Areal Distribution of Ground Effects Induced by Strong Earthquakes in the Southern Apennines (Italy)

AREAL DISTRIBUTION OF GROUND EFFECTS INDUCED BY STRONG EARTHQUAKES IN THE SOUTHERN APENNINES (ITALY) S. PORFIDO1, E. ESPOSITO1,E.VITTORI2, G. TRANFAGLIA3, A.M. MICHETTI4, M. BLUMETTI5, L. FERRELI2, L. GUERRIERI2 and L. SERVA2 1Istituto di Ricerca Geomare Sud - C.N.R., Via A. Vespucci, 9, 80142, Napoli, Italy E-mail: porfi[email protected] 2ANPA – Agenzia Nazionale per la Protezione dell’Ambiente, Via Vitaliano Brancati, 48, 00144, Rome, Italy 3Servizio Idrografico e Mareografico, Via Marchese Campodisola 21, 80133 Napoli, Italy 4Dipartimento di Scienze CC.FF.MM, Universitá dell’Insubria, Via Lucini, 3, 22100, Como, Italy 5Dipartimento Servizi Tecnici Nazionali - Servizio Sismico, Via Curtatone, 3, 00185, Rome, Italy (Received 2 January 2002; Accepted 17 June 2002) Abstract. Moderate to strong crustal earthquakes are generally accompanied by a distinctive pattern of coseismic geological phenomena, ranging from surface faulting to ground cracks, landslides, liquefaction/compaction, which leave a permanent mark in the landscape. Therefore, the repetition of surface faulting earthquakes over a geologic time interval determines a characteristic morphology closely related to seismic potential. To support this statement, the areal distribution and dimensions of effects of recent historical earthquakes in the Southern Apennines are being investigated in detail. This paper presents results concerning the 26 July 1805 earthquake in the Molise region, (I =X MCS, M = 6.8), and the 23 November 1980 earthquake in the Campania and Basilicata regions (I =XMSK,Ms = 6.9). Landslide data are also compared with two other historical earthquakes in the same region with similar macroseismic intensity. The number of significant effects (either ground deformation or hydrological anomalies) versus their minimum distance from the causative fault have been statistically analyzed, finding characteristic relationships. In particular, the decay of the number of landslides with distance from fault follows an exponential law, whereas it shows almost a rectilin- ear trend for liquefaction and hydrological anomalies. Most effects fall within the macroseismic area, landslides within intensity V to VI, liquefaction effects within VI and hydrological anomalies within IV MCS/MSK, hence at much larger distances. A possible correlation between maximum distance of effects and length of the reactivated fault zone is also noted. Maximum distances fit the envelope curves for Intensity and Magnitude based on worldwide data. These results suggest that a careful examination of coseismic geological effects can be important for a proper estimation of earthquake parameters and vulnerability of the natural environment for seismic hazard evaluation purposes. Key words: active tectonics, ground effects, historical seismicity, Italy, seismic hazard, seismic landscape, seismite, Southern Apennines 1. Introduction Tectonic crustal structures capable of producing moderate to strong (surface wave magnitude Ms > 5.5) earthquakes typically generate permanent environmental changes, by the occurrence of peculiar geomorphic features: surface faulting, uplift Surveys in Geophysics 23: 529–562, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 530 S. PORFIDO ET AL. and subsidence, slope failures, drainage changes – including temporary or permanent damming, liquefaction, compaction and hydrogeological anomalies. Just after a large crustal earthquake, these elements characterize the scenery as scattered “ir- regularities”; with time they become integrated components of the landscape. The repeated occurrence of these features (which can be considered as seismites, sensu Vittori et al., 1991), leaves a signature in the recent stratigraphy and topography of an area (paleoseismic evidence), which is related to the potential magnitude and repeat interval of the local seismicity and to the local geological framework. This is the recently introduced concept of “seismic landscape” (Serva, 1995; Michetti and Hancock, 1997), which postulates that, once the geodynamic and climatic environments of an area have been properly taken into account, the geomorphological setting is a reliable indicator of its level of seismicity, and must be included in the assessment of seismic hazard. Therefore, a detailed characterization of permanent and temporary ground effects, for documented earthquakes from selected sample areas with specific tectonic environments, is a basic tool to define the seismic landscape to be expected for a given region and level of seismicity. This also represents also a valid back- analysis tool for assessing the actual vulnerability of the environment, and for predicting its future response to significant releases of seismic energy (Serva, 1994; Esposito et al., 1997a; Jibson et al., 1998; Parise and Jibson, 2000; Keefer, 2000, Wasowski and Del Gaudio, 2000), which is much needed for a proper definition of land use codes and land planning in seismic areas. Depending on the geological environment and the magnitude of the event, the scenery following an earthquake is distinctive. Significant recent examples come from the 17 January 1995, Kobe earthquake in Japan (body wave magnitude Mw = 7.1, intensity X–XI MM – Mercalli Modified scale: EQE Summary report, 1995; Bardet et al, 1995; Sassa et al., 1996) and the 17 August 1999, Kocaeli earthquake in Turkey (Mw = 7.4, intensity X MM: USGS, 1999; EERI, 1999); these events were followed by a wide suite of primary (surface faulting) and secondary (mainly liquefaction, soil settlement and landslides) effects clearly related to the distance from the ruptured faults. Particularly impressive was the coastal submergence in the Goluck area, a common phenomenon along the coastal regions of eastern and central Mediterranean Sea (JSCE, 1999). The detailed description of ground ruptures is a relatively common feature in the historical reports of many destructive seismic events in Southern Italy (Figure 1), such as the 1980 and 1930 Irpinia, the 1857 Basilicata, the 1805 Molise, the 1783 Calabria, the 1694 Irpinia, the 1688 Benevento and the 1456 southern Italy earthquakes (Serva, 1985; Figliuolo, 1988; Porfido, et al. 1991; Michetti et al., 1997; Esposito et al., 2000). As an example, the February 1783 Calabria earthquake (macroseismic magnitude M =6.9,I = XI MCS – Mercalli Cancani Sieberg macroseismic scale) produced what has been called a “geomorphogenetic crisis” (Cotecchia et al., 1986a), changing for ever the geography of large part of the region. Many landslides dammed valley floors producing at least 215 permanent AREAL DISTRIBUTION OF GROUND EFFECTS INDUCED BY STRONG EARTHQUAKES 531 Figure 1. Historical earthquakes of Intensity ≥ IX MCS (from CPTI, 1999) and capable faults (from ITHACA database, Michetti et al., 2000c) in the Southern Apennines, superimposed on a digital elevation model of the region showing an immature basin and range-like morphology. or ephemeral lakes; many ground cracks and extensive liquefaction took place, followed by flood waves and eruptions of ground water with characteristic sand volcanoes. As well, the other large historical earthquakes have left a clear mark in the landscape, mainly, but not only, of their epicentral region. For all these earthquakes, a detailed macroseismic field is available, essentially based on the amount of damages to buildings, according to the MCS or MSK scales (Postpischl, 1985). In order to search for regularities which may help in the interpretation of the correct size of the events and in the assessment of seismic hazard (e.g., the distribution of ground effects with respect to the causative fault), this paper analyzes the characteristics and the spatial distribution of ground effects of events with comparable magnitudes that occurred in the Southern Apennines in the last two centuries. A detailed description is given for the Molise event of 1805 (macroseismic Magnitude M = 6.8), and the Irpinia–Lucania event of 1980 (Ms = 6.9), for which a wide collection of information is available. Some of these data are also compared with those existing for other well studied earthquakes in Southern Italy. 532 S. PORFIDO ET AL. 2. Methods Knowledge the 1980 earthquake and the other strong earthquakes that occurred in Italy during the XX century basically comes from scientific and technical surveys. In contrast, the macroseismic data found in historical documents referring to earthquakes in some way are the best available sources of information on past seismicity before the end of XIX century. Macroseismic intensities are assigned based on distinct degrees of empirical scales, e.g., MCS and MSK; commonly, the uncertainty in intensity assignment is a half degree (Boschi et al., 1995). Thanks to the progress of historical studies in the last two decades, the information on historical seismicity in Italy and Europe has reached satisfactory levels of completeness and homogeneity, by means of procedures that allow verification of the research paths, the methods for data synthesis, and qualified elements for evaluating the data reliability. Such procedures and results are well illustrated in Stucchi (1993), Boschi et al. (1995) and Guidoboni and Ferrari (1995). It is essential to consider that macroseismic data, covering a wide time period, are influenced by the evolving social and cultural environments; therefore, also when they represent the best possible dataset, the description of the event may still be incomplete. Documents from state and local (church and municipal) archives

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