GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 2.1 317 - the considered instrumental data, new and/or updated; - the energy thresholds, lowered to intensity 5 or magnitude 4.0 (instead of 5-6 and 4.5, respectively); - the determination of parameters from macroseismic data, based on a new calibration of the Boxer method (Gasperini et al., 1999); - the instrumental magnitudes, resulting from new sets of data and new conversion relationships. The catalogue covers almost the entire Italian territory together with some neighboring areas and seas. It collects 4584 earthquakes in the time period 1000-2014. Earthquake magnitude is expressed in terms of moment magnitude (Mw). For all earthquakes in the catalogue, the related uncertainty is provided. Originally, the CPTI15 catalogue includes foreshocks and aftershocks. Therefore, it should be deprived of dependent events in order to reflect the Poisson assumption underlying the seismicity process. This was achieved by using a table/map for the completeness/declustering of the catalogue provided by the CPTI15 working group (Fig. 2). For the zonation presented here (A1: Rebez et al. , 2016), a logic tree of six branches is considered (Fig. 2): three branches are considered to account for the epistemic uncertainty in the seismicity model and two branches are related to alternative Mmax values. Concerning the node relative to the seismicity model, one branch accounts for individual rates (I-R) while the remaining two use different approaches to compute the values of the GR coefficients ( a and b -values): one uses the least mean square approach (GR-LS) and one adopts the Maximum Likelihood method of Weicher (1980) (GR-ML). Although the least-squares method is often used, it is not formally suitable since magnitude is not error free, cumulative event counts are not independent, and the error distribution of the number of earthquake occurrences does not follow a Gaussian distribution. The maximum likelihood method has been widely applied: Weichert (1980) proposed a general routine suitable also for different completeness periods of the earthquake catalogue. For these reasons a weight of 0.4 is applied to the ML branch and only 0.2 to the LS one (Fig. 2). The maximum magnitude was evaluated for macro areas representing portions of the Italian territory and surroundings for which it is expected a homogeneous tectonic behaviour. It represents the magnitude value for which the rate of occurrence is considered negligible for 30-5000 year mean return periods. The data used for the maximum magnitude assessments are the historical seismic catalogue (CPTI15) and the composite seismogenic database (DISS 3.2.0). Mmax1 is equal to the highest magnitude observed or computed (from Fig. 2 – A1 zonation model: logic tree.