ALBERT

Description: The evaluation of seismic hazard over wide territories is a basic tool for planning activities aimed at earthquake damage mitigation. This is commonly performed through probabilistic approaches based on the statistical analysis of past seismicity. Among these, due to its wide application worldwide, the Cornell-McGuire approach (Cornell 1968; McGuire 1978) has become a kind of “standard” methodology for probabilistic seismic hazard assessment (PSHA). In Italy, several national seismic hazard maps were produced in recent years (Slejko et al. 1998; Albarello et al. 2000; MPS Working Group 2004) by following this procedure as implemented by Bender and Perkins (1987). Yet despite its widespread application, this standard methodology presents severe drawbacks due to its strong sensitivity to some ill-defined aspects, such as geometry of seismic sources, attenuation of ground motion with distance from the source, completeness of available seismic catalogs, etc. Moreover, this kind of approach does not allow the full exploitation of a large amount of documentary data available at the site about the seismic effects of past earthquakes (Albarello and Mucciarelli 2003). Another drawback is that the standard approach was developed with the assumption that the seismicity database used to feed the computational model is constituted by instrumental data (magnitude, epicentral locations, etc.). However, in many countries (first and foremost, Italy) the bulk of the seismic database is constituted by macroseismic data, and thus the application of the standard method requires a “forcing” of macroseismic information into a para-instrumental format. But macroseismic information is not isomorphic to instrumental data since intensity values are discrete, ordinal, and range-limited. This implies that, in principle, mathematical formalizations suitable to instrumental information cannot be used to manage macroseismic data (see, e.g., Pasolini et al. 2008a, 2008b). To overcome some of these difficulties and to better exploit available information, probabilistic hazard evaluations based on observed intensity data were performed in Europe (Monachesi et al. 1994; Papoulia and Slejko 1997; Azzaro et al. 1999; Albarello et al. 2002) and Japan (Bozkurt et al. 2007) using alternative numerical procedures. An apparent limitation of these studies is the fact that PSH estimates are provided in terms of intensity, and this conflicts with the fact that ground acceleration still remains the traditional output of PSHA devoted to seismic design. However, a new interest has recently grown around macroseismic intensity. In fact, when damage scenarios and post-earthquake emergency planning are of concern, hazard assessment in terms of intensity as ground-shaking measure may be more suitable than conventional estimates based on instrumental parameters (PGA, etc.). A further possible advantage of these kinds of approaches is that they provide hazard evaluations completely independent from the standard ones and more directly linked to empirical observations (local seismic history). Thus, they could represent a useful benchmark for a direct assessment of reliability of traditional PSH estimates (Mucciarelli et al. 2000, 2006, 2008; Bozkurt et al. 2007). In this paper we present the computer program SASHA (Site Approach to Seismic Hazard Assessment), which implements the intensity-based PSHA procedure originally proposed by Magri et al. (1994) and then improved by Albarello and Mucciarelli (2002). It relies on the analysis of the site seismic history, i.e., the dataset of seismic effects (macroseismic intensities) documented during past earthquakes at a given locality. This methodology (hereafter, site approach) has been specifically developed to handle macroseismic data, and thus both the peculiar nature of intensity values (discrete, ordinal, range-limited) and relevant uncertainty (ill-defined intensity values, completeness of site seismic history, etc.) are taken into account by a coherent statistical approach that does not require any assumption about earthquake recurrence model and seismic source geometry. Furthermore, no aftershock removal is required in advance and epicentral data are only considered to integrate (when necessary) felt data at the site. Several PSHA studies have been performed in the last decade in Italy using different versions of the site approach (Mucciarelli et al. 2000; Albarello et al. 2002; D’Amico and Albarello 2003; Albarello, Azzaro et al. 2007; Azzaro et al. 2008). SASHA’s theoretical background is briefly outlined in the next section of the paper. Then, we describe the most important features of SASHA along with a sample application to the Italian area.

Description: Published

Description: 663-671

Description: 4.2. TTC - Scenari e mappe di pericolosità sismica

Description: JCR Journal

Description: reserved

Description: Macroseismic intensity has recently attracted attention as a tool for validating probabilistic seismic hazard assessment (PSHA) studies or as an alternative method for PSHA in countries where the historical catalog is much longer than the instrumental one. In Italy, the new seismic hazard map was recently produced using the Cornell–McGuire approach in terms of the peak ground acceleration characterized by a 10% exceedance probability for an exposure time of 50 yr (Amax). We compare this map with an alternative one, produced using a different approach based on a nonparametric and zonation-free statistical analysis of local seismic histories. In this case, results are expressed in terms of the maximum intensity corresponding to an exceedance probability of not less than 10% for an exposure time of 50 yr (Iref ). In order to compare the two maps, we selected 1401 control sites, where local seismic history includes at least 10 intensity values relative to felt effects documented during past earthquakes. The values of Amax and Iref at these sites have been ranked in the respective domains. The spatial distribution of rank differences of Amax and Iref values shows a strong correlation with the seismogenic zoning used in the calculation of PSHA following the Cornell–McGuire approach. This suggests that the adopted zoning could be incomplete (some further “hidden” sources may exist) and too rough to capture actual seismogenic sources. Because more detailed zoning is prevented by the amount of data available, the results obtained suggest the preference of zonation-free approaches for seismic hazard assessment in Italy. Furthermore, among the possible zonation-free approaches, those that offer better exploitation of local information about the effects of past earthquakes would be preferred.

Description: Published

Description: 2652–2664

Description: 4.2. TTC - Scenari e mappe di pericolosità sismica

Description: JCR Journal

Description: reserved

Description: Nel 2013 il progetto CE “SHARE” (Seismic Hazard Harmonization in Europe; http://www.shareeu. org/) ha prodotto un modello di pericolosità sismica per l’area europea, i cui risultati sono tuttora in via di distribuzione. Questi risultati rappresentano di fatto la prima valutazione di pericolosità sismica completa per il territorio italiano a partire dal 2006, anno in cui è stato rilasciato il modello di pericolosità di riferimento nazionale (MPS04-S1, http://esse1-gis.mi.ingv.it/). In questo lavoro si propongono alcune analisi delle differenze fra i due modelli, che mostrano come ad alcune differenze nei valori di PGA (10% di probabilità di eccedenza in 50 anni) si accompagnino differenze, anche di segno opposto, negli spettri in accelerazione a pericolosità uniforme. Le analisi mostrano anche che i tassi di sismicità di SHARE sono pressochè simili a quelli di MPS04-S1 e che le ragioni delle differenze più significative sembrano da ricercare nei diversi modelli di attenuazione del moto sismico utilizzati nei due studi.

Description: Published

Description: 15-25

Description: 3T. Pericolosità sismica e contributo alla definizione del rischio

Description: N/A or not JCR

Description: restricted

Description: In the frame of the Italian research project INGV-DPC S2 (http://nuovoprogettoesse2.stru.polimi.it/), funded by the Dipartimento della Protezione Civile (DPC; National Civil Protection Department) within the agreement 2007-2009, a tool for probabilistic seismic hazard assessment (PSHA) was developed. The main goal of the project was to provide a flexible computational tool for PSHA; the requirements considered essential for the success of the project included: •ability to handle both stationary and non-stationary earthquake time-occurrence models; •ability to use ground-motion prediction models that are not parametric equations but probabilistic "footprints" of the intensities generated by earthquakes of known magnitude and focal characteristics. Usually, these footprints are results of ground motion simulations. Some commonly used programs (e.g., FRISK, by McGuire, 1978; SEISRISK III, by Bender and Perkins, 1987) and more recent and state-of-the-art tools (e.g. OpenSHA, by Field et al., 2003, http://www.opensha.org; OpenQuake, http://openquake.org) for PSHA were analyzed. It was decided to focus on CRISIS2007, which was already a mature and well known application (e.g., Kalyan Kumar and Dodagoudar, 2011; Teraphan et al., 2011; D’Amico et al., 2012; see also http://ecapra.org/CRISIS-2007), but also suitable for additional development and evolution since its source code is freely available on request. The computational tool resulted in an extensive redesign and renovation of the previous CRISIS2007 version. CRISIS is a computer program for PSHA, originally developed in the late 1980's using Fortran as programming language (Ordaz, 1991). In this format, still without a graphical user interface (GUI), it was distributed as part of SEISAN tools (Ottemöller et al., 2011). Ten years later, a GUI was constructed, generating what was called CRISIS99 (Ordaz, 1999). In this version, all the graphic features were written in Visual Basic, but the computation engine remained a Fortran dynamic link library. The reason for the use of mixed-language programming was that computations in Visual Basic were extremely slow. Around 2007 the program was upgraded, in view of the advantages offered by the object-oriented technologies. An object-oriented programming language was required and the natural choice was Visual Basic.Net. In the new version (called CRISIS2007), both the GUI and the computation engine were written in the same language. Finally, in the frame of the mentioned S2 project, starting from 2008, the program was split into two logical layers: core (CRISIS Core Library) and presentation (CRISIS2008). In addition, a new presentation layer was developed for accessing the same functionalities via Web (CRISISWeb). It is worth noting that CRISIS has been mainly written by people that are, at the same time, PSHA practitioners. Therefore, the development loop has been relatively short, and most of the modifications and improvements have been made to satisfy the needs of the developers themselves.

Description: Italian Presidenza del Consiglio dei Ministri, Dipartimento della Protezione Civile (DPC).

Description: Published

Description: 495-504

Description: 4.2. TTC - Modelli per la stima della pericolosità sismica a scala nazionale

Description: JCR Journal

Description: restricted