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A multidisciplinary approach to the evaluation of the mechanism that triggered the Cerda landslide (Sicily, Italy)

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A multidisciplinary approach to the evaluation of the mechanism that triggered the Cerda landslide (Sicily, Italy)
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  A multidisciplinary approach to the evaluation of the mechanismthat triggered the Cerda landslide (Sicily, Italy) Valerio Agnesi a, *, Marco Camarda  b , Christian Conoscenti a  , Cipriano Di Maggio a  ,Iole Serena Diliberto c , Paolo Madonia c , Edoardo Rotigliano a  a   Dipartimento di Geologia e Geodesia, Universita` degli Studi, Corso Tukory 131, Palermo 90134, Italy  b  Dipartimento CFTA, Universita` degli Studi, Via Archirafi 36, Palermo 90100, Italy c  Istituto Nazionale di Geofisica e Vulcanologia, Sez. di Palermo, Via U. La Malfa 153, Palermo 90100, Italy Received 8 December 2003; received in revised form 20 July 2004; accepted 5 August 2004Available online 28 September 2004 Abstract The present paper describes a multidisciplinary approach to the evaluation of a seismically triggered landslide that occurredin the Cerda area (Italy) on September 6, 2002, about 1 h after an earthquake took place in the south Tyrrhenian Sea. The studywas focused on an analysis of the role of the seismic input in triggering the landslide, in view of the evidence that no other massmovement was recorded in the adjacent areas despite geological and geomorphological spatial homogeneity. The studied area islocated on a slope of the western flank of the Fiume Imera Settentrionale (Northern Sicily), which is made up of clayey–areniticrocks. The slope inclines gently but is not uniform due to fluvial, gravitative, and rainwash processes. Field data dealing withglobal positioning system (GPS), geology, geomorphology, geophysics (vertical electrical sounding, or VES), and geochemistry(soil gas fluxes and composition) were acquired and analysed in order to investigate the cause–effect relationships between theearthquake and the mass movement. The GPS survey allowed us to map the ground failures that have also been classified on the basis of their kinematical meaning (i.e., compressive, distensive, or transcurrent structures). The geological analysis revealedoutcropping rocks and tectonic structures. The geomorphologic survey highlighted the presence of preexisting landslide bodies.The geophysical survey detected a buried surface located at a depth of about 100 m . Finally, the geochemical survey showedthat the gas released from the displaced mass came from a shallow depth and was not related to any active fault system. Theabovementioned information allowed us to interpret the landslide event as a partial reactivation of a preexisting landslide bodythat was triggered by the earthquake. D  2004 Elsevier B.V. All rights reserved.  Keywords:  Landslide; Earthquake; Geochemistry; Geophysics; GPS; Triggering mechanism 1. Introduction On September 6, 2002, at 03:21 am, an earthquakehaving a magnitude of 5.6 (MI) occurred in the south 0169-555X/$ - see front matter   D  2004 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2004.08.003* Corresponding author. Fax: +39 91 704 1041.  E-mail address:  agnesi@unipa.it (V. Agnesi).Geomorphology 65 (2005) 101–116www.elsevier.com/locate/geomorph  Tyrrhenian Sea (INGV, 2002). The epicentre (Fig. 1) was located out at sea about 50 km north–northeast of the city of Palermo (Sicily, Italy). Its focal depth wasabout 18–20 km. The earthquake represents the latest event of a seismic series, historically recorded in thisarea since 1726. The seismicity is caused by theshifting of the African plate, which is slowly movingtoward N–NW, and the European plate.During the earthquake, the macroseismic fieldaffected the northwest area of Sicily. There were novictims or injured people, but there was a lot of damage especially to the oldest buildings. Soon after the main shock, several geological effects caused byseismic shaking were observed, even at distancesgreater than 100 km from the epicentre: changes in physical parameters of some thermal springs (Car-acausi et al., 2002) and the occurrence of a landslidewere the most significant. As far as the first effects areconcerned, the rise in groundwater level elevationscaused the reactivation of dried springs; temperatureanomalies of 1  8 C above the highest value recorded inthe past were measured in two thermal springs of theTermini Imerese basin (Fig. 1). The landslide occurred near the village of Cerda,about 1 h after the earthquake (as attested by peopleliving there and reported by local newspaper s) on theleft slope of the Fiume Imera Settentrionale (Fig. 1). It  affectedanareaofabout1.5km 2 andcaused damagetoseveral country roads and buildings. In order to studythe cause–effect relationships between the earthquakeand the mass movement, the Department of Geologyand Geodesy of the University, and the local Section of the National Institute for Geophysics and Volcanology(INGV)ofPalermocarriedoutamultidisciplinary fieldsurvey whose results are presented in this work.Field surveys and data interpretation processeswere organized in accordance with the systemicapproach shown in Fig. 2. The first step consisted of  the global positioning system (GPS) mapping of the Fig. 1. Location of the studied area.Fig. 2. Logical flow chart of the multidisciplinary approach. V. Agnesi et al. / Geomorphology 65 (2005) 101–116  102  landslide area aimed at evaluating the displacement and direction of the movement.The landslide was then analysed from three differ-ent points of view:1) Geology and geomorphology, in order to ascer-tain the tectonic, lithological, geomechanical, andgeomorphological conditions under which themass movement occurred (i.e., why there and not anywhere else?)2) Geochemistry, with particular reference to theflux and composition of the fluids degassing fromthe soil and their spatial relationships to theobserved ground failures (i.e., are fluids con-nected to active tectonic lineation?)3) Geophysics, in order to individuate and character-ize possible buried sliding surfaces.All the data, independently acquired, were thenanalysed and interpreted under the dichotomic eval-uation key formulated as follows: is this singleinformation coherent with a simple gravitationalmovement, or is it necessary to postulate the presenceof an active/passive tectonic lineation? In other words,what is the most reliable scenario between thefollowing options?1) new gravitational movement triggered by theearthquake, or 2) reactivation of an ancient landslide triggered bythe earthquake.In both cases, causes are:a) landslide driven by passive tectonic structures b) landslide caused by (re)activation of tectonicstructures.A possible answer to these questions will bedeveloped during the critical discussion of the wholeset of field evidences presented in the followingsections. 2. Geological and geomorphological setting of thestudy area The Cerda area (Fig. 3a) is a sector of the MadonieMountains, which represents a segment of the Magh-rebide–Apenninic chain. In the Madonie area, succes-sions of Meso-Cenozoic and Upper Tortonian–Lower Pliocene late-orogenic rocks are present (Grasso et al.,1978; Abate et al., 1988). The Meso-Cenozoicsuccessions are made up of sandy clays and marlyclays ( b Argille Variegate  Q  ; Lower Oligocene–Upper Cretaceous) or marly calcilutites ( b FormazionePolizzi  Q  ; Oligocene–Upper Eocene) belonging to theSicilide Units; clay with intercalations of sandstonelevels ( b Flysch Numidico  Q  ) of the Numidian Units(Lower Miocene–Upper Oligocene); mainly carbonaterocks of the Panormide Units (Middle Oligocene– Upper Trias); alternations of shales, marls, radiolarites,and carbonates of the Imerese Units (Oligocene–Upper Trias); and alternations of marls and calcilutites of theLercara Units (Trias). The late-orogenic units are madeup of fluvial-delta (clays, sandstones, and conglom-erates of the  b Formazione Terravecchia  Q  ; Lower Messinian–Upper Tortonian), reef (coral limestones;Lower Messinian–Upper Tortonian), evaporitic (lime-stones, gypsum, and salt rocks; Messinian), and pelagic (marly calcilutites of the  b Formazione Trubi  Q  ;Lower Pliocene) rocks covering the Meso-Cenozoicsuccessions. Miocene overthrusts and Plio-Pleistocenefault systems are responsible for the existing tectonicsetting (Catalano et al., 1996), which consists of a pile of imbricate tectonic units (thin-skinned tectonicsAuct) that have been folded and faulted.On a large scale, the selective erosion, thanks to thetectonic or stratigraphic superimposition of terrige-nous covers on carbonate rocks, is responsible for astrict congruence between topographic and tectonichighs and lows (Hugonie, 1982). The Madonie Mountains are characterized by four different geo-morphological systems, each marked by a well-defined morphostructural style and a typical associa-tion of landforms (Agnesi et al., 2000): (a) monoclinal faulted blocks (Monte dei Cervi area), mainly producing structural landforms; (b) faulted carbonatemassive blocks (Pizzo Carbonara–Pizzo Dipilo area),affected by intensely karstified planation surfaceslocated at different heights; (c) intensively faultedand fractured relief (Monte Quacella area), wherelandforms caused by deep-seated gravitational slopedeformation phenomena and physical weathering processes prevail; and (d) systems of clayey–areniticslopes, ruled by frequent surface landslides.The Cerda area in particular (Fig. 3 b) belongs to a territory where morphology is characterized by V. Agnesi et al. / Geomorphology 65 (2005) 101–116   103  Fig. 3. (a) Geographical location. (b) Geological map (modified from Abate et al., 1988). (c) Landslide map of the Cerda area. V. Agnesi et al. / Geomorphology 65 (2005) 101–116  104  clayey–arenitic slopes. It is mostly constituted byclayey rocks intercalated by marly, arenitic, or con-glomeratic levels. The rocks can be roughly referred toas Argille Variegate, to the Formazione Lercara, to the Numidian Flysch, to the Formazione Polizzi, to theFormazione Terravecchia, to the reef or evaporiticsuccessions, and to the Formazione Trubi. Its structuralsetting is characterized by the overthrusting of SicilideUnits above Numidian and Lercara Units (Abate et al.,1988). Late-orogenic units cover the Meso-Cenozoicunits with a stratigraphic boundary.In relation to the outcropping terrains, low hillswith gentle slopes dominate the landscape of theCerda area. The slopes are affected by a number of landslides (Fig. 3c) in various degrees of evolutionand activity, as well as by landforms caused byerosive and sedimentary processes due to surfacewater. Furthermore, where intercalations of relativelyresistant rocks are particularly frequent, selectiveerosion gave srcin to structural forms. 3. Analyses and results 3.1. GPS survey Between Septem ber 16 and 24, 2002, a GPSsurvey was made (Fig. 4), with the aim of mapping Fig. 4. Locations of GPS stations, geochemical profiles, and VES. V. Agnesi et al. / Geomorphology 65 (2005) 101–116   105
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