ABSTRACT

Following the April 6 2009 L’Aquila earthquake, a great efforthas been carried out by numerous research institutions for seismicmicrozonation purposes of urban areas in order to evaluate the seismicpotential of the main active faults of the epicentral area. This is thecase of the active Mt. Pettino master extensional fault (hereafterPEF) which controls the tectono-sedimentary evolution of westernL’Aquila Basin. PEF is near to a strongly urbanized area which coin-cides with its pediment. This condition shows a high seismic exposi-tion, so that specifically devoted multidisciplinary investigations,organised in subsequent in-depth steps and whose results are pre-sented here, permitted to fine-tune its seismic hazard evaluation.The investigations were mainly based on a seismic reflection profileorthogonal to the fault strike in its hangingwall. The seismic profilewas interpreted by comparison with the geological outcropping dataand the stratigraphic analysis of numerous boreholes, several ofwhich reached the pre-Quaternary substratum. The main goals weredouble: (i) to improve the integrated geological-geophysical subsurfacemodel for the evaluation of the local seismic amplification and (ii) toestimate the PEF seismic hazard through the study of the sedimentaryinfill evolution and the tectonic subsidence of the PEF hangingwallwhich were strictly interrelated with the PEF seismic history.

KEY WORDS: Quaternary geology, active tectonics, reflectionseismic profile, seismic facies, Western L’Aquila Basin.

INTRODUCTION

In the Pliocene-Quaternary intramontane fault-boundedextensional basins of central-southern Apennines themaster faults commonly dip south-southwestward, borderthe northern-northeastern basin boundary and stronglycontrol the morpho-tectonic evolution of the extensionaldepressions and the consequent basin infill (CAVINATO &DE CELLES, 1999; BOSI et alii, 2003). Moreover basin-boundary faults can be considered active so that theirseismic hazard evaluation should be assessed for the seis-mic microzonation planning (GALADINI & GALLI, 2000;PACE et alii, 2006). Hence, the characterization of thePliocene-Quaternary sediments filling the basins on thehangingwall of the active master faults, in terms of faciesanalysis, geometrical relationships, age determination,and estimated subsidence rate, can provide useful infor-

mation on the seismic history, the kinematics behaviourand, then, the seismic potential of the basin-boundarymaster faults.

Following the April 6 2009 L’Aquila earthquake(Mw: 6.3), a great effort has been carried out by numerousresearch institutions (GRUPPO DI LAVORO MS-AQ, 2010)for seismic microzonation of urban areas in order to esti-mate the seismic potential of the main active faults in theepicentral area. This is the case of the Mt. Pettino masterfault (hereafter PEF) of the western L’Aquila Basin (here-after WAB) which is considered an active fault (BLUMETTI

et alii, 2002; MORO et alii, 2002; BLUMETTI & GUERRIERI,2007; GALLI et alii, 2010). PEF is near to a strongly urbani -zed area which coincides with its pediment. As a conse-quence this area shows a high seismic exposition, so thatspecific multidisciplinary investigations were carried outto fine-tune the seismic hazard evaluation of PEF. Theinvestigations were based on a high-resolution multi-foldwide-angle seismic survey located orthogonal to the PEFstrike in its hangingwall along the fault’s pediment. Theseismic reflection profile was analysed and interpretedthrough the comparison with geological outcropping dataand stratigraphic analysis of boreholes, several of whichreached the pre-Quaternary substratum; this approach istypically carried out in similar Quaternary background(LYONS et alii, 2011). The final profile interpretation per-mitted (i) to provide new constraints on the Quaternarytectonics involving the hangingwall units of PEF forevaluating its seismic hazard and (ii) to elaborate an inte-grated geological-geophysical subsurface model aimed atreconstructing the sedimentary infill evolution and possi-bly the subsidence rate of the PEF hangingwall which canbe strictly interrelated with the seismic history of thismaster fault.

GEOLOGICAL OUTLINE

The study area is located in the Quaternary WNW-ESE oriented WAB which is situated, as an intramontaneplain, between the Gran Sasso Massif and Ocre Mts.. Oro-genic shortening in this sector of the Apennines thrust-fold belt occurred during the upper Miocene-lowerPliocene (DOGLIONI, 1991, with references therein) (fig. 1).From upper Pliocene to Quaternary, the area experiencedWSW-ENE directed extensional regime, well testified bythe presence of mostly S-SW-dipping and NW-SE or W-Etrending normal faults, which conditioned the sedimen-

(1) DICEAA, Università dell’Aquila, L’Aquila.(2) IGAG, CNR, Roma.(3) IAMC, CNR, Napoli.(4) IDPA, CNR, Milano.(5) Presidente Laboratorio di Urbanistica e pianificazione terri-

toriale. Università degli Studi di Napoli “Federico II”, Napoli.

Seismic surveys integrated with geological data for in-depth investigationof Mt. Pettino active Fault area (Western L’Aquila Basin)

MARCO TALLINI (1), GIUSEPPE CAVUOTO (2), FRANCESCO DEL MONACO (1), VINCENZO DI FIORE (3),MARCO MANCINI (2), GRAZIA CAIELLI (4), GIAN PAOLO CAVINATO (2), ROBERTO DE FRANCO (4),

NICOLA PELOSI (3) & ANTONIO RAPOLLA (5)

Ital. J. Geosci. (Boll. Soc. Geol. It.), Vol. 131, No. 3 (2012), pp. 389-402, 9 figs. (doi: 10.3301/IJG.2012.10)© Società Geologica Italiana, Roma 2012

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tary basin filling (BONCIO & LAVECCHIA, 2000; GALADINI

& MESSINA, 2004) such as in the case of WAB. Quater-nary activity of PEF, which borders the WAB northernboundary, produced an asymmetrical graben of triangu-lar shape, 30 km2 wide, entirely filled with continentaldeposits (BIGI, 2006; GRUPPO DI LAVORO MS-AQ, 2010).PEF has an extensional dip- and oblique-slip kinematics;it is about 14 km long, W-E-trending and S-dipping, witha high dip immersion angle and is considered as an activeseismotectonic fault (GALADINI & GALLI, 2000).

The Late Pliocene-Quaternary extensional phase causedthe PEF enucleation and evolution and the consequentdevelopment of the syn-tectonic WAB filled up by conti-nental deposits.

The WAB continental units are related mostly to thelacustrine, fluvial and slope depositional environments(MAGALDI & TALLINI, 2000; MESSINA et alii, 2001). Thesedimentary sequence displays complex architectureswhich reflect the local sedimentary responses to the con-trols exerted on sedimentation and accommodation bythe regional, post-orogenic tectonic regime and by thehigh-frequency Quaternary climate fluctuations (CAVI -NATO et alii, 1994; MAGRI et alii, 2010).

The Aterno River Valley and the surrounding moun-tain ridges are located in the hangingwall of the Gran

Sasso thrust front, which outcrops more northward andcorresponds to the greatest of the Central Apennine thrustand fold structures. The examined area is also charac-terised by sharp topographic contrast with difference inheight of about 2400 metres between the bottom AternoR. Valley and the highest peaks of Gran Sasso Massif.This contrast clearly suggests a geodynamic imbalancewith associated and still active seismotectonic and mor-phogenic phenomena. In the Gran Sasso thrust fronthangingwall generally NW-SE-trending extensional faultsoccur, which cut off the previously built up compressivestructures. These faults control the development of half-graben or horst and graben structures which are locatedin the river valleys at low altitudes (i.e. Aterno River Val-ley) and in the uplifted tectono-glacio-karstic plains atmedium-high altitudes (i.e. Campo Imperatore basin).

The tectonic framework of the substratum of theAterno R. Valley and surrounding slopes is characterisedby monoclines generally dipping to the north-northestwhich are dislocated by several north-verging forethrusts,with associated compressive second order structures (likefolding or duplex- leading-imbricate fan thrust), such asthe so called Ocre Mts. thrust front in the southern slopeof the Aterno R. Valley. The latter caused the superimpo-sition of Mesozoic and Miocene carbonate/ramp platform

390 M. TALLINI ET ALII

Fig. 1 - Geologic outline of the studied area (modified from VEZZANI & GHISETTI, 1998 and GRUPPO DI LAVORO MS-AQ, 2010): 1) Aterno R. alluvialdeposit (Holocene); 2) fine-grained debris slope deposit of PEF pediment (Upper Pleistocene); 3) coarse-grained debris slope deposit of PEFpediment: Pettino unit (Upper Pleistocene); 4) Vetoio R. alluvial deposit (Upper Pleistocene); 5) Pagliare di Sassa alluvial unit (middle Pleistocene);6) calcareous gravel and breccia placed as debris flow (L’Aquila Breccia) (Middle Plesitocene); 7) Scoppito-Madonna della Strada unit sensuBOSI et alii (2003); 8) foredeep terrigenous unit (Messinian); 9) carbonate inner and reef platform units with paraconcordant carbonate rampunits (Miocene and Cretaceous); 10) cherty-calcareous-marly-detrital slope to basin units (with included ramp lithofacies) (from Miocene toLias); 11) normal fault (dashed if presumed); 12) Mt. Pettino active Fault; 13) thrust (dashed if presumed); 14) studied area reported in fig. 2;15) borehole of figs. 3 and 4.

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units on to the Messinian foredeep turbidites (LORÈ &TALLINI, 1997). Hence the pre-Quaternary substratum inL’Aquila area, including WAB, is characterised by fore-thrusts and backthrusts among which the main ones arerespectively that of Ocre Mts. and the one almost com-pletely buried below WAB and cropping out at Colle Mac-chione. But it is worth a mention that the entire L’Aquilaarea has also a paleogeographic and paleotectonic signifi-cance, as it roughly corresponds to the E-W-trending,middle Jurassic to middle Miocene boundary between theplatform and slope to basin domains respectively locatedsouthward and northward of the present-day AternoRiver course.

LOCAL GEOLOGICAL SETTING OF WAB SUBSTRATUM

A fine-scale detailed geological mapping of WAB andspecifically the Mt. Pettino area (fig. 2) and the reinter-pretation of the stratigraphic well logs drilled therebefore, during and after the seismic microzonation activi-ties (GE.MI.NA., 1963; GRUPPO DI LAVORO MS-AQ, 2010)(figs. 3 and 4) provided useful information to constrainthe interpretation of the seismic profile.

The study area is bounded northward by the Mt. Pet-tino ridge, where lithologies belonging to pre-orogenicMeso-Cenozoic slope to basin cherty-calcareous-marlyformations (Corniola, Verde Ammonitico, Marl with

SEISMIC SURVEYS INTEGRATED WITH GEOLOGICAL DATA FOR IN-DEPTH INVESTIGATION OF MT. PETTINO 391

Fig. 2 - Geologic map of the studied area (modified from GRUPPO DI LAVORO MS-AQ, 2010). Quaternary units: Fi) filling anthropic material(Holocene); Cl) fine-grained eluvial-colluvial deposit (Holocene); Db) debris slope deposit (Holocene); Al) Aterno River alluvial deposit (Holocene);Pd) alluvial-debris cone deposit of PEF pediment: Pettino unit (dense and at times poor cemented calcareous gravels with tephra horizonsand well-cemented sometimes micro-karstified calcareous gravels (Pda) (upper Pleistocene); Fd) fine-grained debris slope deposit of PEF pediment(Upper Pleistocene); Ta) terraced alluvial unit of Vetoio Stream: gravels, sand and sandy-clayey silt with tephra horizons (upper Pleistocene);Br) L’Aquila Breccia: dense and poor- to well-cemented calcareous gravels (middle Pleistocene). Meso-Cenozoic carbonate and terrigenousunits: Tt) terrigenous turbidites (Messinian); Ml) Limestone with bryozoa and lithotamnia Fm. (middle Miocene); Ma - mud-supported, micro-detrital and oolitic limestone (detrital Maiolica and Calcarenite and calcirudite with fucoids Fms.) (Cenomanian p.p.-upper Titonic); Co) mud-supported and micro-detrital sometimes dolomitised limestone (detrital Corniola Fm.) (middle Lias): 1) bedding with dip immersion angle;2) horizontal bedding; 3) fault (dashed if presumed), F1, F2, F3, F4: faults described in the text; 4) thrust (dashed if presumed); 5) fault plainwith dip immersion angle (PEF); 6) fault rock; 7) edge of alluvial terrace; 8) alluvial-debris cone; 9) earthquake-induced landslide (not mappable);10) reflection seismic profile; 11) borehole of figs. 3 and 4.

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Fucoids and Maiolica Fms.) all showing recurrent calcare-ous/detrital intercalations, crop out. Besides a 200m-widestrip of those formations in the PEF footwall fault zone isstrongly tectonised and above all dolomitized (BILLI etalii, 2003).

Southward the study area is bounded by the OcreMts. northwestern spurs and the Monteluco Hill. In theseareas Cretaceous carbonate inner and reef platformsequences, on which lay in paraconformity the Miocenecarbonate ramp sequence, crop out. Messinian foredeepterrigenous turbidites cover on top the carbonate sequence.

Besides in the WAB centre it is located Colle Mac-chione, which is a structural and morphological highbounding southward the PEF pediment and the AternoRiver. At Colle Macchione, the above mentioned slope tobasin units are well exposed, as well as a SW-verging back-thrust which caused the superimposition of carbonate for-mations (Maiolica and Scaglia Fms.) on to the Orbulinamarl Fm. and the syn-orogenic Messinian arenaceous tur-bidites (TALLINI et alii, 2002; PETITTA & TALLINI, 2003)Two wells located in the WAB plain (nearby Palombaia di

Sassa: W1 and L’Aquila Hospital: W2, fig. 1), encounteredat about 100 m in depth the above mentioned tectonicsuperposition, thus showing that the Colle Macchionebackthurst structure is buried in part below the WABbasin fill, and extends all over the WAB (GRUPPO DI

LAVORO MS-AQ, 2010). Moreover this backthrust can becorrelated with the outcropping backthrust which charac-terises the Bazzano Hill more eastward to WAB andl’Aquila downtown. In fact also at Bazzano hill the back-thrust causes the superimposition of Scaglia Fm. on tosyn-orogenic Miocene terrigenous formations.

LOCAL STRATIGRAPHY OF THE WAB SYN-RIFT INFILL

The Upper Pliocene-Quaternary stratigraphy of WABis detailed below. Several informal units have been distin-guished, listed in the stratigraphic ascending order andreported in the stratigraphic logs of figs. 3 and 4.

i) Alluvial to lacustrine sequence. An old compo -site, 150-250 m thick, alluvial to lacustrine sequence,

392 M. TALLINI ET ALII

Fig. 3 - Scheme of stratigraphic logs: 1) colluvial, alluvial and talus deposit (Holocene); 2) slope gravel and breccia of Mt. Pettino pediment:Pettino unit (Holocene-upper Pleistocene); 3) Vetoio alluvial unit (upper Pleistocene); 4) Pagliare di Sassa Unit sensu PALOMBO et alii, 2010(middle Pleistocene); 5) L’Aquila Breccia and calcareous tufa of L’Aquila hospital area (middle Pleistocene); 6) Scoppito-Madonna della Stradaunit: pelite with lignite levels and sand or CASL sensu GE.MI.NA. (1963) (lower Pleistocene); 7) CGSA sensu GE.MI.NA. (1963) (lower Plei-stocene); 8) pelite and sand of L’Aquila hill (lower Pleistocene-upper Pliocene?) (w17 and w18 of fig. 1); 9) turbiditic sandstone (Messinian)and Orbulina marl Fm. (middle Miocene); 10) Limestone with bryozoa and lithotamnia Fm. (lower Miocene); 11) detrital limestone (detritalMaiolica and Calcarenite and calcirudite with fucoids Fms.) (Cenomanian p.p. - upper Titonic); 12) pelite; 13) sand; 14) gravel; 15) calcareoustufa; 16) breccia; 17) marly pelite; 18) erosional contact; 19) tectonic contact.

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upper Pliocene-earliest Middle Pleistocene in age andmostly buried in the subsurface, which is tripartite intoapparently concordant sub-units or complexes (GE.MI.NA.,1963; GRUPPO DI LAVORO MS-AQ, 2010). Namely the com-plexes are: i-1) a basal, clayey-sandy-gravelly unit (CASG,sensu GE.MI.NA., 1963), unexposed and resting on thepre-Quaternary bedrock; i-2) an intermediate, gravelly-sandy-clayey unit (CGSA), unexposed; and i-2) an upper,outcropping clayey-sandy-lignitiferous unit (CASL).

i) The formation of “L’Aquila Breccia” Early Plei -stocene-earliest Middle Pleistocene in age, that partlyinterfingers downward, with partial heteropy, with theuppermost levels of the old sequence;

ii) a Middle to Late Pleistocene complex of alluvialterraced deposits, locally interfingered with calcareoustufa, and northward laterally continuous with pedimentdeposits of the PEF;

iii) recent (latest Pleistocene-Holocene) alluvial andcolluvial deposits, mostly related to the Vetoio and AternoRiver systems.

The oldest units of the late Pliocene?-Quaternary basin-fill (i.e. the clayey-sandy-gravelly, and the gravelly-sandy-clayey units, CASG and CGSA) are unexposed and onlyknown from boreholes (GE.MI.NA., 1963; AMOROSO etalii, 2010). They are approximately 40-60 m thick, in total,and are composed of large banks of complexly stackedsands, gravels, conglomerates and pelites. These sedimentsrepresent the early stage of basin infill, and are related tocoarse grained dominated alluvial fan and fluvial environ-ments; being overlain by open lacustrine deposits of theupper clayey-sandy-lignitiferous unit, they may haverecorded the embryonic stage of lake development.

The successive, upper, clayey-sandy-lignitiferous unit(CASL) is up to 200 m thick (figs. 3 and 4), and is preva-lently composed of well bedded and/or massive pelite andsands (with abundant muscovite and clay minerals). Thecommon presence of freshwater molluscs (ESU et alii,1992), such as Bithynia and opercula, allows this forma-tion to be related to the lacustrine environment. This unitis also known as Scoppito-Madonna della Strada unit(BOSI et alii, 2003), and is well exposed both in the WABand in the neaby Scoppito sub-basin, west of Colle Mac-chione. The unit is also characterized by the occurrenceof numerous rests of a late Villafranchian (Early Plei -stocene) mammalofauna, with Mammuthus meridionalis,Hippopotamus antiquus, Stephanorhinus etruscus, Arver-noceros cf. A. giulii (see also AGOSTINI et alii, 2012 andMANCINI et alii, 2012 for further details). In the Scoppitosub-basin and in the lower Raio creek valley, close to theconfluence with the Aterno River in the south-westernWAB, the unit is charac terized by the recurrent occur-rence of several, few metres thick, lignite seams (fig. 5),encased into the sands and pelites, and laterally persistentfor several km (see also GE.MI.NA., 1963).

The typical stratigraphic pattern of the Scoppito-Madonna della Strada unit, resulting from borehole corre-lation, is the one of a series of vertically stacked, metres todecametre thick, tabular bodies of sands and pelites, withminor gravels interposed, almost regularly alternated(MANCINI et alii, 2011). At least five horizons of lignite havebeen recognized from subsurface data in the Scoppito sub-basin, each located approximately at depth intervals of10 metres. The lignite beds are attributed to marshy envi-ronments, in a marginal position with respect to the open

lake deposits found further east in the WAB and in the sub-surface of downtown L’Aquila. The uppermost lignite bedin the Scoppito area has been recently attributed to aninterglacial phase, occurred between about 1.3-1.1 Ma andcorrelated with MIS 37 or 35, on the basis of the integra-tion of paleobotanic and mammal biochronological data(MAGRI et alii, 2010). It is thus possible that older ligniteseams and the intervening lower sandy and pelite beds mayhave recorded a quite long (tens to hundreds of ka) phaseof lacustrine-palustrine sedimentation in the Early Pleisto -cene of the Scoppito sub-basin and WAB.

Besides recent deep boreholes drilled in L’Aquila down-town hill (w17 and w18 in figs. 3 and 4), located immedi-ately east of WAB, evidenced an apparently concordant250 m-thick homogeneous succession composed of clayeysilts and sands, that are totally correlated with the entireupper-Pliocene-earliest Middle Pleistocene sequence. Fromthis drilled succession there is no evidence of those clearstratigraphic and lithologic differences found among com-plexes in WAB and Scoppito sub-basin. The drilled succes-sion lay upon the Mesozoic-Cenozoic carbonate bedrockwith angular unconformity (AMOROSO et alii, 2010).

ii) The L’Aquila Breccia. Latest Early Pleistocene-earliest Middle Pleistocene variably-cemented calcareousbreccias and dense calcareous gravels, the so calledL’Aquila Breccia, outcrop in several areas of WAB, andalso compose the L’Aquila downtown top hill. L’AquilaBreccia, from 50 to more than 90 m in thickness, laysupon the clayey-sandy-lignitiferous upper unit of WAB,and on clayey-sandy sediments in L’Aquila downtown hill,via a few metres thick, interbedded sandy gravel alluvialdeposit. L’Aquila Breccia sometimes is composed byblocks, whose size may reach even some m3 or more, so

SEISMIC SURVEYS INTEGRATED WITH GEOLOGICAL DATA FOR IN-DEPTH INVESTIGATION OF MT. PETTINO 393

Fig. 4 - Stratigraphic logs of the deep wells drilled in the WAB; fortheir location figs. 1 and 2: 1) colluvial, alluvial and talus deposit(Holocene); 2) Vetoio alluvial unit (upper Pleistocene); 3) L’Aquila Brec-cia and calcareous tufa of L’Aquila hospital area (middle Pleistocene);4) Scoppito-Madonna della Strada unit: pelite with lignite levels andsand or CASL sensu GE.MI.NA. (1963) (lower Pleistocene); 5) CGSAsensu GE.MI.NA. (1963) (lower Pleistocene); 6) pelite and sand ofL’Aquila hill (lower Pleistocene-upper Pliocene?); 7) Meso-Cenozoicsubstratum; 8) erosional contact.

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394 M. TALLINI ET ALII

Fig. 5 - A) sand (in yellow) and pelite (in gray) with lignite seam (in black) (Sassa industrial area) referable to the lower Pleistocene Scoppito-Madonna della Strada unit (sensu BOSI et alii, 2003) and corresponding to the LAP’s pattern recognised in the seismic profile; B) lower Pleistocenehorizontal laminated sand and silt of the Scoppito-Madonna della Strada unit (Ponte Peschio site); C) epikarst on L’Aquila Breccia mantledby colluvium and alluvial deposit (S. Antonio site); D) tephra horizon (white arrow) in the colluvium-alluvial deposit mantling the epikarstsurface of L’Aquila Breccia of picture C (S. Antonio site); this tephra can be assigned to the Ancient-Early activity of the Sabatini VolcanicDistrict (0.4-0.8 Ma; SOTTILI et alii, 2004; 2010), so dating, as upper limit, the middle Pleistocene; E) gravel of alluvial Vetoio unit (S. Antonio site);F) calcareous slope deposit of Mt. Pettino pediment.

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that this formation can be attributed to a true megabrec-cia deposit. Generally the clasts of L’Aquila Breccia aremainly composed of lithotypes coming from the Mesozoicslope to basin formations outcropping in the Gran SassoMassif located to the north (BLUMETTI et alii, 1996). Thematrix is abundant and silty-sandy; it lacks of ferroma -gnesian minerals of volcanic origin, which are insteadabundant in younger formations of the area (GALLI et alii,2010). As volcanic products are commonly found after600 ka in the entire L’Aquila area, an upper chronologicalconstrain of about 600 ka can be hypothesized for theL’Aquila Breccia (fig. 5).

L’Aquila Breccia was sedimented into the lacustrinepaleoenvironment via debris-flow, rock-avalanche andlikely flash-flooding processes, which probably occurredunder the effect of extreme morpho-climatic events (DE -MANGEOT, 1965; BLUMETTI et alii, 1996; BLUMETTI et alii,2002; BLUMETTI & GUERRIERI, 2007). Indeed, the Early-Middle Pleistocene transition (between 1.2 and 0.6 Ma),i.e. the inferred interval of the L’Aquila Breccia sedimen-tation, is known to be a period characterized worldwideby intense glacial phases (HEAD & GIBBARD, 2005;EHLERS & GIBBARD, 2007); at local scale the brecciasmay have recorded events of climate deterioration.

SEISMIC SURVEYS INTEGRATED WITH GEOLOGICAL DATA FOR IN-DEPTH INVESTIGATION OF MT. PETTINO 395

Fig. 6 - Seismic tomographic inversion: top: ray tracing; bottom: final tomography model (NE to the left).

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396 M. TALLINI ET ALII

Fig. 7.

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iii) Alluvial terraced deposits. After L’Aquila Brec-cia deposition a warm and humid climatic event wasresponsible for the formation of (i) a diffuse epikarst onL’Aquila Breccia partly colluviated and alluviated andmantling it and (ii) calcareous tufa overlapped on L’Aqui -la Breccia in the hospital area (fig. 5). This epikarst, andso the related L’Aquila Breccia, can be dated back to themiddle Pleistocene because L’Aquila Breccia lay upon theScoppito-Madonna della Strada unit (BOSI et alii, 2003) oflower Pleistocene age. Moreover, at S. Antonio, a fine col-luvium mantling that epikarst contains a ~5 cm-thickthepra horizon of infra middle Pleistocene age, as upperchronological limit (fig. 5); indeed, although its correla-tion to specific events of the Roman Province is uncer-tain, its mineralogical and microtextural features suggestan attribution to the Ancient-Early activity of the SabatiniVolcanic District (0.4-0.8 Ma; SOTTILI et alii, 2004; 2010)during which intense plinian activity took place (personalcommunication of G. Sottili and B. Giaccio).

Then the Quaternary sequence follows upwards withterraced fluvial deposits from the Aterno paleo-River(Vetoio unit) which are laterally in contact with alluvial-debris fan deposits (Mt. Pettino unit) corresponding geo-morphologically to the Mt. Pettino pediment (fig. 5).These sedimentary units are characterised by tephra hori-zons suggesting a middle-upper Pleistocene age, similarlyto equivalent deposits cropping out further east in thePaganica area (GALLI et alii, 2010) (fig. 5).

Besides coarse- and fine-grained deposits pertinent toa large alluvial fan, now deeply eroded, outcrop in thesouthern side of WAB (at Pagliare di Sassa in sandy andsilty sediments remarkable rests of human peopling suchas a notable cherty bifacial and mammals were found:PALOMBO et alii, 2010 and at Lucoli Valley several verycoarse gravels outcrop).

iv) Recent deposits. Finally, the youngest and topo-graphically lowest deposit of WAB corresponds to theHolocene alluvial deposit, which represent the currentsedimentary phase in WAB. The alluvial deposit consistsof alternations of more or less coarse gravels, sands andsilty clays of fluvial and alluvial-fan environments organ-ised in lens-shaped bodies.

THE SEISMIC DATASET

We collected a 1200-m-long seismic profile to investi-gate the basin structure and the fault zone for depthsbelow those reached by trenching and boreholes. Theacquisition layout consisted of an array of 164, 14 Hz-ver-tical receivers deployed with a 5 m interval. The receiversrecorded shots fired with a spacing of 10 m by a Minipulse(2800 Joule) source with 1000 Hz sampling rate. The ini-tial part of the profile (from 0 to 515 CDPx) was also inter-ested by high-resolution seismic tomography (MARTÍ etalii, 2008), in which we used this multi-fold wideanglegeometry since it allows to collect highly redundant turn-

ing waves and deep-penetrating refracted waves, whichcontain information on the velocity distri bution in depth.

The reflection seismic dataset was processed usingPromax and Seismic Unix software. Prior to preliminaryprocessing, a data quality control as well as the definitionof the geometry of the acquisition system and the traceediting has been conducted (SHERIFF & GELDART, 1995).

A qualitatively high velocity analysis was performedby the semblance function that is defined as a normalizedcross-correlation. Spiking pre-stack and post-stack decon-volution was applied to improve temporal resolution. Thefilter coefficients are estimated from the auto-correlo-gram of the seismic trace that contains information onthe source signature.

After deconvolution a time and space variant band-pass filter (TVF) was also applied to remove selectivelythe high and low frequency noise (also boosted by thedeconvolution process) along the sections. Fig. 8 showsthe velocity field obtained by interpolation of some CDPvelocity analysis (red line on the top figure identifies theCDP velocity utilized for the interpolation).

Finally, the stacked data were migrated in depth,using Kirchhoff depth migration (YILMAZ, 1987). The pro-cessing sequence carried out is listed as follows: (i) pre-processing phase: import SEG2 data, geometry setup,quality check trace editing; (ii) processing phase: ampli-tude correction, static correction, spiking deconvolution,velocity analysis (semblance), NMO, stack, Kirchhoffdepth Migration.

We also made use of the trace complex attributes (e.g.TANER et alii, 1979) during structural interpretation. Theinstantaneous attributes A (t), p (t) and f (t) can be con-sidered to be instantaneous estimates of reflectionstrength, phase and frequency at a given seismic sample.We selected the 2D instantaneous frequency section thatrepresent the vector perpendicular to the local wave frontand it is therefore defined by a magnitude and a dip angle(BARNES, 1996).

Data were processed using a Nonlinear Least Squares(BERRYMAN, 1991) inversion technique, while the raytracing was obtained through the Finite Difference Calcu-lation (VIDALE, 1988).

P wave velocity field obtained by refraction dataanalysis were also used for the static correction.

Fig. 6 shows the results of tomographic analysis. TheP wave velocity field range from 0.5 Km/s at the free sur-face up to 3200 m/s at the bottom model.

THE SEISMIC-STRATIGRAPHIC UNITSAND THE SEISMIC FACIES

The seismic facies description was based on reflectorparameters as geometry, amplitude strength (high orlow), continuity, and qualitative reflection frequency, allof which can be described in qualitative terms. A compar-ison scheme of 4 seismic facies (chaotic facies: cf. di -

SEISMIC SURVEYS INTEGRATED WITH GEOLOGICAL DATA FOR IN-DEPTH INVESTIGATION OF MT. PETTINO 397

Fig. 7 - A) 2D depth-migrated reflection seismic line (profile location in fig. 2); B) line drawing of main horizons and faults; C) line drawing ofthe seismic profile with recognised seismo-stratigraphic units. w13, w14 and w10 correspond to the wells located in fig. 2 (legend of litho-stratigraphic unit of w10 as in fig. 3). The profile shows the PEF hangingwall structure characterised by a series of buried and faulted basementblocks. The horizon B, an inferred basal unconformity, illustrates the base of the Quaternary basin infill. The seismo-stratigraphic unitsH, L and S and the faults F1, F2, F3 and F4 are described in the text. MD = main depocentre; HTH = highest thickness of seismo-stratigraphicunit H. PEF and F4 are projected from ground surface.

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srupted reflectors: dr; parallel bedded reflectors with highamplitude: pbr; inclined oblique reflectors and lateralaccretion packages (LAP’s): ior), recognised in the Pettinoarea up to the Aterno R. Valley are listed in fig. 8. Faciescharacteristics correspond to observations between 0 and300 m in depth in the seismic stack of fig. 7. The geologi-cal interpretation of the seismic facies was sustainedthrough several shallow and deep wells located close tothe seismic profile (w10, w11, w12, w13, w14 of figs. 2and 3) and with 3 deep wells (w16, w17, w18) drilled inthe L’Aquila urban area (AMOROSO et alii, 2010; GRUPPO

DI LAVORO MS-AQ, 2010) (figs. 3 and 4).In the studied seismic section, the pre-Quaternary

structural basement and the Quaternary, cover have beenidentified, mostly on the basis of the recognised boundingseismic surfaces and seismic facies, integrated with fieldgeology and borehole interpretation. This led to recognise3 main seismo-stratigraphic units, from top to bottom:H, L, S. Unit L is characterised by the dr, pbr and ior seis-mic facies, while units H and S, above and below unitL respectively, display the same seismic facies cf probablydue to the seismic noise (fig. 9). The top and bottomboundaries of L unit are the well evident: horizon B and Crespectively (fig. 7).

The facies cf is recognised along the entire section inits lower part below horizon B: here, it is composed byconcave/convex up subparallel reflectors and also bychaotic, variable amplitude reflectors situated near thecentral-lower sector of the section. This facies is poorlyresolved probably because of low penetration of waves orsmall impedance contrasts.

The facies dr is recognised only in the southern sectorfrom 890 to 1200 m CDP at depth from 210 to 270 m;they characterize the basal zone with higher reflectivityand amplitude, vertically terminated by a zone with arela tive low-angle toplap.

The facies pbr is characterised by very high ampli-tude, flat topped with relatively concave edging bottomreflectors (CDP: 820-1200 m; depth: 140-210 m); by tabu-lar geometry with well defined sub-horizontal reflectors(CDP: 697-1288 m;); by thinning of reflectors with bicon-cave geometry (CDP: 380-697 m); CDP; by biconvex warpinggeometry with a southward dipping reflection pattern(CDP: 50-380 m).

The facies ior is evidenced by inclined oblique reflec-tors, lateral accretion packages and rare reflectors withbi-directional downlaps occur from 570 to 900 m CDPand at 100-150 m in depth, close to an inflection pointcharacterized by narrow, north-directed and upward con-cave downlapping reflectors.

In the upper part of the entire seismic section again thefacies cf is present above the horizon C within the unit H.

PROFILE INTERPRETATION

The seismic profile interpretation is synthesized inthe line-drawing of fig. 7 and enhanced by field geologyand borehole data analysis.

Considering the geological constraints (fig. 7), thehorizons B and C may assume a geological significance.In particular horizon C could correspond to the litostrati-graphic boundary between the coarse deposits of Pettinoand Vetoio units and L’Aquila Breccia, above, and theupper Pliocene (?)-lower Pleistocene fine-grained mainlylacustrine sequence of the WAB, below (figs. 2, 3, 4), asevidenced by the well logs w10, w13 and w14 (fig. 7). Thehorizon B could correspond to the pre-existing floor mor-phology (the top of Meso-Cenozoic substratum) on whichunit L laps, as evidenced by the well log w10 (fig. 7).

So seismic and geological data points out the influ-ence of pre-existing floor morphology on the accumula-

398 M. TALLINI ET ALII

Fig. 8 - P waves velocity field obtained by the interpolation of some CDP velocity interval. Red line identifies the CDP position utilized for theinterpolation.

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tion of two main sedimentary bodies (units L and H) piledonto previously deformed Meso-Cenozoic carbonate/ter-rigenous substratum (unit S).

The units L and H compose a wedge shaped infill ofthe basin, with the highest thickness located in the south-ernmost portion of the profile, from 900 to 1330 m CDPand at 0-270 m depth. This portion of the seismic linelikely corresponds to the main depocentre of the LatePliocene-Quaternary basin, where the most completestack of infilling units is recorded and preserved.

In the unit L, the facies pbr could correspond to moresub-parallel and laterally extensive reflectors (HARP’s:High Amplitude Reflection packages, or HAC: HighAmplitude Continuous, from POSAMENTIER, 2002) whichare related to finer-grained sediments as evidenced byw10 borehole data (at depth interval: 610-570 m asl) (fig. 9).These fine sediments may correspond to shingled sheet-like deposits; as a whole the internal seismic attributessuggest a lacustrine origin for these sediments, also con-sidering the reference geological setting. In fact boreholedata found a thick sedimentary sequence of fine-grained

lacustrine deposits in the lowermost part of the L’AquilaQuaternary basin (w17 and w18 in figs. 3 and 4; see alsoAMOROSO et alii, 2010).

Concave upward reflectors are confined in the centralportion of the basin (from 730 to 910 m, and between 560and 660 m CDP) forming channelized bodies whose loca-tion slightly shifted laterally to the north. In them,smaller scale, convex up and south dipping reflectors arepresent (ior) in fig. 9), which could resemble the typicalpattern of Lateral Accretion Package or LAP (ABREU etalii, 2003).

The facies dr, in its lower part (200-260 m in depth),shows semi-continuous or disrupted reflectors which maycorrespond to alternated fine- and coarse-grained sedi-ments, as suggested by the occurrence of mainly low-amplitude seismic and irregularly-fringed reflectors (fig. 9).

The overall geometry of unit L changes from thesouthern to northern portion of the basin, where thereflectors lap on and pinch out onto the top of the unit S(from 50 to 400 m CDP, in fig. 7). In several cases thereflectors seem to be also interrupted by faults.

SEISMIC SURVEYS INTEGRATED WITH GEOLOGICAL DATA FOR IN-DEPTH INVESTIGATION OF MT. PETTINO 399

Fig. 9 - Recognised seismic facies types (1) and their location in the seismic profile (2).

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The complex internal seismic pattern of unit H (fig. 9)presumably reflects a high lateral variability of sedimen-tary facies (breccia, gravel, sand, pelite of alluvial lithofa-cies) in the topmost basin infill as inferred also by fieldgeology and well log data (w10, w13 and w14 of fig. 7).

The seismic reflection profile reveals that the subsur-face of Mt. Pettino area exhibits a prominent tectonicimprint composed of a series of buried and faulted blocksin the PEF hangingwall. These blocks and their boundingfaults are parallel oriented to the main strike of the WABand PEF (WNW-ESE), the main tectonic elements in thearea (figs. 2 and 7). Such a feature, with parallel mainand secondary elements, has been already recognised inother nearby intramontane Quaternary basins, such asSulmona, Fucino and Rieti basins (CAVINATO, 1993; CAVI -NATO & MICCADEI, 1995; CAVINATO et alii, 2002).

In detail several main faults, active also during Qua-ternary time, were recognised in the seismic profile bas-ing on the displacement of seismic units and key horizons(B and C in fig. 7). F1 and F2 are southwest-dipping faultsystems and displace units H and L, while F3 is south-west-dipping normal fault system displacing only unit L.

DISCUSSION

Seismic profile interpretation, corroborated also bythe geological mapping (fig. 2) and the analysis of strati-graphic logs (figs. 3 and 4), permitted to remark severalmain considerations.

Concerning the Meso-Cenozoic substratum (seismo-stratigraphic unit S), the Colle Macchione backthrust wasnot detected in the seismic profile probably because ofseismic noise, while the horizon B may represent thebasal unconformity below of the whole Quaternary infillsequence (seismo-stratigraphic units L and H).

The seismic facies pbr in the unit L represents the HighAmplitude Reflections Packages (ABREU et alii, 2003) refe -rable to prevalent fine/medium grained deposits, while unitH is characterised by a more complex and chaotic patterncorresponding to coarser deposits, as evidenced by bore-hole data (w10) (fig. 7). Indeed, a correlation between theseismo-stratigraphic units H, L and S and the lithostrati-graphical ones of WAB is here proposed (figs. 2, 3 and 4).Specifically unit L correspond to the upper Pliocene (?)-lower Pleistocene sequence of the WAB (CASG, CGSA,CASL unit of GE.MI.MA. (1963) or Scoppito-Madonnadella Strada unit sensu BOSI et alii (2003) and MANCINI etalii, 2011) (fig. 7). Whereas, unit H corresponds to thecoarse deposits of Pettino and Vetoio units and L’AquilaBreccia (Pd, Pda, Fd and, probably, Br units of figs. 2, 3, 4),middle to late Pleistocene in age. Such dating is enhancedby the local recovery of 0.8-0.4 Ma old tephra layers withinthese lithostratigraphic units (fig. 5d).

From the above, the principal sedimentary and ero-sive events, from the oldest to the youngest, are:

(i) substratum erosion, with horizon B carving;(ii) medium-fine grained alluvial and lacustrine sedi-

mentation and aggradation (unit L). (LAPs in figs. 7 and 8).(iii) probable horizon C carving, as suggested by par-

tial truncation of reflectors of unit L and undulated shapeof the surface.

(iv) At last, unit H records the final infilling, alluvialdominated, of the Mt. Pettino hangingwall area (fig. 7).

On the tectonic setting it can be deduced from the pro-file interpretation, geological mapping (fig. 2) and strati-graphic logs (figs. 3 and 4), the existence of 4 main faultsystems or single faults in addition to PEF. Fault systemsor single faults F1, F2 and F3 are synthetic, while F4 isantithetic with PEF and with F3. F4 defines, together withPEF and F3, a graben which corresponds to the basindepocentre locate in the Vetoio R. Valley (MD in fig. 7C).

The basal unconformity (horizon B) is downthrownin four steps by PEF and by the fault systems F1, F2 andF3. The step between F1 and F2 dips very gently to thedepocentre (southward), while the other two steps aresub-horizontal, evidencing a differential behaviour ofdeformation (fig. 7).

The displacement of horizon B, due to faults, is about200 m, while the fault steps of F1, F2 and F3 are in theorder of 50 m each (fig. 7).

On the relations between sedimentation and tectonicsadditional features can be recognized (fig. 7):

i) The unit L (upper Pliocene?-lower Pleistocene) isdisplaced by F1, F2, F3 and F4 systems, but apparently,no evident growth strata have been recognized, whichsuggests its sedimentation into a pre-structured graben;

ii) The thickness of the middle-upper Pleistocene unitH is not constant all over the profile as it shows the high-est value at approximately 450 m CDP (HTH in fig. 7C),suggesting a persistent activity of the F2a fault element,syn-depositional to unit H.

The tectono-sedimentary evolution, taking into accountfield geology, borehole data and profile interpretation,can thus be summarised in the following stages, from theoldest to the youngest.

Stage 1: erosion of the Meso-Cenozoic substratumand incision of a concave up surface corresponding to thehorizon B (>2.0 Ma approximately), and activation of F3and F4 antithetic systems, which provided the accommo-dation for unit L in a local small graben (MD of fig. 7C).

Stage 2: deposition of prevalent lacustrine sediments(unit L) with aggradation and lake widening (early Plei -stocene, at about 2.0-0.8 Ma).

Stage 3: i) possible erosion event (probable horizon Ccarving), ii) faulting of PEF and minor systems (F1 andF2) at the hangingwall and iii) syn-tectonic deposition ofthe alluvial unit H, as suggested by local thickening ofunit H at 450 m CDP (middle-late Pleistocene, after 0.8 Ma).

In particular, as concerned the middle-late Pleisto -cene tectonic activity, the (re-)activation of PEF andrelated faults (F1-F2) is in good agreement with the gen-eral enhanced extensional tectonic activity and uplift thatcharacterized the central Apennines since middle Pleisto -cene time (CAVINATO et alii, 1994; D’AGOSTINO et alii,2001 with references therein).

Besides, for the F1 and F2 fault systems, a cumulativevertical displacement of horizon C and overlaying stratais estimated to be some 80 m, occurred in the last 800 ka,and resulting in a 0.10 mm/y of displacement rate. Such avalue is compatible with those calculated for most of thecentral Apennines normal faults (DOGLIONI et alii, 1998;BARCHI et alii, 2000; GALADINI & GALLI, 2000; PIZZI et alii,2002; ROBERTS & MICHETTI, 2004; GALLI et alii, 2008 withreference therein).

Summarising, from 2.0 Ma to the present, geologicaldata and profile interpretation together with paleoseismo-logical observations (GALLI et alii, 2010) shows an appar-

400 M. TALLINI ET ALII

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ent and general trend of migration of faulting activityfrom SW to NE, that is toward PEF: F3 and F4 fault sys-tem activation, than followed by F1 and F2 system andfinally the PEF main fault paleoseismological activity. Sothis suggests that the active seismotectonic strip islocated in close proximity of PEF and F1 (figs. 7 and 10).But, at the moment, a possible reactivation of faults otherthan PEF and F1 cannot be ruled out. Anyway furtherdesirable integrated studies can improve this evaluation.

CONCLUSIONS

The integrated approach between field geology, coredata analysis and reflection seismic investigation allowedus to reconstruct in detail a long Quaternary history (in thelast 2.0 Ma) of a crucial sector of the L’Aquila Basin, at thehangingwall of the active Mt. Pettino normal Fault (PEF).The main conclusive remarks are as in the following:

1) several steps of alternating erosional, sedimentary,and tectonic events have been identified, with the recogni -tion of a relevant post early Pleistocene activity (0.8 Ma toPresent) of PEF and related synthetic faults (F1 and F2),which controlled alluvial syn-tectonic sedimentation (unit H);

2) the evaluated displacement rates is 0.10 mm/yr forthis 800 ka long activity, in agreement with calculatedrates for similar fault systems of central Apennines;

3) the elements, active in late Quaternary times, arelocated in a 600 m broad belt parallel to PEF at its hang-ingwall, providing good pieces of information for seismichazard evaluation at local scale; the closeness of sec-ondary tectonic elements (F1 and F2) to the main one(PEF) resembles a typical structural pattern, recurrentalso for other active normal faults in central Italy (BARCHI

et alii, 2000; GRUPPO DI LAVORO MS-AQ; 2010).

ACKNOWLEDGEMENTS

We would like to thank Giuseppe Naso for his technical supportand availability in the initial stage of the investigation, the directorof CERFIS, Dante Galeota for the encouragement in the investiga-tion of L’Aquila subsoil, CERFIS of L’Aquila University to financethe drilling of the deep wells of L’Aquila downtown and the papercolour printing and CARISPAQ institutions to grant funds for theseismic reflection survey investigation. Thanks are also extended toall the people who helped us during the geophysical acquisitionphase in the field. Discussions with Silvano Agostini, DomenicoCosentino, Paolo Galli, Biagio Giaccio, Elsa Gliozzi, Paolo Messina,Maria Adelaide Rossi and Andrea Sposato on Quaternary geology ofWAB were appreciated. At last we would like to thank FabrizioStorti and the anonymous referee whose comments and suggestionsimproved the final version of the paper.

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Manuscript received 19 July 2011; accepted 28 February 2012; editorial responsability and handling by C. Faccenna.

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