Seismically Induced Ground Ruptures and Large Scale Mass Movements.

The Pollino Range is the southernmost segment of the Southern Apennines, at the boundary with the Calabrian Arc. While several strong earthquakes (magnitude 6.5 7.0) occurred in the nearby regions, apparently the Pollino area has known no historical evidence for seismic events of magnitude > 5. We carried out an airphoto interpretation and a field survey of the Pollino fault (the major Quaternary normal fault of the area) in order to geologically charac- terize the seismic potential of this structure. We dug two sets of trenches across fault scarps within the apex of latest Pleistocene to Holocene alluvial fans at Masseria Quercia Marina (MQM) and Grotta Carbone (GC) sites, in the central segment of the southern Pollino Range front. At both sites we identified two surface faulting events affecting the alluvial fan deposits and two overlaying colluvial units of historical age. The penultimate event produced a vertical offset of 80-90 cm at GC and 50-60 cm at MQM; while the last event produced a vertical off- set of 40-50 cm at GC and few centimeters at MQM. Based on field observations and range front morphology, we hypothesize that the two historical earthquakes reactivated at least the entire length of the Masseria Marzano Civita segment of the Pollino fault (rupture length about 18 km). For such events, the comparison with surface faulting earthquakes in the Apen- nines and abroad indicates a magnitude of 6.5 7.0. Therefore, the maximum potential earth- quake and the seismic hazard of the Pollino area is significantly larger than that suggested by the available historical seismic catalogue. Purpose and scope of the study The Pollino Range is the southernmost segment of the Southern Apennines, at the boundary with the Calabrian Arc. Crustal extension has shaped these regions during the Quaternary with a system of tectonic basins (e.g., Bousquet, 1973; Ciaranfi et al., 1983; Westaway et al., 1989; Scandone et al., 1992; Valensise et al., 1994; Fig. 1) and is also active today as shown by seismological data (cf. Gasparini et al., 1985). While several strong earthquakes (Intensi- ty X MCS or greater, i.e. about magnitude 6.5 to 7.0; Postpischl, 1985; Boschi et al., 1995; Camassi and Stucchi, 1996; Fig. 1) occurred nearby, seemingly the Pollino area has known no historical evidence for seismic events of Intensity greater than VII MCS (cf. Valensise and Guidoboni, 1995), i.e. about magnitude 5 (yet, for low Intensity events along the Pollino fault, see Magri and Molin, 1979). 85 5. Castrovillari (CS). Earthquake-induced ground ruptures and paleoseismology in the Mt. Pollino Area 1 Servizio Sismico Nazionale, Via Curtatone 3, 00185, Roma, Italia 2 APAT (Agenzia Nazionale per la Protezione dell’Ambiente e per i Servizi Tecnici), via V. Brancati 48, 00144, Roma, Italia 3 Dipartimento di Scienze Chimiche Fisiche Matematiche, Universita dell’Insubria, via Lucini, 3, 22100, Como, Italia S E I S M I C A L L Y I N D U C E D G R O U N D R U P T U R E S A N D L A R G E S C A L E M A S S M O V E M E N T S However, the Pollino fault, that is the major neotectonic structure of the area (“a” in Fig. 1; see also Figs 2 and 3), shows evidence of late Quaternary faulting with the same style (nor- mal), and at least the same (but probably a larger) amount, than that observed in the neigh- boring seismogenic structures (e.g., Bousquet and Gueremy, 1969; Bosi, 1987; Russo and Schiattarella, 1993; Ghisetti et al., 1994; Tortorici et al., 1995; Fig. 1). It is reasonable to assume that in the seismotectonic environment of the Central-Southern Apennines and Cala- brian Arc, major Quaternary normal faults do represent the surface expression of seismic sources capable of strong events (magnitude 6.5 to 7.0; e.g. Michetti, 1994; Vittori, 1994; Tortorici et al., 1995); the lack of documented creep along either the Pollino fault and any other normal fault within the Central and Southern Apennines rules out the occurrence of sig- nificant aseismic slip. Therefore, this apparent disagreement between geological and seismological data may have two explanations: (1) the seismic potential is higher than that suggested by the historical record and the last strong earthquake occurred in pre-historical times, since then the Pollino region being seismically quiescent; or (2) the last strong earthquake is historical but unrecog- 86 Fig. 1. Main Quaternary faults (from our own data and from Bousquet, 1973; Bosi, 1987) and historical seismicity (according to Postpischl, 1985, and Camassi and Stucchi, 1996; Intensity is given in the MCS, MercalliCancani-Sieberg scale) in the area between the Apennines and the Calabrian Arc. Magnitude estimates provide values of 7 for I = XI and of 6.1 to 6.6 for I = X (Postpischl, 1985); note the lack of events with Intensity > VII near the Pollino Range, whereas faults marked “a”, “b”, and “c” do show evidence of recent surface faulting events. The Vallo di Diano (VD), Val d’Agri (VA), Mercure Basin (MB), Morano Castrovillari Basin (MCB), and Valle del Crati (VC) are the main Quaternary extensional basins (e.g. Bousquet and Gueremy, 1968; 1969; LippmanProvansal, 1985; Ascione et al., 1993; Russo and Schiattarella, 1993; Tortorici et al., 1995), corresponding (with the exception of MB and MCB) to the main historical seismogenic structures. C A S T R O V I L L A R I ( C S ) . E A R T H Q U A K E S I N D U C E D G R O U N D S R U P T U R E S A N D P A L E O S E I S M O L O G U I N T H E M T . P O L L I N O A R E A nized, the catalogue is not com- plete, and the seismicity of the Pollino region is similar to that of the nearby areas. These hypothe- ses imply completely different sce- narios for earthquake hazard characterization. To address this issue, we firstly car- ried out field survey and airphoto interpretation along the range front generated by the Pollino fault, the related second-order faults (like the Frascineto cross faults, within the Pollino fault hang- ingwall, and the Civita fault, with- in the Pollino fault footwall; “b” and “c” in Fig. 1; Figs 2 and 3) and over the Morano-Castrovillari basin (Fig. 1), and then selected two sites for exploratory trenching across the Pollino fault. In this paper we summarize the results of our analyses in the Pollino region, and discuss the evidence for histor- ical paleoseismicity. The initial findings from these investigations are described in Ferreli et al. (1994) and Vittori et al. (1995). 87 Fig. 2. Geologic map and late Quaternary faults of the Pollino region. Inset shows the location of Fig. 4 Fig: 3. Block diagram showing the structural relations in the Pollino region, near Castrovillari (after Bousquet, 1973, modified); dotted lines show the maximum dip on fault planes; the represented marker is the top surface of the lower Miocene. Post-lower Miocene vertical displacement on the Pollino fault reaches a maximum of ca. 3000 m, while post-lower Pleistocene vertical displacement reaches a maximum of ca. 400 m. Based on available geomorphic and structural data, since the beginning of the Quaternary extensional phase the N-S trending faults in the Castrovillari basin are interpreted as release faults (sensu Destro, 1995) confined within the hangingwall of a major active normal fault, the Pollino fault; likewise, cross faults developed in the Pollino fault footwall. In particular, recent activity is concentrated on the Civita and Frascineto faults. S E I S M I C A L L Y I N D U C E D G R O U N D R U P T U R E S A N D L A R G E S C A L E M A S S M O V E M E N T S a Radiocarbon analyses by GEOCHRON Laboratory, Cambridge, Massachusetts, USA. b Age calibration calculated (cal) following Stuiver and Becker, 1993. c Samples collected, for instance, at Masseria Quercia Marina site, trench 1, are marked as MQM1; Figs 2 and 4 show the map location of the sampling sites. d TOC is for total organic carbon * AMS dating 88 Samples dC Laboratory reported Calibrated Radiocarbon Location c Radiocarbon Age, Age b (cal years at 2s) and Description C years B.P. Table 1 – Radiocarbon samples POL 2 -27,6 575 ± 210 1270-1615 A.D. MQM1, top unit 1, wood