GNGTS 2014 - Atti del 33° Convegno Nazionale

20 GNGTS 2014 S essione 1.1 ESTIMATing THE EXPECTED SEISMICITY RATES of VOLCANO-TECTONIC earthquakes at mT. ���� �������� �� � ������������������� �������� ETNA (ITALY by a geometric-kinematic approach R. Azzaro 1 , S. D’Amico 1 , B. Pace 2 , L. Peruzza 3 1 INGV Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Catania, Italy 2 DiSPUTer Università degli Studi “G. d’Annunzio”, Chieti-Pescara, Italy 3 OGS Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Sgonico (TS), Italy Introduction . ������� ������ ������� ���� ���� ���������� �� ���� ������� �� ��� ���� Seismic hazard studies have been undertaken at Etna volcano in the last years with the aim of estimating the potential of local fault’s activity in generating destructive earthquakes. The target is the mid-term assessment (30, 20, 10 and 5 yrs), as the identification of zones that are exposed to the recurrent seismic shaking may be important for land planning at a local scale, and it represents a valuable complement to establish priority criteria for seismic risk reduction action. The methodologies applied at Mt. Etna area include probabilistic approaches based on the use of historical macroseismic data (the “site approach” by the software code SASHA, see Azzaro et al ., 2008) and fault-based time-dependent models in which occurrence probabilities of major earthquakes are estimated through the Brownian Passage Time (BPT) function and the time lapsed since the last event (Azzaro et al. , 2012b, 2013b). Mean return period of major earthquakes - strong to destructive events with epicentral intensity I 0 ≥ VIII EMS, considered as “proxies” of “characteristic” earthquakes – have been obtained by the fault seismic histories, i.e. the associations “earthquake-seismogenic fault” derived from the historical catalogue of Etnean earthquakes (CMTE Working Group, 2014). Inter-time statistics of major earthquakes have been applied to the Timpe tectonic system, considered as a homogeneous seismotectonic domain (Azzaro et al. , 2013b), obtaining a mean recurrence time ( T mean ) of 71.3 years, and an aperiodicity factor α (σ T mean / T mean ) = 0.42, typical of semi-periodic processes. In the present study we present the preliminary results of an analysis aimed at verifying the variability of the mean occurrence times of major earthquakes generated by the main tectonic systems at Etna (Pernicana and Timpe faults) by using a geological approach based on geometric- kinematic parameters (3D dimensions, slip-rates etc) representative of fault activity. Method and input data. ��� �������� ��� ���� ������� ��� ������� ��� �������� ���� The analysis has been carried out through the software code E rror P ropagation (hereinafter EP), a Matlab® routine produced in the framework of the projects DPC-INGV S2 in order to quantify the seismic activity from geometry and slip-rates of a fault (Peruzza et al. , 2010; Pace et al. , 2013). We used the Beta version 0.15 implemented for this work with new earthquake scaling relationships for volcanic contexts. The adopted approach is based on the assumption of seismicmoment conservation. It imposes the condition that the total amount of seismic moment released by an individual seismogenic fault does not exceed the seismic moment released by the maximum expected magnitude alone. The budget is obtained by fixing the seismic moment rate of the maximum expected earthquake and by scaling the occurrence of each magnitude class properly, and can be adopted for both gaussian and G-R linear distributions. In the probabilistic procedure for calculating the seismic hazard, the mean recurrence time ( T mean ) of the maximum magnitude ( M max ) expected on a fault, together with the quantification of its variability, are the basic ingredients to compute occurrence earthquake probabilities, both under Poissonian assumptions as well as in a time-dependent perspective. The best situation for a given fault segment is to have a long list of associated events, so that mean and variability derive directly from observations. The real situations in Italy of multiple characteristic events occurring on the same fault segment are definitely few, mostly represented by recent active sources along the central Apennines (e.g., Paganica fault, Galli et al. , 2010; Cinti et al. , 2011; Moro et al. , 2013). More favourable conditions are present at Etna, where some ten major earthquakes ( M l 4.3-5.2) repeatedly occurred along fault segments of the Timpe system (Azzaro et al. , 2012b).