GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.1 35 Seismic hazard map in the Umbria and Marche Regions: from the present underestimated assessment to a new more reliable evaluation E. Mantovani 1 , M. Viti 1 , D. Babbucci 1 , N. Cenni 2 1 Dipartimento di Scienze Fisiche, della Terra e dell’Ambiente, Università di Siena, Italy 2 Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Bologna, Italy Introduction. Some zones of the Umbria and Marche Regions have been hit since 1000 A.D. by numerous strong earthquakes that have reached intensity XI (MCS), involving an high number of victims and very large damages (Tab.1). Tab. 1 - List of major earthquakes (M≥5.5 or I≥VIII) occurred in the Umbria and Marche Regions since 1000 A.D. (from Rovida et al. , 2011). Year Epicentral zone M Imax 1276 Orvietano 5.6 VIII-IX 1279 Camerino 6.3 X 1298 Reatino 6.2 X 1328 Norcia 6.4 X 1352 Monterchi 6.4 IX 1353 Sansepolcro 6.0 IX 1389 Bocca Serriola 6.0 IX 1458 Val Tiberina 5.8 VIII-IX 1599 Valnerina 6.0 IX 1639 Amatrice 5.9 X 1672 Riminese 5.6 VIII 1690 Anconetano 5.6 VIII-IX 1695 Bagnoregio 5.7 IX 1703 Appennino 6.7 XI Umbro-Reatino 1703 Aquilano 6.7 X 1730 Valnerina 5.9 IX 1741 Fabrianese 6.2 IX 1747 Nocera Umbra 5.9 IX Year Epicentral zone M Imax 1751 Appennino 6.3 X Umbro-Marchigiano 1781 Cagliese 6.4 X 1785 Umbria 5.7 VIII-IX meridionale 1789 Val Tiberina 5.8 IX 1799 Appennino 6.1 IX-X Marchigiano 1832 Valle Del Topino 6.3 X 1859 Norcia 5.5 VIII-IX 1916 Alto Adriatico 6.0 VIII 1916 Alto Adriatico 6.1 VIII 1917 Valtiberina 5.9 IX-X 1930 Senigallia 5.8 VIII-IX 1943 Marche meridionali 5.8 IX Abruzzo 1979 Valnerina 5.9 VIII-IX 1997 Appennino 6.0 IX Umbro-Marchigiano 1997 Appennino 5.7 VIII-IX Umbro-Marchigiano This seismic history would suggest the opportunity of adopting an adequate strategy of defence from the eventual occurrence of shocks comparable to the past strongest ones. However, this reasonable intention is not encouraged by the hazard map (Fig.1) which has been obtained by the probabilistic procedure presently adopted in Italy (Gomez Capera et al. , 2010). When expressed in term of macroseismic intensity, such procedure predicts for the above Regions a low probability (<10%) for the occurrence of shocks with I >VIII in the next 50 years. Only for a small zone of southeastern Umbria an higher intensity value (IX) is predicted. Since the damage associated to intensity VIII is not high and above all can hardly provide victims (unless very week buildings are involved), the present hazard map could produce a dangerous drop of attention for seismic risk. This is mainly due to the fact that the approach presently adopted for hazard assessment (Cornell, 1968) arbitrarily assumes that the probability