GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.1 73 with the geometric dimensions of the possible ruptures along the fault. According to this model, Wells and Coppersmith (1994) developed scaling laws between the fault-rupture dimensions and the related characteristic magnitude. The difficulty of associating earthquakes to faults does not allow the seismologists to ultimately decide if this model is valid worldwide, only for some faults, or nowhere. Entering into details of our work, the characteristic earthquake magnitude (considering an uncertainty of only +/- 0.3) and recurrence interval reported in DISS for the sources in NE Italy have been used (Tab. 2), removing the related range of seismicity from the rates of the FRI zones, that are used, in this case, as a sort of “background zone” as they collect the low to medium seismicity not linked to any specific fault. More precisely, the events in each “background zone”, with a magnitude less than the minimum characteristic magnitude of the faults inside that zone, have been fitted according to the G-R model and this seismicity has been treated in the PSHA in a zone-source fashion. Tab. 2 - The seismogenic sources of the DISS (DISS Working Group, 2010), used in this work, and their main characteristics. IDSOURCE SOURCENAME LENGTH WIDTH MIN MAX REC. INT. LATEST MAX DEPTH DEPTH EQ MAG ITIS101 Montello 22 11.2 1.0 8.2 513 Unknown 6.5 ITIS102 Bassano-Cornuda 18 9.5 1.0 6.4 1724 25/02/1695 6.6 ITIS108 Maniago 8 5.5 0.5 3.3 1941 10/07/1776 5.9 ITIS109 Sequals 16.5 9.0 1.0 6.8 5000 Unknown 6.5 ITIS112 Tramonti 6 4.5 1.0 3.6 660 07/06/1794 5.8 ITIS113 Monte Grappa 5 3.9 0.5 2.7 400 12/06/1836 5.5 ITIS119 Tarcento 6 4.5 2.0 4.3 793 11/09/1976 5.7 ITIS120 Gemona South 16 9.0 2.0 6.5 1148 06/05/1976 6.5 ITIS121 Montenars 8 5.5 2.0 5.2 1361 15/09/1976 6.0 ITIS122 Gemona East 10 6.4 6.5 10.2 1230 15/09/1976 6.1 ITIS124 Cansiglio 10 6.4 1.5 6.4 1154 18/10/1936 6.1 ITIS125 Polcenigo-Montereale 15 8.5 2.0 7.5 1282 29/06/1873 6.4 ITIS126 Medea 16 9.0 0.5 6.9 3571 Unknown 6.4 ITIS127 Thiene-Bassano 18 9.5 1.0 5.8 1500 Unknown 6.6 SIIS001 Idrija 50 12.6 1.0 13.4 2000 26/03/1511 6.8 SIIS002 Bovec-Krn 13 6.3 3.0 9.2 360 12/04/1998 5.7 Individual seismicity rates have been computed following the “higher not highest” method, developed for the seismic hazard map of the Italian territory developed in the framework of the activities of the Gruppo Nazionale per la Difesa dai Terremoti (Slejko et al. , 1998). Then, to be used in Crisis 2012 (Ordaz et al. , 2012), the parameters of the G-R relation (Aki, 1965; Utsu, 1965, 1966) have been derived by linear regression analysis. Different methodologies for assessing the b -value of the G-R relation are available in literature: in this work the maximum likelihood method has been applied (Aki, 1965; Utsu, 1965) according to the general routine proposed by Weichert (1980) which is suitable also for different completeness periods of the earthquake catalogue. For the computation of the maximum magnitude ( M max ), the “one step beyond” approach has been used. It extrapolates the observed seismicity rates by one step (in the present work 0.3 magnitude units) according to the G-R b-value of the SZ when the corresponding return