) is the correlation coefficient of the residuals of the GMPEs for the spectral acceleration at the two periods {T,T*}
Peak-over-threshold: Quantifying ground motion beyond design
No full textIn performance-based seismic design, as adopted by several building codes worldwide, the structural performance is verified against ground motions that have predetermined exceedance return periods at the site of interest. Such a return period is evaluated by means of probabilistic seismic hazard analysis (PSHA), and the corresponding ground motion is often represented by the uniform hazard spectrum (UHS). The structural performance for ground motions larger than those considered in this design approach is, typically, not explicitly controlled under the assumption that they are sufficiently rare. On one hand, this does not achieve uniform safety at sites characterized by different design ground motions corresponding to the same return period; on the other hand, exceedances of the design spectra are systematically observed over large areas, for example in Italy. The latter issue is because of the nature of UHS, the exceedance of which is likely-to-almost-certain when the construction site is in the epicentral area of moderate-to-high magnitude earthquakes (ie, the design spectrum may be not conservative at these locations), especially if PSHA is based on seismic source zones. The former is partially because of the systematic difference of ground motions for return periods larger than the design one at the different sites. Quantification of the expected ground motion given the exceedance of the design ground motions (ie, the recently introduced as the expected peak-over-threshold or POT) can be of help in quantitatively assessing these issues. In the study, a procedure to compute the POT distribution is derived first; second, POT spectra are introduced and used to help understanding why and how seismic structural reliability of code-conforming structures decreases as the seismic hazard of the site increases; third, expected and 95th percentile POT maps are shown for Italy to discuss how much high hazard sites are exposed to much larger peak-over-threshold with respect to mid-hazard and low-hazard sites; finally the POT is discussed with respect to the slope of the hazard curve (in log-log scale) at the threshold, a known proxy for ground motion beyond design. All data presented in the maps are made available for the interested reader as a supplemental archive
Rarity, proximity, and design actions: mapping strong earthquakes in Italy
No full textAt the state-of-the-art of structural codes, seismic design actions are based on probabilistic seismic hazard analysis (PSHA). In the performance-based earthquake engineering framework, the return period of exceedance of the reference ground motion is established based on the desired performance of the structure. It is easy to show and recognize that exceedance of elastic spectra, for the most common return periods considered for design, is very likely for some earthquakes if they occur close to the site of interest, and that this does not necessarily contradict the results of PSHA. Therefore, it might be relevant to gather insights about: (i) the probability that the site is in proximity of earthquakes of magnitude that can imply exceedance; (ii) the probability that earthquakes occurring close cause exceedance of design actions; (iii) the minimum magnitude of close-by events that are likely to cause exceedance of design actions, which are then referred to as the strong earthquakes; (iv) the accelerations that structures could be exposed to in the case of exceedance of design spectra. These results, which are produced for Italy in this study, may be considered by-products of PSHA, and are helpful in determining what to expect in terms of elastic actions for code-conforming structures in countries where probabilistic seismic hazard lies at the basis of structural design
Conditional hazard for simplified multi-site seismic hazard and risk analyses
No full textThe joint probability distribution of different intensity measures (Formula presented.) at different sites in one earthquake is needed to define the stochastic process regulating the exceedance (in time) of the (Formula presented.) at the ensemble of the sites, which in turn is a funding element of the so-called multi-site probabilistic seismic hazard analysis (MSPSHA). The simplest model for the joint distribution of the (Formula presented.) in one seismic event requires the mean vector and the covariance matrix of the (Formula presented.) at all sites, conditional to the magnitude and location of the earthquake, which may need a large amount of data to be calibrated. The conditional hazard (CH) approach, originally developed for single-site surrogate vector-valued probabilistic seismic hazard analysis, may be a simplified option for MSPSHA, as it explicitly models only part of the covariance matrix of the (Formula presented.) at the sites, while the rest forcefully follows the working assumptions. The presented paper compares the CH approach for MSPSHA against the benchmark in which the complete covariance matrix is modelled, using a testbed in which one-hundred sites are considered. When the comparison metric is the probability of a given number of (Formula presented.) exceedances observed at the sites in some time intervals, it is found that CH is a viable alternative to MSPSHA, although the degree of approximation is sensitive to which and how many (Formula presented.) are considered (e.g., which spectral acceleration). When the analysis is taken all the way to the risk, considering the fragilities of a portfolio of hypothetical buildings, and taking as the metric the probability of observing a given number of structural failures in a time interval, it is found that the approximation introduced by CH with respect to the benchmark is further reduced
Differential response of Mytilus galloprovincialis hemocytes to in vivo and in vitro bacteria challenge
No full textEmpirical assessment of seismic design hazard’s exceedance area
No full textDesign ground motion intensities determine the actions for which structures are checked, in the conventional approach of seismic codes, not to fail the target performances. On the other hand, due to inherent characteristics of probabilistic seismic hazard analysis (PSHA), it is expected that site-specific design intensity based on PSHA is exceeded in the epicentral area of moderate-to-high magnitude earthquakes. In the context of regional seismic loss assessment and of the evolution of seismic codes from the regulator perspective, it is useful to gather insights about the extent of the zone around the earthquake source where code-conforming structures are expected to be systematically exposed to seismic actions larger than those accounted for in design. To assess such areal extent based on empirical evidence is the scope of the study presented in the paper. To this aim, peak ground acceleration ShakeMap data for Italian earthquakes from 2008 to 2020 were compared to the current design intensities in the same areas for which the maps are available. This allowed, first, to develop simple semi-empirical models of the exceedance area versus the magnitude of the earthquakes. Second, it allowed to model the probability that an earthquake of given magnitude causes exceedance of the design intensity via logistic regressions. Coupling the first and second class of models provides an approximation of the expected exceedance (logarithmic) area upon occurrence of an earthquake of given magnitude. Such an area can be of several thousand square kilometers for earthquakes occurring relatively frequently in countries such as Italy
Exceedance of design actions in epicentral areas: insights from the ShakeMap envelopes for the 2016–2017 central Italy sequence
No full textShakeMap is the tool to evaluate the ground motion effect of earthquakes in vast areas. It is useful to delimit the zones where the shaking is expected to have been most significant, for civil defense rapid response. From the earthquake engineering point of view, it can be used to infer the seismic actions on the built environment to calibrate vulnerability models or to define the reconstruction policies based on observed damage vs shaking. In the case of long-lasting seismic sequences, it can be useful to develop ShakeMap envelopes, that is, maps of the largest ground intensity among those from the ShakeMap of (selected) events of a seismic sequence, to delimit areas where the effects of the whole sequence have been of structural engineering relevance. This study introduces ShakeMap envelopes and discusses them for the central Italy 2016–2017 seismic sequence. The specific goals of the study are: (i) to compare the envelopes and the ShakeMap of the main events of the sequence to make the case for sequence-based maps; (ii) to quantify the exceedance of design seismic actions based on the envelopes; (iii) to make envelopes available for further studies and the reconstruction planning; (iv) to gather insights on the (repeated) exceedance of design seismic actions at some sites. Results, which include considerations of uncertainty in ShakeMap, show that the sequence caused exceedance of design hazard in thousands of square kilometers. The most relevant effects of the sequence are, as expected, due to the mainshock, yet seismic actions larger than those enforced by the code for structural design are found also around the epicenters of the smaller magnitude events. At some locations, the succession of ground-shaking that has excited structures, provides insights on structural damage accumulation that has likely taken place; something that is not accounted for explicitly in modern seismic design. The envelopes developed are available as supplemental material
Sequence-Based Hazard Maps for the United Kingdom
No full textThe current practice of probabilistic seismic hazard analysis (PSHA) does not take into account that earthquakes actually occur in time–space clusters. The input for PSHA is based on declustered seismic catalogs, used to characterize only the mainshocks, that is, the largest magnitude events within each cluster. However, the so-called sequence-based PSHA (SPSHA; Iervolino et al., 2014) allows us including the effect of aftershocks in hazard analysis, that is, the events following the mainshock, still conveniently resourcing from declustered catalogs. In the United Kingdom (UK), the seismic source model developed for the national seismic hazard assessment has been recently updated by the British Geological Survey (BGS, 2020). In this study, the source model developed by the BGS (one directly derived from it, in fact) is used to implement SPSHA in the UK. The calibration of the model for the occurrence of aftershocks, that is, the modified Omori’s law, is fitted on a few sequences and under some simplifying assumptions. The results, represented by hazard maps for selected spectral ordinates and exceedance return periods of interest for structural engineering, are compared to the PSHA counterparts to discuss the increase in the design seismic actions when the effects of aftershocks are considered. The maps show that, based on the modeling of aftershock sequences considered in the study, in the UK this increase can be up to 14%, at least for the spectral ordinates and exceedance return periods herein investigated. The discussed maps are provided as supplemental material to this article
Comparing alternative models for multisite probabilistic seismic risk analysis
Full text linkThe risk assessment for a building portfolio or a spatially distributed infrastructure requires multi-site probabilistic seismic hazard analysis (MSPSHA). In fact, MSPSHA accounts for the stochastic dependency between the ground motion intensity measures (IMs) at the sites. Multi-site hazard needs to define the correlation structure for the same IM at different sites (spatial correlation), that of different IMs at the same site (cross-correlation) and that of different IMs at different sites (spatial-cross-correlation). Literature shows that such models usually require a significant amount of regional data to be semi-empirically calibrated. An approximated yet simpler-to-model alternative option is the conditional-hazard approach. The latter, originally developed for single-site analyses as an alternative to vector-valued PSHA, allows computing the distribution of a secondary IM given the occurrence or exceedance of a value of a primary IM. Conditional hazard considers the spatial correlation of the primary IM and the cross-correlation at each site for the two IMs, thus, if it is adopted for MSPSHA, the spatial correlation of the secondary IM as well as the spatial-cross-correlation between the two IMs descends from these two models. In the study, the conditional hazard procedure for MSPSHA is discussed and implemented in an illustrative application. Results in terms of distribution of the total number of exceedances of selected thresholds at the sites in a given time interval are compared with the case of complete formulation of MSPSHA and the differences are quantified. It appears that conditional hazard is a solid, yet simpler alternative for MSPSHA, at least in the considered cases
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