- © 2012 by the Seismological Society of America
The effort for reducing the uncertainties in the location and size of historical earthquakes, even moderate‐size ones, is not a peripheral issue, as it plays a major role in the distribution of earthquake recurrence times that can affect the maps of seismic hazard of a territory. The L’Aquila area (Abruzzo, Central Italy) struck by the 6 April 2009 Mw 6.3 seismic event is a typical example of an earthquake‐prone region, the location of which historical seismicity needs to be better located. Thanks to a large body of geological, seismological, and geodetic evidence, the deep source of the 2009 mainshock has been imaged as a ∼15‐km‐long, northwest‐trending, southwest‐dipping, almost‐pure normal fault coinciding with the Paganica fault system at the surface (Fig. 1). Conversely, so far, very few papers have addressed the issue of reconstructing the seismic history of the Paganica fault, charting the relationships between this fault and the adjacent ones, and finding out the causative source for each of the earthquakes listed in the Italian earthquake catalog (CPTI Working Group, 2004; Rovida et al., 2011).
In particular, the 1461 M 6.4 and the 1762 M 6.0 earthquakes (Fig. 1) took place in the same general area and display a comparable magnitude with that of the 2009 earthquake. According to recently provided paleoseismological data (Cinti et al., 2011), the shock of the 1461 earthquake was the penultimate event that ruptured the Paganica fault. The case of the 1762 earthquake was more open to debate: Tertulliani et al. (2009) suggested that it could have had the same fault source as that of the 2009 shock (west of the catalog epicenter) on account of some similarities in the distribution of the maximum intensities and in the temporal evolution of the 1762 and 2009 sequences. At the same time, however, Tertulliani et al. (2009) were aware of, and cautioned against, the very short recurrence interval implied by their hypothesis. The issue of the too‐short recurrence interval, along with some observations on the too‐low throw rate measured across the Paganica fault, induced Roberts et al. (2010) to invoke the role of other neighboring sources and to propose for the 1762 shock an alternative location some 10 km east‐southeast of the epicenter adopted by the catalog.
Such a variance of opinions concerning the epicentral location of the 1762 earthquake is explained by the scantiness of the data set on which investigations must rely and which consists of the shaking intensities at three localities only. As Di Bucci et al. (2011) pointed out, such a data set made it impossible to associate the 1762 event to any fault “beyond mere speculation”. We are left from this stark and somewhat frustrating consideration to systematically gather all potential primary sources of information available for the 1762 earthquake in the area to attempt to improve its intensity map and to compare it with the 2009 one.
In the following, we will (1) set out the evidence newly collected from archives in L’Aquila and Naples and from other repositories of contemporary sources; (2) explain the rationale that allowed us to turn demographic and historical information into data sensitive to macroseismic analyses; (3) show the new intensity map for the 1762 earthquake and discuss its main differences from the previous intensity data sets (Boschi et al., 2000 and the most recent versions of the Italian macroseismic database, i.e., Stucchi et al., 2007; Locati and DBMI Working Group, 2009; Locati et al., 2011); (4) provide a more accurate location and propose new source parameters for the event; and (5) discuss the constraints on its epicenter location and source parameters with the possible seismotectonic implications.
IMPROVING THE HISTORICAL DATA SET
In the last 30 years, the 1762 earthquake has been studied several times (Spadea et al., 1985; Monachesi and Castelli, 1992; Boschi et al., 1997; Guidoboni et al., 2007). All studies were based on a scanty data set that included very few contemporary sources (official records, newspapers); none seems to have attempted a thorough search for contemporary information. In the latest catalogs (Stucchi et al., 2007; Locati and DBMI Working Group, 2009; Locati et al., 2011), the 1762 earthquake parameters are derived from Guidoboni et al. (2007). This study is based on the perusal of a single folder of archive file Archivio di Stato dell’Aquila (ASAQ, 1762–1779), which was put together in October–November 1762 by officials charged with assessing earthquake damage in the villages of Castelnuovo and Poggio Picenze, whose inhabitants had, on account of the earthquake, pleaded for a temporary exemption from taxes. On the basis of this information, Guidoboni et al. (2007) concluded that the 1762 earthquake reached its highest effects in Castelnuovo (I IX–X Mercalli‐Cancani‐Sieberg [MCS]) and Poggio Picenze (I IX MCS), whereas it was only felt in two nearby villages (Barisciano and San Demetrio ne’ Vestini). According to Guidoboni et al. (2007), L’Aquila was also damaged, evidence for this being derived from nonlocal newspapers (Bologna, 1762; Diario Ordinario, 1762). As a look at the terrain will show (Fig. 1), this picture of the 1762 earthquake effects seems quite unsatisfactory because of the closeness between the few highly damaged sites and the undamaged ones and also because of the presence in the area of many other localities about which there is no evidence of any earthquake effect at all.
Seeking to improve the 1762 earthquake historical data set, we carried out a systematic, cost‐effective search for contemporary written records of this event. We defined the potential field of investigation and located the main repositories of documentary and narrative sources to be searched. For original narratives, that is, writings produced for preserving and/or disseminating the remembrance of events, we extensively sampled contemporary diaries and newspapers (Table 1). For archive documents, produced for administrative purposes by public and private bodies, we decided to consider only public records (i.e., those produced by the main lay and ecclesiastic bodies entrusted with the administration of the territory; see Table 1) because extending the investigation to private records would be too costly in terms of time and effort without guaranteeing adequate results in a short period. We then located the main archives to be investigated by taking into account the history and administrative structure of the affected territory. In 1762, the Abruzzo provinces belonged to the Kingdom of Naples, an absolute monarchy within which the provincial administrators had very little freedom in the decision‐making processes concerning local affairs, all of which had to be submitted to the central government. For this reason, public documents concerning the 1762 earthquake were sought not only in local archives but also in the central archives of the kingdom, preserved in Naples. Accordingly, we carried out our search not only in the public archives of L’Aquila (Archivio di Stato for state records and Archivio Diocesano for church records) but also in Naples (Archivio di Stato). Our search took into account the records of all the provincial and central offices that, by reason of their allotted tasks and prerogatives (financial, managerial, and so on), were likely to have been involved with handling the consequences of the 1762 earthquake. The results of this investigation far surpassed our expectations, bringing to light information on earthquake effects in 10 localities, seven of which had never been considered by any of the previous studies.
TURNING HISTORICAL EVIDENCE INTO MACROSEISMIC EVIDENCE
Having completed an exploration of the repositories most likely to have preserved contemporary evidence of the 1762 earthquake and retrieved a remarkable amount of previously unknown historical information, it remained to translate the information acquired into macroseismic data to assess intensity values for all the earthquake‐affected sites. To this end, we needed to know with a good approximation (1) how many buildings there were in each site at the time of the earthquake; (2) how many buildings in each site were damaged by the earthquake; and (3) the damage levels as many as possible of the damaged buildings.
The new 1762 historical data set includes detailed damage surveys made for official purposes by experts of building techniques in three localities (Barisciano, Castelnuovo, and Poggio Picenze); only part of the data concerning Castelnuovo and Poggio Picenze had been considered in previous studies (Boschi et al., 1995; Guidoboni et al., 2007).
To assess macroseismic intensities from historical documents such as the ones used in this study, it is necessary to find a correct translation of the technical or vernacular terms employed to describe earthquake effects into the standard word pictures proposed by the EMS98 scale (Grünthal, 1998). The 1762 accounts of damage in Barisciano (Fig. 2), Castelnuovo, and Poggio Picenze include both sworn depositions made by house owners whose structures had sustained damage and who were asking for financial relief and surveys made by skilled master masons charged with checking those depositions on behalf of the public authorities (ASAQ, 1762–1779). Owing to the cultural divide then existing between the ruling class, whose language was a formal, convoluted Italian language with Neapolitan overtones, and the common people, who spoke the local vernacular (i.e., Carrer and Federici, 1827; Fanfani, 1865), the descriptions of the type, amount, and severity of damage employ quaint nouns and adjectives best intelligible within a regional or even local context and are often quite difficult to interpret by modern Italian speakers.
To use in the best possible way the considerable amount of information available, we decided to cross check the depositions made by damaged house owners with those made by assessors so as to construct a pattern to translate and organize the original descriptions of damage into modern damage categories (Table 2). At the same time, we took care to distinguish between the cases in which damage accounts were related to whole buildings and those related to parts (rooms) of a building, as the sources mention both cases (e.g., “Pietro Sidonio’s four‐roomed house…one [room] is fallen to the ground, the others are quite extensively damaged” or “Francesco Gasbarro’s house is completely fallen down and ruined”).
The next step in the translation of damage information into EMS98 intensity values was to find out how many buildings there were in each of the damaged localities at the time of the earthquake to obtain reliable percentages of damage. By crossing the contemporary data provided by standard demographic studies (De Matteis, 1973; Giannangeli, 1989) on the number of taxable dwellings (called “fuochi” or hearths) existing in the L’Aquila countryside with those provided by a mid‐eighteenth century census of the Kingdom of Naples (De Matteis, 1973), we were able to infer the approximate number of buildings in Castelnuovo (120), Poggio Picenze (170), and Barisciano (450). In some cases, direct information on the building stock benefited from the discovery of a contemporary detailed image of a village (see Fig. 3). The building typologies most likely to be found in these villages can all be referred to the A class of vulnerability as defined in the EMS98 scale (Grünthal, 1998). Local traditional buildings, as it has been pointed out both by literature (Carocci and Lagomarsino, 2009; Ortolani, 1961) and by direct observation of historical buildings in situ, tended to be constructed out of simple stone cemented with poor‐quality mortar, with barns or stables on the ground floor and living quarters above, roof coverings of wood or rushes, and very large masonry spans at the ground floor. The results of the reevaluation of the data and the assessment of the macroseismic intensities are summarized in Table 3.
Detailed reports of damage are available only for the three localities mentioned earlier (Archivio di Stato di Napoli [ASNA], 18th century, 1763a, b, c, e, 1763–1773). For another 10 localities (five of which completely unknown to previous studies), summary descriptions of earthquake damage are available; for seven more localities (all of them unknown to previous studies), only indirect evidence of earthquake effects has been found (Fig. 4). One interesting discovery was that Barisciano, Castelnuovo, and Poggio Picenze were not the only communities that applied for a tax exemption on account of earthquake damage. Similar pleas were made by the communities of Pescomaggiore, Picenza, and Sant’Eusanio Forconese (ASNA, 1763d, 1764), but we have found no evidence of damage surveys carried out in these localities to check the soundness of these claims. It is impossible to tell whether this means that these pleas were not taken into consideration or were considered, but the surveys made in these localities are missing. In these cases, we supposed that the localities in question were damaged but to a lesser degree than that of Barisciano, Castelnuovo, and Poggio Picenze. Table 4 shows examples of the main information related to these localities and the intensity values we assessed to them.
The general picture of effects is completed by a number of reports found in the Visite Pastorali (i.e., the accounts of the periodical inspections carried out by the Bishop of L’Aquila on church properties) from 1762 to 1769 and in other archival records of the L’Aquila diocese (Archivio Diocesano dell’Aquila [ADAQ], 1762). These accounts mention the state of disrepair of many inspected buildings, without linking it explicitly with the 1762 earthquake. Nevertheless, it is interesting to note that these references are very close, both in time (1762–1766) and in space, to the 1762 event. Although those accounts could not be used to assess intensities, we think that a real correlation between them and the earthquake is highly possible and can contribute to shaping the general scenario of damage. We accordingly treated such accounts as indirect sources (Fig. 4).
There are also silent sources, that is, records that, although produced within the earthquake time frame and dealing with localities likely placed within the earthquake‐affected area, do not mention the earthquake at all, which implies a likelihood that the earthquake in those localities was either not felt or inconspicuously felt.
The intensity map produced after the revision of all the available sources (Fig. 5) confirms that the epicentral area of the 1762 earthquake is placed 15–20 km southeast of L’Aquila. The major damage area is well constrained to a limited sector of the Middle Aterno Valley. The city of L’Aquila suffered much less damage from the 1762 earthquake than from the 1461, 1703, and 2009 earthquakes. The high level of damage undergone by Castelnuovo in 1762 appears to be due to local large amplifications of ground motion (as occurred following the 2009 earthquake, see Tertulliani et al., 2009; Lanzo et al., 2011), showing its proclivity to be extensively damaged. Conversely, the village of Monticchio appears to have been preserved from heavy damage in 1762, just as it was also in 1703 (Morelli, 1962) and in 2009 (Galli et al., 2009).
Previously, no information was found on the duration of the whole 1762 seismic sequence (Amato and Ciaccio, 2012), apart from some generic evidence describing a period of 24 days of continuous shocks. New information retrieved from this study (ASNA, 1762, 1763b) shows that the sequence was active up to the end of December 1762 (approximately for three months) and included frequent aftershocks that alarmed the population at L’Aquila and Castelnuovo.
Figure 5 shows all the macroseismic information available at the end of this study. The intensity data set has been more than doubled by our investigation (rising from 6 to 13 intensity data points), and it is now possible to attempt a more reliable estimation of the epicentral parameters. To allow for a robust and objective comparison with the source parameters estimated by previous studies, we decided to use the Boxer 3.3 code (Gasperini et al., 1999) adopted by the latest Italian earthquake catalog (Rovida et al., 2011). The Boxer 3.3 code correlates the magnitude to the available isoseismal areas defined as those of circles with radii equal to the average epicentral distance of sites with a given intensity (Gasperini and Ferrari, 2000). Clearly, we are aware that an algorithm using intensity observations provides an estimation of the center of strong shaking rather than the location where the quake starts (Bakun et al., 2011). Table 5 shows a comparison between the parameters calculated by Rovida et al. (2011) and by this study.
The newly calculated magnitude in this study has dropped to 5.3 from the earlier value of 6.0 (Rovida et al., 2011) due to the decrease of the maximum intensity; the epicentral location has not changed (as was to be expected, the epicenter in both cases having been calculated with the same code and using an almost unchanged distribution of highest intensity points). However, location and magnitude are now considerably more robust than before because they are based on a larger set of data than previously used in analyses of this event.
After carrying out a painstaking historical study of the 1762 earthquake, we confirm the epicentral coordinates that had been previously determined. At first glance, this might be considered having taken much pain for little gain, were it not for the obvious fact that, far from missing a chance to improve the general knowledge on this historical event, we were in fact able to put new and better constraints on the epicenter position, which provides a better insight on seismic hazard assessment in the study area.
CONSTRAINTS ON THE LOCATION OF THE EPICENTER
The CPTI (Rovida et al., 2011) epicenter of the 1762 earthquake was calculated on the basis of a very sparse and uneven data set; out of eight localities reporting news from the event, only three could be used for calculating the epicenter; a fourth one was judged to be well outside (∼90 km) the epicentral area; and in the last four localities, the earthquake was barely felt. The distribution of these points was also quite puzzling, with the two highest intensity sites immediately juxtaposed by another two sites where intensity seems to have been at its lowest. The 1762 epicenter calculated by our study relies on a data set that includes 13 intensity points, 11 of which are useable for calculating the epicenter. The highest intensities cluster in a 5‐ to 7‐km‐wide area that includes the villages of Poggio Picenze, Barisciano, Castelnuovo, and San Demetrio ne’ Vestini; intensities outside this area tend to decrease as one gets progressively farther from the highest intensity area. Mapping the distribution of indirect and silent sources delimits the earlier mentioned area of maximum damage and limits the possibility of shifting the epicenter any farther northwest or southeast. A southeast shift seems particularly unlikely as an extensive perusal of contemporary sources shows that the life in most villages located southeast of Castelnuovo simply was not affected at all by the occurrence of the earthquake.
The new data set suggests that the epicentral location of the 1762 earthquake cannot have been the same as that of the 2009 earthquake, as the respective macroseismic epicenters are 5–6 km apart, a distance higher than the errors associated with the output of the Boxer 3.3 code (see Fig. 5 and Table 5). To further compare the 1762 and 2009 epicenters, we run the Boxer 3.3 code of localization using the intensities of the 2009 earthquake only on the localities affected by the 1762 event. The result is again a location (named 2009b in Fig. 5) adjacent to the 2009 instrumental epicenter (2009a in Fig. 5).
NEGATIVE AND POSITIVE EVIDENCE ON THE EARTHQUAKE SOURCE
The most important result of this study is the marked decrease of the 1762 earthquake magnitude from 6.0 to 5.3 relative to earlier estimates of the size of this event. Merging this information with the earlier described constraints on the earthquake location, some of the source locations proposed in the past for this event should be no longer considered accurate, as they were based on the old data set and on the quest for a M 6, ∼15‐km‐long source.
First, it can be reasonably ruled out that the 1762 earthquake source was the Paganica fault as it ruptured in 2009 (compare the 2009 source in Fig. 6), the two earthquakes being markedly unlike in terms of fault dimensions and damage scenarios. This assumption is confirmed by the lack of paleoseismological signatures of the 1762 earthquake in the trenches so far dug along the Paganica fault (Cinti et al., 2011; Galli et al., 2011), although the M 5.3 proposed in this study sets the 1762 earthquake at the lower limit for paleoseismological identification.
Second, similar considerations lead us to rule out the S. Pio fault as a possible source for the 1762 earthquake (Fig. 6). Apart from the questionable activity of this structure (Messina et al., 2011), neither the magnitude proposed for a potential earthquake originated by this source (Mw 6.2, Di Bucci et al., 2011) nor the hypothetical intensity map associated with it would fit with the historical evidence recently retrieved for the 1762 earthquake.
A third possibility, suggested by Tertulliani et al. (2009), was that the 1762 earthquake originated on the Fossa fault (Fig. 6), a northeast‐dipping structure antithetical to the Paganica fault and showing evidence of Holocene activity (Emergeo Working Group, 2009). The seismic potential of the Fossa fault could be comparable with the magnitude of the 1762 earthquake; we think, however, that the damage scenario produced by this structure would be totally different.
Placing the 1762 epicentral location in the context of the active faults of the area (Fig. 6), we note that the epicenter is situated at the southern end of the Paganica fault system, in a zone of extremely distributed deformation that coincides with the southern boundary of the 2009 earthquake source and with the limits of most of its aftershocks’ sequence. This is the area where the Paganica complex fault system (composed of several 2‐ to 5‐km‐long segments) has a zone of overlap and stepover with the Middle Aterno fault system (Cinti et al., 2011; Galli et al., 2011). Therefore, from all the previously described evidence and constraints, a possibility is that the 1762 earthquake was located on the southernmost segment of the Paganica fault system, between San Demetrio ne’ Vestini and Poggio Picenze.
When we started this work, our aim was to improve the characterization of the 1762 earthquake in terms of epicentral location and source dimensions. After a systematic search for contemporary evidence which retrieved a great amount of new data, the main findings of our study can be summarized as follows:
a noticeable improvement of the macroseismic database (from 6 to 13 localities) and a wider distribution of the intensities
new information on the duration of the sequence, which lasted approximately three months instead of about 25 days
a marked decrease of the estimated magnitude of the event, from ∼6.0 to ∼5.3
a reduction of the uncertainties associated with the position and the size of the earthquake source, which allowed us to rule out some of the hypotheses about the 1762 event (e.g., twin of the 2009, northeast‐dipping source, and so on) proposed before this study
the constraints on the epicentral location and on the earthquake source dimensions, which suggest that the 1762 earthquake ruptured only the southernmost segment of the Paganica fault
This study demonstrates that improving the macroseismic data set even of a single historical earthquake can influence the definition of damage scenarios and can also affect the knowledge of the seismic hazard of an area. We believe that historical investigation can vigorously contribute to the knowledge of past seismic events and that there is still much to be gained from the in‐depth study of any single historical earthquake.
We warmly thank the staff of the Archivio di Stato in L’Aquila and in Naples and of the Archivio Diocesano in L’Aquila for professional assistance and guidance in the retrieval of historical documents and Fabrizio Galadini for having read and corrected the first draft of the manuscript. Comments and suggestions from a thorough review by an anonymous referee greatly improved the paper.