GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.1 99 Boschi et al. (1995) modelled historical seismicity by Poissonian and Gaussian models, in order to calculate the probability of strong earthquakes ( M ≥5.9) in various Italian source zones for the next 5, 20 and 100 years. However, as pointed out by Marzocchi (2008), such attempt presents some ambiguities and several discrepancies between the predicted and actual seismicity. In the aftermath of theApril 6, 2009L’Aquila destructive shock, the International Commission on earthquake prediction appointed by the Italian Civil Protection decided to give priority to probabilistic approaches (Jordan et al. , 2009). In this context, the International Working Group CSEP (Collaboratory for the Study of Earthquake Predictability) has sponsored an experiment of long-term forecasting for Italy that involves several research groups (e.g., Schorlemmer et al. , 2010). Since the prediction intervals were 5 and 10 years, we are now able to evaluate the results obtained for the first time interval (2009-2014), as described in the next section. In the next section we point out some possible critical points of probabilistic predictions by focusing on a specific methodology (Double-Branching ETAS). In the third part we discuss the problems common to all probabilistic approaches, including the crucial issue of the various interpretations of the probability of earthquakes. For reasons of space, we cannot provide here a complete review of the various forecasting techniques and results. An exhaustive description of the most relevant probabilistic algorithms is presented by Tiampo and Shcherbakov (2012). Recent prediction attempts for the Italian region. Among the various contributions to the CSEP experiment (Schorlemmer et al. , 2010), we have chosen to discuss in some detail the paper by Lombardi and Marzocchi (2010a). Indeed, this work adopts an improved version of the Epydemic-Type Afterhock Sequence (ETAS) methodology, which is one of the most frequently adopted forecasting procedures, as pointed out by Tiampo and Shcherbakov (2012). Moreover, the Authors involved are certainly at the top of the Italian research on probabilistic forecasting of earthquakes. Therefore, the above-mentioned work can be considered an important and representative example in order to evaluate the state-of-art of probabilistic predictions in Italy. The ETAS approach (e.g., Ogata, 2011 and references therein) mostly relies on the phenomenology of the aftershocks, presumably induced by a strong earthquake due to the stress redistribution on the fault surface. In the context of the ETAS model, seismicity can be either spontaneous (i.e. or controlled by long-term and large-scale plate motions) or induced by previous earthquakes. Moreover, each earthquake may be a potential trigger for the next seismic events. The calculation of the probability of future shocks takes into account such possibilities (e.g., Console et al. , 2010; Tiampo and Shcherbakov, 2012). As part of the aforementioned CSEP experiment, the ETAS procedure have been used to obtain long-term (5 to 10 years) forecasts for the Italian region. The adopted stochastic procedure (Double Branching Model or DBM) would represent a significant improvement in the ETAS model (Lombardi and Marzocchi, 2010a). In particular, DBM algorithms would more adequately take into account the influence of long-term processes (possibly related to post- seismic perturbations and interactions between faults), which operate alongside the short-term concentrations (clustering) of events observed in the aftershocks phenomenology. The DBM model has been applied to a suitably filtered (declustered) Italian seismic catalog in order to predict moderate to large, shallow earthquakes ( M ≥4.5; h ≤30 km). The results obtained, which refers to the first forecast period (2009-2014), are reported in Fig. 1. This map shows that the highest probability of strong earthquakes is assigned to the areas recently hit by major shocks, in particular to the sector of the Central Apennines affected by the 2009 seismic crisis. Other relative maxima of probability are located in Friuli, Parma and Romagna Apennines, Irpinia, Calabria and eastern Sicily (Fig. 1). However, it should be noted that within the forecast interval, no shock with M ≥4.5 have occurred in Abruzzo (see e.g. the seismic archive at http://cnt.rm.ingv.it/tdmt.html) . Also, no significant shallow event has took place in the Parma and Romagna Apennines. The Friuli region and southern Italy have been affected by few notable events (4.5≤ M ≤5): Pordenone September 6, 2012, Cosenza October, 25