The ShakeMap Atlas for the City of Naples, Italy

Transcription

1 The ShakeMap Atlas for the City of Naples, Italy by Licia Faenza, Simona Pierdominici, Romano Camassi, Alberto Michelini, Emanuela Ercolani, and Valentino Lauciani E Online Material: Tables of felt earthquakes in Naples and analysis for historical completeness; overview of geological settings of Naples; ground ShakeMap for the 84 earthquakes in Naples with I s > 4 INTRODUCTION 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 The first event (M w 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 beneathvesuvius (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) specific ground-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. DATA One crucial point underpinning the atlas is the accurate collection of seismological knowledge for the area under study. doi: / Seismological Research Letters Volume 84, Number 6 November/December

2 Central Apennines Tyrrhenian Sea km Gargano Calabria Adriatic Sea Apulia Ionian Sea Figure 1. Locations of the 84 earthquakes of the ShakeMap atlas. Black stars, earthquakes from DBMI11; white stars, earthquakes from CAMAL11; and gray stars, earthquakes from MOLAL08. The years printed close to some epicenters indicate the locations of the earthquakes listed in text. This implies the analysis of all databases, catalogs, and bulletins with macroseismic data and ground-motion records available in Italy, and the integration of all this information with the earthquake parametric data in terms of epicentral location, magnitude, and origin time. In Italy there is a very long tradition of historical seismology with productive schools that have led to the definition of seismic databases that go back several centuries, making the knowledge of the seismic history of the peninsula among the most complete in the world (Albarello et al., 2001). On this basis, we analyze: one parametric catalog (i.e., The Parametric Catalogue of Italian Earthquakes, Rovida et al., 2011); one macroseismic database (i.e., The Italian Macroseismic Data Base, Locati et al., 2011); two macroseismic sets of data (i.e., Material for an Italian catalog: Unknown, reappraised or rediscovered events, Camassi et al. [2011]; Material for the catalog of Italian earthquakes: A reappraisal of minor seismology, Molin et al. [2008]), and one strong motion archive (i.e., The Accelerometric Italian Data Base, Luzi et al., 2008) to create the atlas based on trustable and authoritative seismic sources. The Parametric Catalog of the Italian Earthquakes CPTI11 The Parametric Catalog of the Italian Earthquakes CPTI11 (Rovida et al., 2011) contains information about location and magnitude for damaging earthquakes in Italy. It is available at (last accessed September 2013). CPTI11 is based on the updated Italian Macroseismic Data Base (DBMI11, see subsequently) and on the instrumental databases. The main difference with respect to previous versions of the catalog is the inclusion of some foreshocks and aftershocks; in fact, the catalog was previously declustered in time and space to include only the main earthquakes. The catalog includes more than 3000 earthquakes, with magnitudes above M w 4:5, except for some hundreds of earthquakes, reported in the catalog even if with lower magnitude values and with shallow depth (above 60 km). In our application, we include in the analysis the epicentral location and the magnitude from the CPTI11. The Italian Macroseismic Data Base DBMI In Italy there is a large and homogeneous macroseismic intensity database. The newly released DBMI11 (Locati et al., 2011) has become available in 2011 ( DBMI11, last accessed September 2013). This database is a revised collection of all the macroseismic analysis done for the Italian peninsula. The new database includes a total of 86,071 macroseismic data points, from 1684 earthquakes at more than 14,000 locations. The improvement of this version over the previous one follows from the inclusion of known aftershock earthquakes that were previously disregarded, and new analysis of previously unknown earthquakes or known earthquakes with little or no associated intensity data. The intensities reported in the DBMI11 follows the MCS scale in classes spaced by one intensity unit (e.g., I s 4; 5; 6; ), and the uncertainty values (e.g., I s 4=5; 5=6; 6=7; ). In this application the latter values, rather than uncertainty values, are interpreted as intermediate value (e.g., I s 4:5; 5:5; 6:5; ). In addition, in this study, the intensity values of F (felt) and D (damage) have been encoded using the numerical values of 4 and 6, respectively. From the seismic history of Naples, for 115 earthquakes an intensity data point is available, but only for 61 earthquakes an MCS value greater than or equal to 4 have been assigned. E Table S1 (available in the electronic supplement to this paper) lists the 61 earthquakes. Because the DBMI11 does not report earthquake locations, they are taken from the Parametric Catalogue of Italian Earthquakes, CPTI11 (Rovida et al., 2011). The earthquakes are located in a broad area (Fig. 1, black stars) from the central Apennines to the Calabrian arc and the Apulia region. The northernmost earthquake is the 1979 M w 5.9 Valnerina event (central Apennines), whereas that farthest south is the 1743 M w 7.1 offshore event (Ionian sea). Magnitudes range between M w 7.2 for the 1456 Molise event and M w 3.9 for the 1999 event at Vesuvius. Material for an Italian Catalog: Unknown, Reappraised, or Rediscovered Events CAMAL11 Large-scale studies of historical seismology carried out in Italy and Europe in recent decades were mostly aimed at improving knowledge about earthquakes that were already listed in existing parametric catalogs, rather than at identifying earthquakes missing from the list. Recently, new research focused on the study of unknown earthquakes not included in Italian seismic catalogs has been made available (Camassi et al., 2011, CAMAL11 hereinafter). The analysis has been carried out adopting various strategies, including systematic surveys of historical sources and extensive investigation of local records in selected areas. The paper presents the analysis of 227 damaging earthquakes, 155 of which are unknown to the seismological tradition, whereas the remaining ones were listed in previous 964 Seismological Research Letters Volume 84, Number 6 November/December 2013

3 catalogs (e.g., Postpischl, 1985), but not in the new CPTI11, because no evidence of damage was available. Such evidence has been now found and, in 21 cases at least, it allows for a significant re-evaluation of the macroseismic intensity. For Naples, 10 new earthquakes with I s 4 are included (see E Table S2 in the supplement). CAMAL11 does not provide the epicentral information and magnitude, and these earthquakes are not listed in CPTI11; therefore, their locations and magnitudes are evaluated ad hoc for this work. The location is based on the BOXER code (Gasperini et al., 1999, 2010) and the magnitude has been converted from I 0 adopting the same linear regression used in CPT11. Most of the events are local and have moderate magnitudes (4:3 M w 5:6). These earthquakes are shown in Figure 1 as white stars. Material for the Catalog of Italian Earthquakes: A Reappraisal of Minor Seismicity MOLAL08 Last, we analyzed the database with the review of minor seismicity of the peninsula (Molin et al., 2008; MOLAL08 hereinafter). The 851 earthquakes have epicentral intensities ranging from 5.5 to 7 MCS scale, with epicentral data from Postpischl (1985). The adoption of a rigorous review procedure allows the epicentral parameters of 741 events to be improved and updated, whereas for 84 earthquakes the historical information available does not allow the definition of epicentral parameters. Twenty-six earthquakes either do not exist or are extremely doubtful. E Table S3 (in the supplement) summarizes the results for the city of Naples; MOLAL08 provides the macroseismic regions, the locations, and magnitudes of these earthquakes. There are 13 earthquakes with magnitudes ranging from M w 3.9 to M w 5.1 and I 4 that are included in the analysis, and are represented as gray stars in Figure 1. The Accelerometric Italian Archive ITACA ITACA is the accelerometric Italian database (Luzi et al., 2008; Pacor et al., 2011). It contains data acquired in Italy by different national institutions, namely, Ente Nazionale per Energia Elettrica (ENEL, Italian electricity company), Ente per le Nuove tecnologie, Energia e Ambiente (ENEA, Italian Energy and Environment Organization), and the Dipartimento della Protezione Civile (DPC, Italian Civil Protection); see itaca.mi.ingv.it (last accessed September 2013) for additional details. The new version of ITACA, ITACA1.1 has become available in July ITACA contains 2182 three-component waveforms generated by 1004 earthquakes spanning the time interval from 1972 to The largest earthquake represented is the 1980 Irpinia event with M w 6.9. In addition to the digital strong-motion recordings, ITACA provides various estimates of peak ground motion (e.g., PGA, peak ground velocity [PGV], peak ground displacement [PGD], and Housner and Arias Intensities). To meet the requirements of ShakeMap, the values of PGA, PGV, and spectral acceleration, SA at 0.3, 1.0, and 3.0 s, respectively, are considered in the analysis. Strong-motion records are only available for four events (E Table S1, see supplement): the 1979 M w 5.9 Valnerina event, with eight stations; the 1980 M w 6.9 Irpinia event with twenty stations; the 1981 M w 4.9 Baiano event with two stations; and the 1984 M w 5.9 event that occurred in the Abruzzo region, with four stations (Fig. 1). THE AD HOC SHAKEMAP IMPLEMENTATION Since 2006, the USGS-ShakeMap software has been operational at the INGV to generate maps of the peak ground-motion parameters and of the instrumentally derived intensities for M l 3:0 earthquakes occurring in Italy and neighboring areas (Michelini et al., 2008), and maps are made available in near-real time in the web portal (last accessed September 2013). Currently, the version 3.5 of the USGS Shake- Map package is implemented (Worden et al., 2010). We adapted the code defined for the Italian seismological settings, as described in Michelini et al. (2008) for our test case, with the ad hoc definition of regionalized ground-motion prediction equations and site correction based on geological V S30 (i.e., the mean shear velocity of the uppermost 30 m). Regionalized Ground-Motion Prediction Equation The earthquakes that can be felt in Naples cover a broad area, from the central Apennines to the Calabrian arc, and the Apulia region, as shown in Figure 1. In the ShakeMap implementation at INGV, we adopt the regionalized ground-motion prediction equation (GMPE); for small- to medium-size earthquakes (M w <5:5), we adopt the regionalized equations of Malagnini et al. (2000) and Morasca et al. (2006), and for M w 5:5 the regression of Akkar and Bommer (2010). Following the seismic-hazard map of Italy (Mappadi Pericolosità Sismica [MPS] Working Group, 2004; Stucchi et al., 2011), the national territory is divided into six macro regions (Michelini et al., 2008). The area of interest of this paper corresponds to Zones 3, 4, and 5 of that zonation, as shown in Figure 2. In addition, we include three small regions for the volcanic area: Vesuvius, Campi Flegrei, and Ischia island (Fig. 2). Earthquakes occurring in volcanic areas have different ground attenuation properties with rapidly decreasing intensity with distance, and generally large values of ground motion at short distances. The GMPE by De Natale et al. (1988) has been specifically derived for volcanic areas in Italy and it has been implemented in the seismic-hazard map of Italy (MPS Working Group, 2004; Stucchi et al., 2011) to quantify the shaking in these zones. Similarly, in our application for earthquakes occurring in the volcanic area of Vesuvius, Campi Flegrei, and Ischia island we use the De Natale et al. (1988) GMPE, calibrated using small earthquakes; the regression of Akkar and Bommer (2010) is used in case of M w 6:5 earthquakes. Site Characterization The site characterization is the other important ingredient in the definition of ground motion. In ShakeMaps it is defined using V S30 and the amplification factors of the Borcherdt relation (Borcherdt, 1994). In Michelini et al. (2008), the classification based on the 1:100,000 geological map of Italy, compiled and published by the Servizio Geologico d Italia Seismological Research Letters Volume 84, Number 6 November/December

4 Apenninic belt Gargano CAMPOBASSO ZONE 3 ZONE 5 41 Campi Flegrei NAPOLI ZONE 4 41 Ischia Island Vesuvius POTENZA km Tyrrhenian Sea Figure 2. Regionalization of the GMPE. Zones 3, 4, 5 correspond to the zones in figure 2a in Michelini et al. (2008) with the regionalized GMPE; the three boxes correspond to the volcanic areas in which the attenuation of De Natale et al. (1988) is applied; inset: black box, location of the region around Naples; the gray lines, regionalization as in Michelini et al. (2008). (sgi.isprambiente.it/geoportal/catalog/sgilink/map100k.page, last accessed September 2013), is adopted. The geologic units are gathered into five different classes A, B, C, D, and E according to the EuroCode8 previsions, EC8 (e.g., eurocodes.co.uk/eurocodedetail.aspx?eurocode=8, last accessed September 2013) and for the classification, following lithological and age criteria are assigned the following velocities A 1000; B 600, C 300, D 150, ande 250 m=s (see table 1 in Michelini et al., 2008). The step from the three-class to the five-class Geological Map has been performed to adhere to the EC8 guideline for soil characterization. The adopted map has been sampled at a space interval of 1 min for the ShakeMap program. The four velocity classes have been depicted as four main geological groups: (1) pyroclastic deposits of Campi Flegrei, (2) tuff deposits of Vesuvius, (3) soil and alluvial pyroclastic deposits, and (4) deposits of the Tyrrhenian sea (Fig. 3). To apply this to Naples, we have refined the V S30 grid. The Earth Science Department of the University of Naples Federico II has proposed a seismic microzonation for the city of Naples ( last accessed September 2013), based on the works of Nunziata (2004), Nunziata et al. (2004), and Nunziata (2007). The urban area of Naples has been divided into six zones with geologically homogeneous subsurfaces (Fig. 3) on the basis of data from geotechnical and stratigraphic investigations (Comune di Napoli, 1994, and references therein). For each zone, the authors provide a geological description of the zone itself, and some physical parameters, such as V S velocity profiles and spectral amplification. To obtain a refined and more detailed V S30 for Naples, we have cross-matched the information 15 Figure 3. Geological sketch of the study area (bold line). The outcropping deposits have been grouped in three main clusters: (a) Pyroclastic deposits of Campi Flegrei (Ignimbrite Campana deposits and Neapolitan Yellow Tuff); (b) Deposits of Vesuvius; (c) Soil and alluvial pyroclastic deposits (in bright green color) and sea sand (in dark green color) and (d) sea. For our groundshaking analyses the deposits of the Tyrrhenian Sea have also been taken into account. The different patterns indicate the six zones that have been identified and characterized by homogeneous geological deposits on the basis of deep stratigraphic investigation (Nunziata et al., 2004). available from this microzonation, the articles of Nunziata and co-workers (Nunziata et al., 2000; Nunziata, 2004; Nunziata et al., 2004; Nunziata, 2007), the 1:100,000 geological map of the city, and our one-minute V S30 grid. For more details on the geological setting and zonation, the reader can find E an overview (see supplement). Map Generation For each earthquake we have used all the data available in the literature to generate the ground-shaking maps. The new release of ShakeMap has the advantage that different kinds of information can be included in the maps, from intensity to ground-motion values and estimates from GMPEs, using a weighted-average scheme based on their variance. Each map (e.g., intensity, PGA, PGV maps) can be a combination of three types of data: native data, such as macroseismic observations and measured ground-motion ones; converted data, such as ground-motion data converted into intensity data and vice versa; and estimated data, from empirical models (Worden et al., 2010). For example, in the PGA map, the data from strong-motion measurements are the actual native data, whereas the macroseismic field data are considered to be converted. In addition, each map is associated with an uncertainty map to maintain control on each grid point and assess the reliability of the information available (e.g., the converted data are subject to the propagation error of the regression law). The weighting scheme balances the contribution of a datum with a function of its uncertainty. In its full implementation, 966 Seismological Research Letters Volume 84, Number 6 November/December 2013

5 ShakeMap computes a weighted sum of different types of information for each grid point, based on their variance. In this application, the conversion between ground motion and intensity is done using the regression of Faenza and Michelini (2010), calibrated for MCS intensity data in Italy. The maps have been calculated using the option, strictbound, to force the map to have the boundaries of the urban area of Naples. RESULTS For each of the 84 calculated shake maps, we provide the intensity map and the PGA map as 16th, 50th, and 84th percentile. To calculate the percentile, we have randomly generated 10,000 values from a 3 σ upper-bounded normal distribution of the PGA. The median and standard deviation of the distribution change at every point of the grid. The median value is derived from the ShakeMap PGA map, whereas the standard deviation is taken from the ShakeMap PGA uncertainty maps. The E atlas is available (see supplement). Here, we only report two significant examples: the 1688 M w 7.0 (Fig. 4) and the 1999 M w 3.9 (Fig. 5) earthquakes. The first example has been chosen because it is the earthquake causing the largest level of shaking in Naples, although the event epicenter was in the southern Apennines; the second one is a small earthquake (M w 3.9) that occurred in the Vesuvius area. Looking at Figures 4 and 5, the contribution of V S30 in the characterization of the shaking is evident; a similar pattern, even if only with a rough spatial resolution, was found using the one-minute V S30 grid currently adopted in ShakeMap. For comparison, maps based on rock-type soil only are also reported (Fig. 6 for the 1688 and 1999 earthquakes in MCS intensity scale). In these maps the effects of amplification and de-amplification of the ground motion disappear. Afundamentalingredientinhazardstudiesistheevaluation of the magnitude time windows of completeness. Stucchi et al. (2004) present an overview of the different strategies adopted to quantify the window of completeness. They propose as well a methodology for the quantification of the historical completeness, which has been adopted in national seismic-hazard maps (MPS Working Group, 2004; Stucchi et al., 2011), that is suitable for extended areas. In the work, we focus on methodologies for the evaluation of the completeness of single cities. In this regard, we adopt two strategies: the first is mainly based on statistical criteria, the second is based on historical analysis. The seismic history of an area, in general, and of Naples in particular, is a collection of heterogeneous pieces of information, from instrumental records to macroseismic observations. To infer the statistical completeness of a parametric seismic catalog, we use the methodology developed by Albarello et al. (2001). It has the aim to define the probability P C the completeness which represents, for each possible time span ΔT, the level of confidence associated to the hypothesis that the seismic catalog for that period is representative of actual seismicity (Albarello et al., 2001). The method is based on the assumption of statistical independence of interevent times (e.g., Poisson character of the seismicity in the temporal domain), and it is developed in two stages. The first nonparametric analysis quantifies the conditional probability P CjR, which is the probability that the catalog is complete [P C ] given that the catalog is representative [P R ], that is, the time ΔT is long enough to guarantee the occurrence of major seismological processes. In the second step, P C is calculated making assumption about the reliability of P R (further details in Albarello et al., 2001). The statistical completeness for the city of Naples is reported in Figure 7 for different intensity thresholds. For intensity above 7 (M w 5:3) the catalog is complete since 1420, whereas for smaller intensity (I 0 4) the date of completeness is around Whereas the basis for statistical completeness is the assumption that seismicity is stationary over time and that the numbers of historical sources progressively increase as time approaches the present, the historical approach is free of any preconceived assumptions about standard behavior of historical sources (Camassi and Castelli, 2000). The aim of this strategy is to determine a temporal limit within which reports of the damage effects of and earthquake in a particular location have not been lost. In general, for Italy, destructive earthquakes have left traces in historical sources since 1200 (Albini et al., 2001); if this is not true, the reasons can be that this particular event is an exception and it did not leave traces, or those sources have been lost, or those sources are available but have not been analyzed for seismological purposes. The starting point for this methodology is the E seismic history of Naples (see Tables S1, S2, and S3 in the supplement), that provides all the available information on the known earthquakes felt in Naples (i.e., the I s at Naples). For a broad area of interest (Fig. 1), we calculate the predicted intensity data in Naples. In this application we adopt the regression of Pasolini, Albarello, et al. (2008) and Pasolini, Gasperini, et al. (2008) as the intensity prediction equation. To evaluate the historical completeness a comparison is made between the predicted and observed site intensity values. The results are summarized in E a table (see the supplement). For each earthquake entry in the catalogs and databases (CPTI11, CAMAL11, and MOLAL08) we check if the macroseismic value for Naples is available in the three macroseismic databases (DBMI11, CAMAL11, MOLAL08) I so intensity site observation at Naples and we evaluate as well the intensity at Naples I sc intensity site calculated. In E Tables S4, S5, and S6 (see supplement) are reported all the events with I so or I sc larger than or equal to a specific intensity value (i.e., 7.5, 6, and 4, respectively). The definition of the completeness is done with the expert judgment of a historical seismologist. For each table, the epicentral information of the earthquakes is reported, a column is given to identify whether the observed intensity at the city of Naples (I so ) is greater than or equal to the intensity value under analysis or if it is less; a column to identify whether the calculated intensity (I sc ) is greater than or equal to the intensity value under analysis or if it is less; and a column to see if I so is missing (i.e., I sc is greater than the intensity value of Seismological Research Letters Volume 84, Number 6 November/December

6 INTENSITY: Event of with M w 7.0 PGA 50 percentile: Event of with M w 7.0 MCS PGA 16 percentile: Event of with M w 7.0 PGA 84 percentile: Event of with M w 7.0 PGA Uncertainty: Event of with M w 7.0 Ln(PGA) Figure 4. Shake Map for the 1688 M w 7.0 earthquake. The event is located in the Apennines, shown as the red star in the intensity map inset. 968 Seismological Research Letters Volume 84, Number 6 November/December 2013

7 INTENSITY: Event of with M w 3.9 PGA 50 percentile: Event of with M w 3.9 MCS PGA 16 percentile: Event of with M w 3.9 PGA 84 percentile: Event of with M w 3.9 PGA Uncertainty: Event of with M w 3.9 Ln(PGA) Figure 5. ShakeMap for the 1999 M w 3.9 earthquake. The event is located at Vesuvius, shown as the red star in the intensity map inset. Seismological Research Letters Volume 84, Number 6 November/December

8 (a) (b) INTENSITY: Event of with M w 7.0 INTENSITY: Event of with M w 3.9 MCS Figure 6. ShakeMap in MCS intensity scale using rock type soil only for: (a) 1688 M w 7.0 earthquake, and (b) 1999 M w 3.9 earthquake. Figure 7. Statistical completeness for the city of Naples. Dots, the median values; two lines, the lower and upper bounds as the 25th and 75th percentile. interest but no information on I so is available). From E Table S4 (see supplement), we can see that no data are missing for this intensity level and that for I s 7.5 the record is complete. The same information is shown in E Table S5 (see supplement), but for the lower intensity level, I s 6. In this case, only two events are missing; one back in the past, in 1275 at Ischia island; and the second is a local event in Pozzuoli (in Campi Flegrei). For the events in the volcanic island of Ischia (with dates ; , and ) and for one in Pozzuoli (with date ), the I so is smaller that the I sc. This suggests a possible overestimation of Pasolini, Albarello, et al. (2008) and Pasolini, Gasperini, et al. (2008) for earthquakes in volcanic areas. This is reasonable, because Pasolini, Albarello, et al. (2008) and Pasolini, Gasperini, et al. (2008) is not constrained for volcanic areas, but in the literature the attenuations in terms of intensity for volcanic areas are not available. Similar conclusions can be reached for the two events in volcanic areas with I so missing, that we listed above. The other two earthquakes with I so < 6 but I sc 6 have I so equaling 5.5 and 5, and correspond to the 1505 Naples and 1915 Avezzano earthquakes, respectively. Summarizing the results in E Table S5 (see supplement), the completeness of I s 6 could go back to E Table S6 (A and B; see supplement) summarizes the results for I s 4. The situation is more complex that in the previous cases, and a detailed analysis is required for each case with I so equals to NO or I so missing. We start from the analysis of the six events with I so missing, considering only the ones that occurred after the sixteenth century. They are the 1537 I Pozzuoli earthquake, the 1765 I Casertano earthquake, the 1779 I Medio Tirreno earthquake, the 1769 I 0 8 Casamicciola Terme earthquake, the 1933 I 0 9 Maiella earthquake, and the 1960 I Roccamonfina earthquake. The 1537 Pozzuoli and the 1769 Casamicciola Terme earthquakes are in volcanic areas (Casamicciola Terme is a village located in Ischia island). As previously noted, the attenuation of Pasolini, Albarello, et al. (2008) and Pasolini, Gasperini, et al. (2008) overestimates the intensity for volcanic events, therefore having I sc > 4 could be an artifact of the regression that will not affect the completeness of the catalog. As evidence, the 1828 I 0 8 Casamicciola Terme earthquake has I so < 4 and I sc > 4. A peculiar situation is found for the 1779 I Medio Tirreno earthquake, in the gulf of Naples. The macroseismic field has only one data point, which corresponds to the epicentral area. This is a dubious earthquake for which historical seismologists have not yet reached a final conclusion. The remaining I so missing earthquakes occur in areas at large epicentral distances (i.e., 1765 Casertano, 1933 Maiella, and 1960 Roccamonfina earthquakes). For the Casertano earthquake the macroseismic field has three data points and the closest data point to Naples ( 30 km in the direction north to Caserta) has I so 4; I sc at Naples is 4. For the 1933 Maiella earthquake the closest intensity point is Ischia island, with I so 2.5; I sc at Naples equals 4.3. For the 1960 Roccamonfina event (located between Lazio and Campania regions, in an 970 Seismological Research Letters Volume 84, Number 6 November/December 2013

9 extinct volcano) the macroseismic field seems to have a rapidly decreasing shape, and the locations closest to Naples have I so 1. We then analyzed the earthquakes with I so < 4 but I sc 4. Besides two earthquakes on the volcanic island of Ischia (e.g., 1828 and 1881 earthquakes) for which we can reach the same conclusion as for the previous cases, all the other events have large epicentral distances from Naples, and the intensity values calculated at these distances could be not well constrained. This analysis leads us to define the date of the historical completeness for I s 4 at CONCLUSION We calculated 84 USGS-ShakeMap-based shakemaps for the earthquakes with I s 4 listed in the three macroseismic databases and catalogs. The shakemaps are calculated using all available data, and an ad hoc implementation of the shakemap code in terms of specific ground-motion evaluation and site classification. The shakemaps can be a starting point for a Bayesian approach to the evaluation of the seismic hazard in Naples (Selva and Sandri, 2013). The analysis is correlated with the evaluation of the time intensity threshold of completeness, using two different strategies, one based on statistical principles and the other on the historical one. Our evaluations provide completeness values that go back to 1420 and 1550 for I s > 6 and I s > 4, respectively, for the statistical completeness, and 1300 and 1500 for I s > 6 and I s > 4, respectively, for the historical one. Our study provides a time magnitude threshold of completeness larger than the one proposed in MPS Working Group (2004) and Stucchi et al. (2011). The reasons could be ascribed to two principal factors. First, this paper uses databases that were not available at the time the hazard map was compiled. In fact, the DBMI and CPTI catalogs have been extensively revised and CAMAL11 and MOLAL08 allow the inclusion of small events. Second, our analysis is focused on a single city, rather than on an extended area. Naples is in seismic ZS928 zone as from MPS Working Group (2004; hereinafter MPS04) and (Meletti et al., 2008), which includes an area that extends from the Apennines to Ischia island. This implies that the long and detailed seismic history of Naples is weighted with an area in which historical studies are not as detailed. The area around Naples has been the focus of an intensive and detailed repertory published in 1691 by Marcello Bonito (Bonito, 1691). In his work, Bonito completely copied testimonies drawn from books and archive documents that are now lost, making him one of the most extraordinary compilers of Italian seismological history. The procedure we propose, in terms of data collections and analyses, is suitable for specific studies in target areas, such as big hazardous cities, and it does not aim to replace or modify the assessment of MPS04, because we are conscious of the different spatial extension of the two approaches. For instance, Naples is the test city of at least two projects for multi-hazard and multi-risk assessment: Matrix New Multi-Hazard and Multi-Risk Assessment Methods for Europe ( last accessed September 2013), founded by the European Seventh Framework Programme, and ByMur Bayesian Multi-Risk assessment ( last accessed September 2013), financed by MIUR, the Italian Ministry for Research and Education. DATA AND RESOURCES All data used in this article come from the published sources listed in the references. Some plots are made using Generic Mapping Tools version ( last accessed September 2013; Wessel and Smith, 1991). ACKNOWLEDGMENTS The work was supported by the Futuro in Ricerca 2008 FIRB Project ByMur [RBFR0880SR] financed by MIUR, the Italian Ministry for Research and Education. The authors thanks T. Nunziata for helpful comments. The authors thank the Editor S. Hough, P. Hellweg, and one anonymous reviewer for improving the manuscript. REFERENCES Akkar, S., and J. J. Bommer (2010). Empirical equations for the prediction of PGA, PGV, and spectral accelerations in Europe, the Mediterranean region, and the Middle East, Seismol. Res. Lett. 81, no. 2, Albarello, D., R. Camassi, and A. Rebez (2001). Detection of space and time heterogeneity in the completeness of a seismic catalog by a statistical approach: An application to the Italian Area, Bull. Seismol. Soc. Am. 91, no. 6, Albini, P., R. Camassi, V. Castelli, and M. Stucchi (2001). Miglioramento della Qualità delle Informazioni Macrosismiche per un Loro Utilizzo nella Valutazione della Pericolosità Sismica, Tech. Rep. (in Italian). Allen, T. 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Seismol. Soc. Am. 100, no. 6, , doi: / Licia Faenza Istituto Nazionale di Geofisica e Vulcanologia Centro Nazionale Terremoti via Donato Creti Bologna, Italy licia.faenza@ingv.it Alberto Michelini Valentino Lauciani Istituto Nazionale di Geofisica e Vulcanologia Centro Nazionale Terremoti Via di Vigna Murata , Roma, Italy Simona Pierdominici Helmholtz-Zentrum Potsdam Deutsches Geo Forschungs-Zentrum GFZ D Potsdam, Germany Romano Camassi Emanuela Ercolani Istituto Nazionale di Geofisica e Vulcanologia Sezione di Bologna via Donato Creti Bologna, Italy 972 Seismological Research Letters Volume 84, Number 6 November/December 2013