Seismogenic sources in Central Italy: from causes to effects (original) (raw)
Comparison of probabilistic seismic hazard estimates in Italy
Bulletin of the …, 2008
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 (A max). 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 (I ref). 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 A max and I ref at these sites have been ranked in the respective domains. The spatial distribution of rank differences of A max and I ref 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.
In this paper the sensitivity analysis is performed as the preliminary step towards the construction a logic tree useful for probabilistic seismic hazard analyses of western Liguria (northwestern Italy). The sensitivity analysis is conducted for six strategic sites within the Imperia district, following a multi-parameter approach, and accounts for both mean hazard values and hazard values corresponding to different percentiles (e.g. 16 th and 84 th percentile). In this way not only is the effect of a parameter on mean hazard values determined but also its sensitivity for two percentiles. In the analysis presented in this paper, the influence of different seismic catalogues (epicentral parameters), source zone models, frequency-magnitude parameters, maximum earthquake magnitude values and attenuation relationships is considered. As a result, the sensitivity analysis can be used as objective criterion for the construction of a logic tree focusing efforts on the parameters with a greater effect on the hazard. The results, valid just for a medium-low-seismicity and short historical information area such as western Liguria and for a similar set of assumptions in the logic tree, show that the seismic catalogues and the frequency-magnitude parameters have the highest influence on mean PGA values corresponding to a 475-year return period and the 16 th percentile while the attenuation relationships have the highest effect on the 84 th percentile. Furthermore, we found that the seismogenetic zonations characterized by smaller source zones contribute most to the uncertainty in the hazard.
Bulletin of the Seismological Society of America, 2010
A seismic hazard map, in terms of macro seismic intensity with 10% probability of exceedance in 50 years, is proposed for the Italian territory. The input elements used to evaluate the seismic hazard are: the seismogenic zoning ZS9 (Meletti et al., 2007), the earthquake catalogue CPTI04 (Gruppo di lavoro CPTI04, 2004) and intensity attenuation relationships. The first two elements and the historical and statistical completeness of the catalogue are those used in the national seismic hazard map for Italy MPS04 (Gruppo di Lavoro MPS, 2004). Two intensity attenuation models are used: 1) one national relationship obtained with a new approach by Pasolini et al. (2006) and a relationship for the Etna volcanic zone proposed by Azzaro et al. (2006) 2) a set of regional relationships derived from a previous cubic model (Berardi et al., 1993) which is recalibrated in the present study using the macro seismic intensity database DBMI04 (Stucchi et al., 2007), which was used for compiling CPTI04. The computer code adopted to evaluate the seismic hazard, with the elements cited above, is SeisRisk III (Bender and Perkins, 1987), which has been modified within this study to incorporate the aleatory variability of the ground motion (macroseismic intensity). A logic-tree framework allowed to explore some possible alternatives of epistemic character. The seismic hazard map obtained in terms of intensity was subsequently transformed into PGA by means of a linear relation between intensity and PGA, in order to compare it with the recently national seismic hazard map MPS04.
Dowrick/Earthquake Resistant Design and Risk Reduction, 2009
The aim of the last task of the project was to combine all the products from other tasks and to perform a series of sensitivity studies on models and/or parameters to evaluate their impact on the seismic hazard assessment, according to the approach of the seismotectonic probabilism, originally proposed by Cornell (1968). The final step will be to evaluate the impact of the employment of the hybrid GMPEs in the assessment of the seismic hazard. Some significant test sites in Southern Italy has been selected because of the presence of critical infrastructures (Milazzo ME, Priolo Gargallo SR, Gioia Tauro RC) to test the variability of the results at different return periods, depending on the relevance of the infrastructure studied and corresponding to the Serviceability Limit State for damage control (SLD) and the Ultimate Limit State for collapse prevention (SLC) of the European EC8 (CEN, 2004) and Italian NTC08 (CS.LL.PP., 2008) seismic codes. Any result included in the document is based on the available scientific knowledge and is devoted to qualified users. Every risk due to the improper use of data or the use of inaccurate information is assumed by the user.
Some aspects of seismic hazard assessment when comparing different approaches
Natural Hazards, 1994
The Cornell and Gumbel approaches to probabilistic seismic hazard assessment have been applied in northeastern Italy to test the influence of various input parameters. Problems related to earthquake source geometry, seismicity descriptors, and attenuation of two ground-motion parameters of engineering interest (peak ground acceleration and macroseismic intensity) have been analyzed. The results seem to be very sensitive to this last variable (attenuation), while different methodologies can lead to very similar evaluations, if properly applied. Properly applied means that all the input parameters are prepared to satisfy the conditions of the chosen approach. In addition, the seismotectonic knowledge of the study region conditions the choice of approach to be applied.
Layered Seismogenic Source Model and Probabilistic Seismic-Hazard Analyses in Central Italy
Bulletin of The Seismological Society of America, 2006
We defined a seismogenic model for central Italy based on three layers of sources and computed the relative seismic-hazard maps. One layer is constituted by individual structures liable to generate major earthquakes (M Ն5.5). We defined them as seismogenic boxes by using geological information in terms of plan projection of active faults; the seismicity rates associated with an individual source are based on the geometry and kinematics of the fault; the recurrence model is controlled by the earthquake-source association, and, when possible, we defined the occurrence time of the last major event, using it in a time-dependent approach. Another layer is given by the instrumental seismicity analysis of the past two decades, which allows us to evaluate the background seismicity (M ϳϽ5.5); using a sliding-window selection of events, we defined a model of regular adjacent cells of variable a and b values of the Gutenberg-Richter relation. The last layer utilizes all the instrumental earthquakes and the historical events not correlated to known structures (4.5 Ͻ M ϳϽ6), by separating them into seismotectonic provinces shaped on a geological-structural basis. The seismic-hazard computations first use this layered model in a traditional probabilistic scheme. The results indicate a narrow belt of peak ground acceleration (PGA) higher than 0.30g (with standard deviation in attenuation functions) in the axial part of the Apennine chain, with a maximum spot of PGA Ͼ0.40g southeast of the area damaged by the 1997-1998 Umbria-Marche sequence (PGA expected not to be exceeded in 50 years at 90% probability level). The background seismicity gives a nonnegligible contribution to the hazard, at least for first damage levels. Then, a simplified time-dependent hypothesis has been introduced for the individual sources alone, computing the conditional probability of occurrence of characteristic earthquakes for each source by Brownian passage time distributions. Adopting equivalent fictitious seismicity rates, we obtained maps referring to the next 50 years by using traditional codes. These results show that the contribution of the recently active sources vanishes, and the most hazardous sites are now located south of L'Aquila and in the Sulmona area. We consider that the methodology and results obtained are useful for seismic risk reduction strategies.
Seismic hazard mapping for administrative purposes
2001
Local soil conditions, roughly summarised by considering a reference soil for each municipality of the Friuli -Venezia Giulia region in NE Italy, are introduced into probabilistic seismic hazard estimates: the subsequent improvement is checked by comparing these new results and the maximum observed intensities in each municipality to investigate if the major differences between probabilistic estimates and actually observed data can be explained by local site effects and/or by the geometry of the seismogenic zones used in the computation. In addition, a comparison between the new probabilistic hazard results, and the standard ones referred to rock is made for the present and the proposed Italian seismic zonation. The results underline the influence of the seismogenic model used, but are not determinant on the role of site effects.
Bollettino di Geofisica …, 2011
The identification of the causative faults in south-eastern Sicily (Italy) is a complicated task although the seismicity of the area has been well studied for years. For this reason a seismic hazard assessment was carried out for the Catania and Siracusa towns providing a comprehensive re-examination and re-processing of all the available seismic data. The site approach and the seismotectonic one were used and compared. The hazard assessment, using both methods, was performed following a logic-tree approach in order to consider and reduce the epistemic uncertainties. Data available in the literature, coming out from recent studies concerning the parametric earthquake catalogue and a complete database of site intensity in Italy were used. For the seismotectonic approach, two source zone models were considered for southeastern Sicily. In the first model, the same source zone defined for the current Italian reference seismic hazard map is used; in the second model, two separate zones are considered, one includes the normal-fault structures located in the eastern part of the Hyblean area, and the other roughly corresponding to the western portion of the Hyblean front. The combined use of these approaches allowed us to obtain useful elements to define the seismic hazard in Catania and Siracusa. When the seismic history site is used, the town of Catania shows hazard values higher than the ones found for Siracusa, for each considered time interval. On the contrary, when the seismotectonic method is used, the hazard curves show a different behaviour according to the different geometry and size of the adopted source zones. The comparison between the results obtained through the two approaches is recommended since it allows us to verify the robustness of the hazard estimates performed.