Abstract
Abstract. The northernmost part of Apulia, in Southern Italy, is an emerged portion of the Adriatic plate, which in past centuries was hit by at least three disastrous earthquakes and at present is occasionally affected by seismic events of moderate energy. In the latest seismic hazard assessment carried out in Italy at national scale, the adopted seismogenic zonation (named ZS9) has defined for this area a single zone including parts of different structural units (chain, foredeep, foreland). However significant seismic behaviour differences were revealed among them by our recent studies and, therefore, we re-evaluated local seismic hazard by adopting a zonation, named ZNA, modifying the ZS9 to separate areas of Northern Apulia belonging to different structural domains. To overcome the problem of the limited datasets of historical events available for small zones having a relatively low rate of earthquake recurrence, an approach was adopted that integrates historical and instrumental event data. The latter were declustered with a procedure specifically devised to process datasets of low to moderate magnitude shocks. Seismicity rates were then calculated following alternative procedural choices, according to a "logic tree" approach, to explore the influence of epistemic uncertainties on the final results and to evaluate, among these, the importance of the uncertainty in seismogenic zonation. The comparison between the results obtained using zonations ZNA and ZS9 confirms the well known "spreading effect" that the use of larger seismogenic zones has on hazard estimates. This effect can locally determine underestimates or overestimates by amounts that make necessary a careful reconsideration of seismic classification and building code application.
Highlights
Apulia region is the south-eastern end of the Italian peninsula and is constituted by an emerged portion of the Adriatic microplate, representing the foreland-foredeep area of the Southern Apennine chain
From a structural geological point of view, Northern Apulia consists of three different zones (Fig. 1): a) A foreland area to the NE constituted by the Gargano promontory, a horst elongated towards the Adriatic sea, generated by the uplift of a carbonate plateau and delimited by steep scarps
The aforementioned seismotectonic data suggested the possible identification in Northern Apulia of four separate seismogenic zones (Fig. 1): two foreland areas, corresponding to the Fortore lower course – Lesina lake – Tremiti Islands (Zone 1) and to the Gargano promontory (Zone 2), a foredeep area corresponding to the Tavoliere plain (Zone 3) and the external front of the Apennine chain corresponding to the Dauno Sub-Apennine (Zone 4)
Summary
Apulia region is the south-eastern end of the Italian peninsula and is constituted by an emerged portion of the Adriatic microplate, representing the foreland-foredeep area of the Southern Apennine chain. We were interested in evaluating the influence that the proposed local modification of seismogenic zonation would have on the hazard estimates for the study area and whether this influence is significant in comparison to the effect of other epistemic uncertainty factors affecting seismicity rate calculation. For this purpose estimates of seismicity rates based on integrated historical and instrumental datasets were carried out both for the Northern Apulia seismogenic zones defined in the new zonation and for the zone no. Comparisons were carried out between the results obtained by using the ZS9 zonation and that locally modified
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