Probabilistic seismic hazard from historical macroseismic data : an application at the volcanic region of Mt. Etna (Southern Italy)

Abstract

Earthquakes are, by far, the most relevant source of hazard for the densely urbanised areas of Mt. Etna region. Local communities living in the eastern and southern flanks of the volcano continuously suffer social and economic losses due to the very high occurrence of damaging earthquakes, which produce intensities up to degree X EMS despite of low energy (M<5.0). Nevertheless, the assessment of seismic hazard at Etna is neglected in the practice at a national scale, being aimed to evaluate the areas more exposed to large crustal earthquakes in the perspective of improving seismic building codes. The state of art on the recent studies devoted to a detailed assessment of seismic hazard in the Etna region, is presented. The seismic hazard is performed following probabilistic approach (PSHA) by using macroseismic data (D’Amico and Albarello, 2007) as an alternative to the Cornell-McGuire methodology. The input dataset is the CMTE earthquake catalogue (Azzaro et al. 2000), covering a time-span of ca. 180 yrs, from which the site seismic histories, i.e. the database of intensity data available for a given locality, are derived. When historical information is missing, the completeness of the site seismic history is improved by integrating observed data with values calculated according to the decay of the intensity with distance from the epicenter. This step is faced with a procedure based on the Bayesian statistics (Zonno et al., 2009), which provides the probabilistic mode of binomial distribution of the intensity at a given site. Then, by a probability distribution considering the completeness of the catalogue and the uncertainty of intensity data, the seismic hazard is expressed in terms of the maximum expected intensity characterised by a 10% probability of exceedance over different exposure times (10 to 50 yrs). The results shows that if shorter exposure times are considered, the expected intensity due to local very frequent earthquakes reaches the same values (Iexp = IX EMS) of those determined by large, but rare, regional events.