The ShakeMap Atlas for the City of Naples, Italy

Abstract

Naples is one of the most vulnerable cities in the world because it is threatened by several natural and man-made hazards: earthquakes, volcanic eruptions, tsunamis, landslides, hydrogeological disasters, and morphologic alterations due to human interference. In addition, the risk is increased by the high density of population (Naples and the surrounding area are among the most populated in Italy), and by the type and condition of buildings and monuments. In light of this, it is crucial to assess the ground shaking suffered by the city. To create a ShakeMap atlas for the region and to reconstruct the seismic history of the city from historical to recent times, we gather information from the most reliable and complete databases of macroseismic intensity records dating back to the eleventh century. The events felt in Naples cover a time span ranging from 1293 to 1999. The first event (Mw 5.8) was an earthquake in 1293, located in the southern Apennines, at a distance of 100 km from Naples. The most recent event was an earthquake of moderate magnitude in 1999, located beneath Vesuvius (Fig. 1). In the previous release of the macroseismic databases, two additional events associated with the volcanic activity of Vesuvius in 62 and 79 A.D. were included. They are not included in the new release of the databases because they occurred before 1000 A.D., and likewise they have been not included in this atlas because they are too ancient to be incorporated into any time and magnitude window of completeness. For instrumental events (e.g., after 1980), we merge these macroseismic records with strong-motion data. Basically, we integrate information from five Italian databases and catalogs. This gives us the opportunity to explore several sources of information, expanding the completeness of our data set in both time and magnitude. A total of 84 earthquakes have been analyzed. For each event, we compute the shakemap set (Wald et al., 1999; Michelini et al., 2008; Worden et al., 2010) using an ad hoc implementation developed for this application, with (1) specificground-motion prediction equations (GMPEs) accounting for the different attenuation properties in volcanic areas compared with the tectonic ones, and (2) detailed local microzonation to include the site effects. These shakemaps are provided in terms of Mercalli–Cancani–Sieberg intensity (MCS hereinafter) and peak ground acceleration (PGA). For PGA, the maps are provided in terms of median values and 16th and 84th percentiles, to quantify the epistemic uncertainties associated with the ground-motion measurements. In our prospective, the ShakeMap atlas has a dual application. On one hand, it is an important instrument in seismic risk management because it quantifies the level of shaking suffered by a city during its history, and it could be implemented to the quantification of the number of people exposed to certain degrees of shaking (Allen et al., 2009). Intensity data provide the evaluation of the damage caused by earthquakes; the damage is closely connected with the ground shaking, building type, and vulnerability, and it is not possible to separate these contributions. On the other hand, the atlas can be used as starting point for Bayesian estimation of seismic hazard. This technique allows for the merging of the more standard approach adopted, for example, in the compilation of the national hazard map of Italy used in this Bayesian framework as prior mode, with the site-type approach to the purpose of likelihood function (Selva and Sandri, 2013). The site-type technique is based on ground shaking data recorded in a given area; because the majority of earthquakes occurred when no seismometers were available, site data are mainly from macroseismic evaluation, that is, the felt effect is reconstructed from historical documents. The first two sections of the paper describe the databases and catalogs used, and the specific shakemap configuration applied. In the final section, we analyse the completeness of the atlas in terms of time for different magnitude/intensity thresholds, adopting and comparing two different strategies, one based mainly on historical analysis and the other on statistical evaluation.