Rebez, Alessandro

In recent years, new approaches for developing earthquake rupture forecasts (ERFs) have been proposed to be used as an input for probabilistic seismic hazard assessment (PSHA). Zone- based approaches with seismicity rates derived from earthquake catalogs are commonly used in many countries as the standard for national seismic hazard models. In Italy, a single zone- based ERF is currently the basis for the official seismic hazard model. In this contribution, we present eleven new ERFs, including five zone-based, two smoothed seismicity-based, two fault- based, and two geodetic-based, used for a new PSH model in Italy. The ERFs were tested against observed seismicity and were subject to an elicitation procedure by a panel of PSHA experts to verify the scientific robustness and consistency of the forecasts with respect to the observations. Tests and elicitation were finalized to weight the ERFs. The results show a good response to the new inputs to observed seismicity in the last few centuries. The entire approach was a first attempt to build a community-based set of ERFs for an Italian PSHA model. The project involved a large number of seismic hazard practitioners, with their knowledge and experience, and the development of different models to capture and explore a large range of epistemic uncertainties in building ERFs, and represents an important step forward for the new national seismic hazard model.

In the last two decades, several studies addressed the revaluation and homogenization of the Italian instrumental seismic catalog, but all of them refer to the time interval from 1981, that is, the starting year of the Catalogo Strumentale dei Terremoti Italiani (CSTI). At the time, the CSTI was conceived as the continuation of the catalog of the Progetto Finalizzato Geodinamica (PFG) but, over time, the PFG catalog was almost totally forgotten, and presently it is even difficult to obtain because it is not provided by any website. In this work, we integrate a genuine copy of PFG, with additional locations from the bulletins of the Istituto Nazionale di Geofisica (ING, now known as INGV) and of the International Seismological Centre (ISC) and with local magnitudes from two couples of Wood–Anderson (WA) seismometers operational in Italy in the 1970s and 1980s, mostly derived from a careful scrutiny of paper bulletins of the Osservatorio Geofisico Sperimentale (OGS) and of the ING. We restrict our analysis to the time interval from 1960 to 1980 because, based on various evidence, we can infer that within such period most instrumental magnitudes reported by the PFG catalog are reasonably coherent with the Richter’s definition. Magnitudes provided by WA stations and other data sources are calibrated with respect to Mw by general orthogonal regressions. The final catalog from 1960 to 1980 contains 8536 earthquakes, of which we compute a true or proxy Mw magnitude with related uncertainty for 6407. The analysis of the frequency–magnitude distribution indicates completeness for about Mw ≥4:0. This work extends the time coverage of the Italian instrumental catalog to about 55 yrs before the present, allowing the statistical study of some important seismic periods that occurred, for example, in 1962 (Irpinia), 1968 (Belice Valley), 1976 (Friuli), 1979 (Umbria), and 1980 (Irpinia).

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.

A seismic hazard map, with 10% probability of exceedance in 50 years in terms of macroseismic intensity, is proposed for the Italian territory. The input elements used to evaluate the seismic hazard are: the seismogenic zoning ZS9, the earthquake catalogue CPTI04, the historical and statistical completeness. These elements are those used in MPS04 (Gruppo di Lavoro MPS, 2004). Two new intensity attenuation models were used: 1) a set of regional relationships derived from a previous cubic model (Berardi et al., 1993) which has been recalibrated (Gómez Capera, 2006); 2) a relationship obtained with a new approach (Pasolini et al., 2006). The intensity attenuation models were obtained using the macroseismic intensity database, which was used for compiling CPTI04. The computer code adopted to evaluate the seismic hazard, with the elements cited above, is SeisRisk III, which has been modified to be used with macroseismic intensity data, i.e. allowing to consider the normal distribution of the residuals. A logic tree approach has been used to explore some possible alternatives of epistemic character.

The assessment of the completeness of historical earthquake data (such as, for instance, parametric earthquake catalogues) has usually been approached in seismology - and mainly in Probabilistic Seismic Hazard Assessment(PSHA) - by means of statistical procedures. Such procedures look «inside» the data set under investigation and compare it to seismicity models, which often require more or less explicitly that seismicity is stationary. They usually end up determining times (Ti), from which on the data set is considered as complete above a given magnitude (Mi); the part of the data set before Ti is considered as incomplete and, for that reason, not suitable for statistical analysis. As a consequence, significant portions of historical data sets are not used for PSHA. Dealing with historical data sets - which are incomplete by nature, although this does not mean that they are of low value - it seems more appropriate to estimate «how much incomplete» the data sets can be and to use them together with such estimates. In other words, it seems more appropriate to assess the completeness looking «outside » the data sets; that is, investigating the way historical records have been produced, preserved and retrieved. This paper presents the results of investigation carried out in Italy, according to historical methods. First, the completeness of eighteen site seismic histories has been investigated; then, from those results, the completeness of areal portions of the catalogue has been assessed and compared with similar results obtained by statistical methods. Finally, the impact of these results on PSHA is described.

The Italian "Gruppo Nazionale per la Difesa dai Terremoti" has conducted a project in recent years for assessing seismic hazard in the national territory to be used as a basis for the revision of the current seismic zonation. In this project the data on the major earthquakes were reassessed and a new earthquake data file prepared. Definition of a seismotectonic model for the whole territory, based on a structural-kinematic analysis of Italy and the surrounding regions, led to the definition of 80 seismogenic zones, for which the geological and seismic characteristics were determined. Horizontal PGA and macroseismic intensity were used as seismicity parameters in the application of the Cornell probabilistic approach. The main aspects of the seismic hazard assessment are here described and the results obtained are presented and discussed. The maps prepared show the various aspects of seismic hazard which need to be considered for a global view of the problem. In particular, those with a 475-year return period, in agreement with the specifications of the new seismic Eurocode EC8, can be considered basic products for a revision of the present national seismic zonation.