ALBERT

Description: Results of probabilistic seismic hazard analysis (PSHA) depend on the soil conditions of the site investigated. Consequently, it is generally expected that disaggregation, usually employed to gather further information about hazard levels of interest, changes with the soil class. This short note discusses the relationship between hazard curves and disaggregations computed for different soil conditions at the same site. In particular, it is analytically proven that there are cases, depending on the structure of the ground-motion prediction equations employed, in which disaggregations for different soil conditions are necessarily invariant. It is also demonstrated that, in these situations, hazard curves for different soil conditions can be immediately obtained from a curve developed for a reference soil class. The analytical proofs derived do not require any further testing or validation; nevertheless, the results are illustrated via simple case studies to show how they may imply applicative advantages both in the cases of single-model and logic tree PSHA.

Description: Seismic hazard assessment, in its classical format, models the stochastic process of occurrence of earthquakes causing the exceedance of ground-motion intensity measure (IM) thresholds at a specific site. This is because civil structures typically require probabilistic seismic hazard analysis (PSHA) for one location. On the other hand, there are cases in which it may be required to count the number of exceedances of a vector of IM thresholds at multiple sites over time. In these situations, in general, a form of stochastic dependence arises among the processes counting multiple earthquake exceedances of IM at the sites. The present study analyzes this dependence and shows how it is linked to the correlation among IMs at the sites in one earthquake. Indeed, it provides a formalization and derives closed-form solutions for multisite PSHA, showing that hazards at multiple sites are independent if and only if exceedances at the sites in one earthquake are mutually exclusive. The other key results of the work are: (1) probabilistically rigorous insights into the form of dependence among hazard at multiple sites are derived, and (2) it is shown that site-specific and multisite PSHA are unified, that is, how and why the latter is a special case of the former when only one location is considered. In addition, the applicative value of the formalization provided is illustrated by means of simple examples, including a loss assessment for a portfolio of structures and a hazard validation exercise.

Description: 〈span〉〈div〉Abstract〈/div〉Sequence‐based probabilistic seismic hazard analysis (SPSHA) allows us to account for the effect of aftershocks in the assessment of seismic structural‐design actions (〈a href="https://pubs.geoscienceworld.org/bssa#rf10"〉Iervolino 〈span〉et al.〈/span〉, 2014〈/a〉, 〈a href="https://pubs.geoscienceworld.org/bssa#rf9"〉2018〈/a〉). In fact, it generalizes classical probabilistic seismic hazard analysis (PSHA; 〈a href="https://pubs.geoscienceworld.org/bssa#rf5"〉Cornell, 1968〈/a〉), combining it with aftershock‐PSHA (〈a href="https://pubs.geoscienceworld.org/bssa#rf19"〉Yeo and Cornell, 2009〈/a〉). 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).〈/span〉

Description: Earthquakes are typically clustered both in space and time. Only mainshocks, the largest magnitude events within each cluster, are considered by classical seismic hazard, which is expressed in terms of rate of exceedance of a ground-motion intensity measure at a site of interest ( Cornell, 1968 ). This kind of probabilistic seismic hazard analysis (PSHA) is used for structural design or assessment in the long term. Recently, for short-term risk management purposes, a similar approach has been adopted to perform aftershock probabilistic seismic hazard analysis (APSHA), conditional to mainshock occurrence ( Yeo and Cornell, 2009 ). PSHA often refers to a homogeneous Poisson process to describe event occurrence, whereas APSHA models aftershock occurrence via a conditional nonhomogeneous Poisson process, the rate of which depends on the magnitude of the mainshock that has triggered the sequence. On the other hand, the clusters, each of which is composed of the mainshock and the following aftershocks, may be seen as single events occurring at the same rate of the mainshocks. This may allow accounting for aftershocks in hazard analysis in a relatively simple manner, as first argued by Toro and Silva (2001) and further investigated by Boyd (2012) . In fact, this short note, focusing on the probabilistic aspects, shows the feasibility of analytically combining results of PSHA and APSHA to get a seismic hazard integral accounting for mainshock–aftershocks seismic sequences, which was still missing from the mentioned studies. The results of the application presented help to preliminarily assess the increase in seismic hazard in terms of rate of occurrence of events causing the exceedance of an acceleration threshold (e.g., that considered for structural design) also considering aftershocks. That is a relevant aspect from the earthquake engineering perspective.

Description: Quantification of regional seismic risk is based on spatially correlated random fields and requires modeling of the joint distribution of ground-motion intensity measures at all sites of interest. In particular, when a portfolio of buildings or a transportation/distribution network (lifeline) is of concern, correlation models for elastic spectral acceleration (SA) may also be required in order to estimate the expected loss in case of seismic events. The presented study focuses on semi-empirical estimation of spatial correlation as a function of intersite separation distance. In fact, this paper complements, and is based on, preceding work of the authors referring to spatial correlation of peak ground acceleration and velocity ( Esposito and Iervolino, 2011 ). The evaluation of correlation for ground-motion residuals was performed on data from multiple earthquakes, considering different ground-motion prediction equations fitted to the same records. Correlation analyses, carried out through geostatistical tools, considered two datasets: the Italian Accelerometric Archive and the European Strong-Motion Database. Results appear generally consistent with previous research on the same topic. Finally, simple relationships providing the correlation range of intraevent residuals of SA, as a function of structural period, were derived for each dataset. The developed models are useful for earthquake engineering applications where spatial correlation of peak ground motion is required.

Description: The study presented in this paper is among the first in a series of studies toward the engineering validation of the hybrid broadband ground-motion simulation methodology by Graves and Pitarka (2010) . This paper provides a statistical comparison between seismic demands of single degree of freedom (SDoF) systems subjected to past events using simulations and actual recordings. A number of SDoF systems are selected considering the following: (1) 16 oscillation periods between 0.1 and 6 s; (2) elastic case and four nonlinearity levels, from mildly inelastic to severely inelastic systems; and (3) two hysteretic behaviors, in particular, nondegrading–nonevolutionary and degrading–evolutionary. Demand spectra are derived in terms of peak and cyclic response, as well as their statistics for four historical earthquakes: 1979 M w  6.5 Imperial Valley, 1989 M w  6.8 Loma Prieta, 1992 M w  7.2 Landers, and 1994 M w  6.7 Northridge. The results of this study show that both elastic and inelastic demands from simulated and recorded motions are generally similar. However, for some structural systems, the inelastic response to simulated accelerograms may produce median demands that appear different from those obtained using corresponding recorded motions. The magnitude of such differences depends on the SDoF period, the nonlinearity level, and, to a lesser extent, the hysteretic model used. In the case of peak response, these discrepancies are likely due to differences in the spectral shape, while the differences in terms of cyclic response can be explained by some integral parameters of ground motion (i.e., duration-related). Moreover, the intraevent standard deviation values of structural demands calculated from the simulations are generally lower than those given by recorded ground motions, especially at short periods. The assessment of the results using formal statistical hypothesis tests indicates that, in most cases, the differences found are not significant, increasing the trust in the use of simulated motions for engineering applications.

Description: Since August 2016, central Italy has been struck by one of the most important seismic sequences ever recorded in the country. In this study, a strong-motion data set, consisting of nearly 10,000 waveforms, has been analyzed to gather insights about the main features of ground motion, in terms of regional variability, shaking intensity, and near-source effects. In particular, the shake maps from the three main events in the sequence have been calculated to evaluate the distribution of shaking at a regional scale, and a residual analysis has been performed, aimed at interpreting the strong-motion parameters as functions of source distance, azimuth, and local site conditions. Particular attention has been dedicated to near-source effects (i.e., hanging wall/footwall, forward-directivity, or fling-step effects). Finally, ground-motion intensities in the near-source area have been discussed with respect to the values used for structural design. In general, the areas of maximum shaking appear to reflect, primarily, rupture complexity on the finite faults. Large ground-motion variability is observed along the Apennine direction (northwest–southeast) that can be attributed to source-directivity effects, especially evident in the case of small-magnitude aftershocks. Amplifications are observed in correspondence to intramountain basins, fluvial valleys, and the loose deposits along the Adriatic coast. Near-source ground motions exhibit hanging-wall effects, forward-directivity pulses, and permanent displacement.