GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.1 49 for any PSHA. Richer and more homogeneous is the information about past earthquakes and better and more stable will be the results of the seismic hazard computation. To obtain this, we started from the available data, that are reported in Tab. 1. The first step consisted in merging the CPTI04 (Gruppo di Lavoro CPTI, 2004) and the recent CPTI11 (Rovida et al. , 2011) catalogues for the study area, limited by the corner coordinates 38.3 N and 15.3 E to 40.15 N and 17.70 E. The CPTI11 catalogue (Rovida et al. , 2011) has been the preferred choice in case of duplicity. We have obtained in such a way a catalogue of 264 earthquakes, including 42 events from CPTI04 (Gruppo di Lavoro CPTI, 2004). These latter records refer to deep events or earthquakes which occurred before 1000 A.D. Regarding the recent instrumental part of the catalogue, available data from the Istituto Nazionale di Geofisica e Vulcanologia (INGV) [i.e., CSI (Castello et al. , 2006) and ISIDE (ISIDe Working Group, 2011) catalogues] and the hypocentral locations reported in the Regional Seismic Network Catalogue of the University of Calabria (Università della Calabria, 2009) have been analyzed, fixing the threshold magnitude at M ≥2.5. To identify the common events between the INGV data set (2093 records) and the Calabria Network list (1588 records), a Fortran code based on the event origin time comparison has been applied. Several tests with a “matching time key” of 4 s, 3 s, 2 s and 1 s have been performed. At the end of the selection phase, 802 earthquakes were identified as common events, 1256 individual events from the INGV catalogue, and 787 individual events extracted from the catalogue of the Calabria Network. Concerning the double entries, the preference was given to the earthquakes located by the Calabria Network, considered to be more accurate due to the larger number of local stations used (including data from the INGV network). The final instrumental catalogue refers to the years 1981 to 2011 and consists of 2845 events. Tab. 1 - Historical and instrumental catalogues and data sets considered in this study. Catalogue type Name Time span Historical CPTI04 -217-2002 Historical CPTI11 1005-2006 Instrumental CSI - INGV 1981-2002 Instrumental ISIDe - INGV 2001-2011 Instrumental Rete Calabria (RSRUC) 1986-2009 After assembling the catalogue, it was necessary to remove the aftershocks because the standard probabilistic approach (Cornell, 1968) treats seismicity as a Poisson process, i.e., composed only by main events. More problematic is the foreshocks elimination, because no indication on how to do it automatically is available in literature and they are very limited in number, if any. The elimination of the dependent events has been done according to the Gardner and Knopoff (1974) approach (with the original space and time windows calibrated for California), i.e., by defining a space – time window within which the events are not considered independent. The final declustered catalogue for the studied area consists of 2306 events. Several time intervals have been considered to analyze the temporal distribution of the earthquakes in the catalogue. In this analysis of completeness, it was noted that earthquakes with magnitude greater than 6 are listed since very ancient times. In particular, earthquakes greater than, or equal to, magnitude 7 have been reported since 1600. The information on earthquakes with magnitudes between 4 and 5 appears to be continuous only from the beginning of the last century. Smaller earthquakes and those acquired by modern seismic networks provide continuous information from the late 1980s. The logic tree for PSHA . The quantification of the uncertainties (McGuire, 1977) is a crucial point in the calculation of PSHA. The aleatory variability accounts for the natural randomness in the earthquake process while the epistemic uncertainty (McGuire and Shedlock,