1,721,106 research outputs found
Ground-Motion Observations and Probabilistic Seismic Hazard: Frequently Asked Questions
No full textObserved exceedances of ground-motion intensity from probabilistic seismic hazard analysis (PSHA), in countries where it is used for structural design, spark significant public attention, rekindle scientific debates, and are sometimes discussed in trials about the accountability for structural failures and other earthquake-related losses. This short article addresses, in a question-reply format, some recurring issues and related research findings that should be carefully taken into account by those who author or face these reasonings. It considers Italy as a reference, yet the discussed issues are common to several other countries worldwide and thus may be interesting at an international level. The arguments provided, mainly stemming from the fact that observed cases of exceedance should not necessarily be considered a failure of PSHA, can possibly help in gaining a more informed perception of seismic hazard assessment and structural design as implemented in building codes
Generalized earthquake counting processes for sequence-based hazard
No full textSequence-based probabilistic seismic hazard analysis (SPSHA) allows us to account for the effect of aftershocks in the assessment of seismic structural-design actions (Iervolino et al., 2014, 2018). In fact, it generalizes classical probabilistic seismic hazard analysis (PSHA; Cornell, 1968), combining it with aftershock-PSHA (Yeo and Cornell, 2009). SPSHA associates in time aftershocks to mainshocks and, therefore, retains a desirable property of classical PSHA; that is, events (earthquakes in PSHA and mainshock–aftershock sequences in SPSHA) occur according to homogeneous Poisson processes (HPPs). Nevertheless, the number of earthquakes in SPSHA is not Poisson-distributed. This is addressed herein, in which the probability distribution is formulated and discussed for the following random variables: (1) the count of all earthquakes pertaining to sequences originating in any time interval; (2) the count of all earthquakes occurring in any time interval; (3) the count of all earthquakes that cause exceedance of an arbitrary ground-motion intensity threshold at the site of interest, generated by sequences originating in any time interval. An application referring to central Italy is also developed to help the discussion. The three main findings are that: (1) the formulated SPSHA counting processes further generalize PSHA; that is, they degenerate in the corresponding mainshock HPPs, if aftershocks are neglected; (2) to associate the aftershocks to the corresponding mainshocks in time is fit for hazard assessment purposes; and (3) the variance-to-mean ratio of the counting distributions is significantly larger than one; consequently, the occurrence processes cannot be approximated by Poisson processes. These results, which complete the SPSHA framework, can be a reference for model calibration exercises when SPSHA is computed via simulation and in those cases in which the probability of an exact number of exceedances is of interest, rather than that of observing at least one exceedance (e.g., for seismic damage accumulation studies)
Asymptotic behavior of seismic hazard curves
No full textHazard curves from probabilistic seismic hazard analysis (PSHA) are plots of the rate of earthquakes exceeding ground motion intensity values vs such threshold values, for a site of interest. In classical PSHA, these curves can be transformed to provide the probability of exceedance of ground motion intensity values in any time interval, utilizing the properties of the homogeneous Poisson process (HPP). In turn, these probability curves can be seen as the plot of the complementary cumulative distribution function of the maximum intensity observed, at the site, in the time interval of interest. One consequence of the HPP framework, within which PSHA is developed, is that, for large time intervals, it can be argued that these curves could asymptotically lead to a probabilistic model for extreme value (EV) random variables. This is discussed, with a simple engineering approach, in this short note, where it is found – via case studies – that exceedance hazard curves seem to converge towards an EV distribution (i.e., the EV type II or Fréchet), with a pace that is impacted by the discontinuity inherent to the curves. It is also seen that other common models, typically used to provide an analytical format to probabilistic curves, do not show the same level of convergence. Besides providing further insights on the results of PSHA, this study can possibly be useful for those cases where a closed-form equation for the hazard curve could be needed, such as reliability-based calibration of building codes, or seismic risk studies involving seismic hazard approximation/extrapolation
How different PSHA is different enough?
No full textProbabilistic seismic hazard analysis (PSHA) is widely employed worldwide as the rational way to
quantify the uncertainty associated to earthquake occurrence and effects. National-scale PSHA
has its results typically expressed in the form of maps of ground motion measures intensities
that all have the same exceedance return period. Classical PSHA relies on data that continuously
increase due to instrumental seismic monitoring, and on models that continuously evolve with
the knowledge on each of its many aspects. Therefore, it can happen that different, equally
legitimate, hazard maps for the same region can show apparently irreconcilable differences,
sparking public debate. This situation is currently ongoing in Italy, where the process of
governmental enforcement of a new hazard map is delayed. The discussion is complicated by the
fact that the events of interest to hazard assessment are intentionally rare at any of the sites the
maps refer to, thus impeding empirical validation at any specific site. The presentation will show
the result of two recent studies, which pursue a regional approach, regarding three different
authoritative PSHA studies for Italy. The first one entailed formal tests on the output of PSHA
against the observed ground shaking exceedance frequencies, obtained from about fifty years of
continuous monitoring of seismic activities across the country (Iervolino et al., 2023a). The
second compares the areas in which exceedance of PSHA-postulated ground motion intensity
threshold is estimated according to ShakeMap for twelve years of instrumental earthquakes,
with what expected from the considered PSHA models (Iervolino et al., 2023b). The bulk of
analyses reveals that, apparently alternative hazard maps are, in fact, hardly distinguishable in
the light of observations and ShakeMap estimations. This perspective, which may be relevant for
the current debate, may be strengthened by the fact that recent studies (Baltzopoulos et al.,
2023) also show that structural design, for example for reinforced concrete moment-resisting
frames, is strictly dominated by seismic actions only in a fraction of the country, owing to the
effect of building-code-prescribed minima and design for gravity loads
Estimation uncertainty for some common seismic fragility curve fitting methods
No full textThis technical note illustrates and makes available some simple procedures to assess the estimation uncertainty for the parameters of seismic fragility curves. The considered fragility fitting methods refer to the lognormal assumption and are supposed to be based on the results of multi-stripe dynamic analysis of a deterministic non-linear structural model, so that the uncertainty in the fragility parameters arises from the so-called record-to-record variability. The discussed procedures are based on the statistics approach of resampling with substitution, which is commonly referred to as bootstrap. It is also briefly discussed how the estimation uncertainty depends on the maximum value of the probability of failure given seismic intensity that is observed from structural analysis. This work may aid earthquake engineering practice because, both the curve fitting and estimation uncertainty algorithms are implemented in a major update of an application-ready software tool made available at https://www.reluis.it/it/progettazione/software/r2r-eu.html
Risk‐targeted seismic design: Prospects, applications, and open issues, for the next generation of building codes
No full textImplications of GMPE’s structure for multi-site seismic hazard
No full textPrevious research has discussed the implications of the structure of a classical ground motion prediction equation (GMPE) on single-site probabilistic seismic hazard analysis and disaggregation. Classical refers to a GMPE where local site conditions (or any other factor) are accounted for via constant (with respect to magnitude and source-to-site distance) terms added to the mean and that do not affect the distribution of the residuals. Herein, the implications of such a structure of the GMPE are briefly discussed with respect to multi-site hazard assessment that, typically, requires a large number of simulations of random fields of ground motion intensity measures. It is shown that this type of GMPEs enables to run the simulations only once, independently of the soil conditions (or any other factor modeled in a similar way) eventually assigned to each site, which can represent a significant computational advantage in the case of spatially distributed assets
Fatality rates implied by the Italian building code
No full textThe project Rischio Implicito – Norme Tecniche per le Costruzioni (RINTC) assessed the seismic structural reliability, in terms of the annual rate of earthquakes causing failure, of a large set of code-conforming buildings, designed to be located in three different sites, representative of low, mid, and high seismic hazard in Italy. It was found that seismic reliability tends to decrease significantly as the site's hazard increases, despite the design actions having the same return period at all sites. Because this is a consequence of the code's approach, the simple study presented in this paper aims to contribute to the discussion on whether the code-implied safety is yet acceptable. To this end, the annual fatality rates due to the seismic failure of the buildings from the mentioned project are computed, in a simplified manner, and compared with the annual risk from other common causes of death in Italy; the latter obtained based on data from the Italian Statistical Institute. The results, although subjected to the conventionality of the working assumptions, seem to indicate that seismic fatality risk is generally lower than that of other causes of death, by one or more orders of magnitude at the lower hazard sites. This can contribute to the discussion on seismic structural safety due to the characteristics of the Italian code that are common to state-of-the-art codes internationally
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
È possibile evitare il superamento delle azioni di progetto nell’area epicentrale di terremoti forti?
No full textNo, praticamente non è possibile
- …
