Evidence for strong middle Pleistocene earthquakes in the epicentralarea of the 6 April 2009 L'Aquila seismic event from sedimentpaleofluidization and overconsolidation

F. Storti,1 L. Aldega,2 F. Balsamo,1 S. Corrado,3 F. Del Monaco,4 L. Di Paolo,3

M. Mastalerz,5 P. Monaco,4 and M. Tallini 4

Received 28 November 2012; revised 23 March 2013; accepted 17 June 2013; published 24 July 2013.

[1] Obtaining constraints on maximum earthquake energy from fossil geological evidencehas a primary importance for complementing historical information toward the propermitigation of seismic hazard in a region. In this paper, we describe a newly discoveredextensional fault zone in the western L'Aquila basin (Italy), associated with largepaleofluidization structures. Paleofluidization-bearing Calabrian sediments provideevidence for very shallow burial from mineralogical, organic matter thermal maturity, andflat dilatometer test data, whereas density of organic matter shows anomalously high values.We propose that all these pieces of information can be reconciled into a paleoseismologicalscenario where sediment liquefaction and fluidization in early middle Pleistocene times, aswell as dewatering and overconsolidation, were coseismically triggered by strongearthquakes with magnitude approaching or slightly exceeding 7.0.

Citation: Storti, F., L. Aldega, F. Balsamo, S. Corrado, F. Del Monaco, L. Di Paolo, M. Mastalerz, P. Monaco, andM. Tallini (2013), Evidence for strong middle Pleistocene earthquakes in the epicentral area of the 6 April 2009 L’Aquilaseismic event from sediment paleofluidization and overconsolidation, J. Geophys. Res. Solid Earth, 118, 3767–3784,doi:10.1002/jgrb.50254.

1. Introduction

[2] The study of sediment paleoliquefaction and paleofluidi-zation [e.g., Hurst et al., 2011] has been proved to providemeaningful information for paleoseismic analysis [Obermeier,1996, 2009; Galli, 2000]. When appropriate age constraintsare available, paleofluidization structures can complementmorphotectonical and paleoseismological investigations to re-construct the early evolutionary stages of seismogenic faultzones, which may suffer of scarce information. This is the caseof the L'Aquila basin in Central Italy (Figure 1), i.e., the enve-lope of the tectonic depressions in the hanging wall of theUpper Aterno Valley fault system and the Middle AternoValley-Subequana Valley fault system [e.g., Moro et al.,

2013]. Paleoseismological investigations following the 6 April2009 main shock (Mw=6.1) [e.g., Herrmann et al., 2011] pro-vided evidence for three [Galli et al., 2010; Moro et al., 2013]to four [Cinti et al., 2011] paleoearthquakes that affected theL'Aquila basin from 1703 to about 2900–760 BC.Stratigraphical and morphotectonical evidence allowed Galliet al. [2010] to attempt a reconstruction of the slip rate evolutionof the 2009 seismic event causative fault zone for the last~500ka. However, no information on the coseismic activityand earthquake magnitude is available for the westernL'Aquila basin in Calabrian-middle Pleistocene times.[3] In this work, we describe large paleofluidization-

induced features associated with a previously unmapped ex-tensional fault zone cutting through Quaternary strata whichare exposed in the western sector of the L'Aquila basin.These Quaternary deposits include lacustrine lignites andmudstones which show a strong selective overconsolidationthat we quantitatively describe, as well as their mineralogicaland organic matter features. When inserted into the availableseismotectonic framework, these new data have importantimplications about the relations between the structural archi-tecture of the L'Aquila basin and the inferred occurrence ofstrong earthquakes that affected this region in early middlePleistocene times.

2. Seismotectonic Framework of theL'Aquila Basin

[4] The L'Aquila basin is the largest of several small basinsthat have been developing in the hanging wall of the GranSasso carbonate thrust sheet since mainly Calabrian times

Additional supporting information may be found in the online version ofthis article.

1NEXT - Natural and Experimental Tectonics Research Group,Department of Physics and Earth Sciences “Macedonio Melloni”, ParmaUniversity, Parma, Italy.

2Department of Earth Sciences, La Sapienza University, Rome, Italy.3Department of Geological Sciences, Roma Tre University, Rome, Italy.4Department of Civil and Environmental Engineering and Architecture,

CERFIS Research and Teaching Centre for Earthquake Engineering,L'Aquila University, L'Aquila, Italy.

5Indiana Geological Survey, Indiana University, Bloomington, Indiana,USA.

Corresponding author: F. Storti, NEXT - Natural and ExperimentalTectonics Research Group, Department of Physics and Earth Sciences“MacedonioMelloni,” ParmaUniversity, Parma, Italy. (fabrizio.storti@unipr.it)

©2013. American Geophysical Union. All Rights Reserved.2169-9313/13/10.1002/jgrb.50254

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JOURNAL OF GEOPHYSICAL RESEARCH: SOLID EARTH, VOL. 118, 3767–3784, doi:10.1002/jgrb.50254, 2013

[e.g.,Galadini, 1999; Blumetti et al., 2013]. It is an asymmet-ric half graben produced by the southwestward dippingL'Aquila basin-boundary fault system located on its north-eastern side [e.g., Galli et al., 2010]. The L'Aquila fault systemis strongly segmented and consists of four overlapping majorfault zones (Figures 1 and 2): Marine, Pettino, Paganica, andBarisciano, respectively [e.g., Cinti et al., 2011; Lavecchiaet al., 2012]. Fault-trapped wave analysis indicates that at least,the Paganica and Barisciano fault zones join at depth[Calderoni et al., 2012]. The roughly N-S striking Ovindoli-Pezza fault system, characterized by left-lateral strike-slip to

transtensional motions [Pantosti et al., 1996; Cello et al.,1997; Galadini, 1999], occurs to the south of L'Aquila townand is expected to contribute to the three-dimensional structuralcomplexity in the western sector of the basin (Figure 1).[5] Available gravity data [Blumetti et al., 2002] help in

identifying a major depocenter in the hanging wall of thePaganica fault, which is separated by a relative high fromanother main minimum in the hanging wall of the Bariscianofault (Figure 2). The latter, however, has a trend almostperpendicular to the fault strike, and this suggests a morecomplex buried structural pattern. A gravity minimum also

Figure 1. Tectonic sketch map of the L'Aquila region [after Vezzani and Ghisetti, 1998], with location ofthe 6 April earthquake (black star). Thick fault segments were re-activated in 2009 [Boncio et al., 2010].Coseismic interferogram fringes are from Lanari et al. [2010]. Black dots: A, Pagliare di Sassa field site;B, Ponte Peschio field site. Open triangles: 1, Sassa well; 2, Campo di Pile well; 3, Roio well. Black dia-monds: 4, DMT site 1; 5, DMT site 2; 6, DMT site 3; 7, DMT site 4.

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occurs in the hanging wall of the Pettino fault, again trendingperpendicularly to the fault strike. This minimum is separatedfrom the Paganica fault by a structural high in the overstepzone. This segmentation can help explain the position ofthe 2009 L'Aquila earthquake hypocenter at the northwest-ern tip of the Paganica fault (Figure 1), where a higher stressconcentration could have occurred. The preferential loca-tion in this area of the afterslip that occurred in the first daysfollowing the main shock, as obtained by laser interferome-try [Amoruso and Crescentini, 2009], supports this infer-ence. The highest concentration of aftershocks along thePaganica fault is also located at its northwestern tip(Figure 3), providing further evidence for a main asperitybetween the Pettino and Paganica fault zones (Figure 2).[6] The paleoseismological work following the 2009

L'Aquila earthquake indicates a complex seismotectonicbehavior that can be associated with the strong segmenta-tion of the L'Aquila basin-boundary fault system. Seismicactivation of the northwestern half of the L'Aquila faultsystem (Marine, Pettino, and Paganica fault zones) hasbeen proposed as the reason for the year 1703 Mw ~ 7.0earthquake, whereas only the central segment (Paganicafault) was activated in years 1461 and 2009, producing

earthquakes with Mw ~ 6.0–6.3 [Galli et al., 2010, 2011].A possible reason for this behavior is provided by theevidence that in the L'Aquila region, many Pleistoceneto recent extensional fault systems developed by negativeinversion of preexisting contractional deformation struc-tures [e.g., D'Agostino et al., 1998]. The resulting com-plex along-strike segmentation and overlap patterns arethus governed by the interplay between the present-dayextensional stress field and the structural inheritance[Galadini, 1999; Ghisetti and Vezzani, 1999; Di Luccioet al., 2010]. The role of structural inheritance is con-firmed by the distribution of the aftershock epicenters forthe 2009 L'Aquila event, which highlights three mainclusters (Figure 3): the first one is located in the epicentralarea and is elongated in a NW-SE direction (i.e., parallelto the main shock causative fault zone); the second onehas the same orientation and is located to the N ofL'Aquila town, along the Campotosto Fault where a Mw

5.4 earthquake occurred in 9 April; and the third one islocated to the east, where a Mw 5.6 earthquake occurredin 7 April [Chiarabba et al., 2009; Lavecchia et al.,2012], and is elongated in a N-S direction.

Figure 2. Bouguer gravity anomalies (mGal) in the L'Aquila basin [after Blumetti et al., 2002]superimposed on the fault pattern of the region, as in Figure 1 (in grey). The inset shows a simplified sketchof the geometric relations between gravity anomaly minima and active fault zones; focal mechanisms of thetwo main shocks are from Chiarabba et al. [2009]. Note that the northward prosecution of the Ovindoli-Pezza fault aligns with the southern tip of the Pettino fault and bounds to the west the main gravity anomalyminimum in the hanging wall of the Paganica fault.

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3. Stratigraphical Framework of theL'Aquila Basin

[7] The L'Aquila basin sedimentary infill includes continen-tal deposits of mainly lacustrine, marsh, fluvial, and scree en-vironments which are characterized by complex architecturesreflecting the interplay between Pliocene-Quaternary tectonicactivity and climate variations [Cavinato et al., 1994; Bosiet al., 2003]. Three main sedimentary units, in ascendingorder, can be identified from both field and borehole datain the late Pliocene (?)-Pleistocene infill of the westernL'Aquila basin (Figure 4a) [Magaldi and Tallini, 2000;Tallini et al., 2012]: (i) clayey-sandy-gravelly unit (unexposedand resting on the pre-Quaternary bedrock); (ii) gravelly-sandy-clayey unit (unexposed); (iii) clayey-sandy- and lignite-bearing unit, in which the large paleofluidization structureswere found [“Gemina” Geomineraria Nazionale (Ge.Mi.Na.),1963; Tallini et al., 2012]. The Quaternary sequence followsupward with middle and upper Pleistocene terraced fluvialdeposits from the Aterno paleoriver, laterally interfingered withthe alluvial-debris fan deposits of the Pettino Fault pediment.Finally, modern alluvial sedimentation occurs in the present-day Aterno River plain.

[8] Several very recent findings of Pleistocene mammals inthe western L'Aquila basin provide good chronological con-straints on the analyzed sedimentary succession [Agostiniet al., 2012; Palombo et al., 2010]. In the Pagliare di Sassaarea, the sedimentary succession (Figure 4b), from top to bot-tom, starts with a ~24m thick alluvial fan sands characterizedby normal magnetic polarity (Brunhes Chron) with mammalsremains dating back to the lower part of the middlePleistocene (about 0.6Ma) [Palombo et al., 2010]. An ero-sion surface separates the fluvial sands from at least 8m ofclays and fine sand of marshy environment. In the lower partof this succession, the Brunhes/Matuyama paleomagneticboundary (0.78Ma) was found [Palombo et al., 2010]. Thismarshy succession overlies ~12m of fluvial fine sands withpaleosoils characterized by the remains of plant roots. Thepaleofluidization structures occur in a ~5m thick braidedchannel sandy deposit of river bank and riverbed environ-ment, located below the paleosoil-bearing sedimentary unitand also in the underlying peat/lignite-bearing clays. Thefluidization-bearing strata are dated back to the Calabrianage (about 1.0Myr) from mammal remains [Magri et al.,2010; Mancini et al., 2011]. It follows that the age of thepaleofluidization structures of Pagliare di Sassa is constrained

Figure 3. Contoured distribution of aftershock epicenters of the April 2009 L'Aquila earthquake swarm.A northwest trending concentration occurs to the east of Montereale (Campotosto Fault), a major oneoccurs south of Arischia along the Paganica Fault, and a north-south trending concentration occurs nearFossa. Contouring was done using a network of 200m2 cells and assigning to each of them the averagevalue obtained from a circle having a radius of 1000m, centered in the cell center itself.

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between ~1.0Myr (i.e., the age of the sediments) and ~0.6Myr(the age of the alluvial fan succession that unconformably liesupon the fluvial deposit).

4. Soft Sediment Deformation Structures

4.1. The Pagliare Fault Zone

[9] Availability of new large artificial cuts produced in thewestern sector of the L'Aquila basin during the constructionof new buildings soon after the main shock (Figure 4c)allowed us to recognize a previously unidentified faultzone (Pagliare Fault) synthetic to the Pettino fault zone(Figure 1). This fault zone is exposed in Calabrian continen-tal deposits and has a N63°W mean strike, a southwestwarddip, and extensional kinematics (Figures 5 and 6). The faulthanging wall is folded in a northeastward facing, multistepanticline consisting, from the southwest to the northeast, offour dip domains: (i) a flat-lying sector, (ii) the first mono-cline, (iii) another flat-lying sector in the central part of theoutcrop, and (iv) the main monocline abutting the masterfault surface. Northeastward increasing bed thickness indi-cates synkinematic deposition against the southwestwarddipping master slip surface. Several antithetic faults affectthe rollover anticline with a maximum displacement valueof ~3.5m. Noticeably, faults affecting loose sands consist

of thin cores with a sharp transition to the adjacent undeformedsediments (Figure 5d).[10] The most striking feature of the Pagliare outcrop are

irregularly shaped clastic intrusions and dikes [e.g., Hurstet al., 2011] cutting through the hanging wall strata. Theyare preferentially located at the transition zones between flat-lying and inclined strata of the rollover anticline, adjacent tothe master slip surface (Figures 5a and 5b). Clastic dikes arediscordant to bedding, with the exception of some mush-room-shaped, upward terminations where intrusions and dikesinject laterally for ~1m into the sandy layers. Intrusions arevolumetrically more significant than dikes and show an inter-nally chaotic texture. Intrusions are up to ~22mwide and char-acterized by rapid width changes, whereas subvertical dikesare ~1–2mwide. In the southwestern side of the outcrop, thereis clear evidence of at least two intrusive events (Figure 5b).The intrusions and dikes have erosive, irregular, and sharp ver-tical margins (Figure 5e), with convex-up scallops. There is noevident grading and banding parallel to dike margins.[11] Larger intrusions and dikes consist of mudstone clasts

embedded in a fine-grained matrix (Figure 5f). Clast sizeranges between ~10 and ~1000m, with modal value of64mm (coarse pebble) (Figure 7a). They are irregularly shapedwith angular margins and aspect ratio between 1 and 4 (seeAppendices A1 and A2). There is no clear relationship between

Figure 4. (a) Stratigraphic sketch of the Quaternary succession infilling the western L'Aquila basin.(b) Detail of the sedimentary pile exposed in the Pagliare di Sassa area. (c) Panoramic view of the Pagliare diSassa field site.

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clast size and aspect ratio (Figure 7b), although clasts withequivalent diameter <350mm are generally more elongatethan coarser clasts. The long axes of clay clasts within the intru-sions are roughly subhorizontal and do not show any signifi-cant preferential orientation (Figure 7c), whereas in the dikes,they are mostly subvertical and aligned parallel to their verticalmargins (Figure 7d). The fine-grained sandy matrix shows var-iable grain size distributions. Host sediments have a unimodalgranulometric curve and mean grain size diameter of 234μm(Figure 8a). The grain size distribution curves of the fine-grained matrix are either unimodal or bimodal and generallyshow a poor sorting, as indicated by the higher span values(Figures 8b–8j). Intrusions consisting of irregular bodies of flu-idized gravel in a fine-grained sandy matrix also occur(Figures 5a, 5b, and 5c). In thin section, the sandy matrixwithin the intrusions consists of coarse quartz + feldspar crys-tals embedded in fluidized clay-size particles without anypreferential orientation (Figure 9a). Dike boundaries locallydisplay a subvertical preferential alignment of elongate parti-cles in the matrix, which tend to be parallel to the dike margins(Figure 9b). The granular material is poorly sorted, andparticles are mainly angular to very angular and are frequently

broken by intragranular fractures. Preferential alignment iseven more pervasive within the fault core, where fine-grainedmaterial shows preferential orientation of quartz grains andphyllosilicate minerals (Figure 9c).

4.2. The Ponte Peschio Site

[12] The Ponte Peschio site is located in a housing-related ar-tificial cut to the East of the Pagliare di Sassa site (Figure 1).Calabrian lignite-bearing lacustrine sediments are exposedand characterized by NW-SE striking large vertical fracturescutting through both lignite and clay layers (Figure 10). Theyare typically filled by inorganic clastic sediments coarser thanthose in the adjacent host rocks. In particular, large pebbles typ-ically occur at the bottom of the fractures (Figure 10b). This in-dicates that they developed as open fractures at the ground leveland were then rapidly filled by sediments deposited from high-energy flowing water. The NW-SE striking large fractures areparallel to the most abundant joint set, well developed in lignitelayers where a NE-SW joint set also occurs (Figures 10c and10d). Some extensional fault zones are exposed at this siteand strike almost parallel to the two joint sets (Figures 10cand 10e). Finally, decimetric scale detachment folds occur

Figure 5. (a) Photomosaic and (b) line drawing of the Pagliare di Sassa outcrop. In the line drawing,occurrences of intruded and faulted gravel are in black, while clasts within bodies of intruded and faultedbreccia are highlighted in gray. Numbers in circular envelopes indicate the position of samples for grainsize distribution analysis; those in rectangular envelopes indicate samples for thin section analysis. Therelative chronology of paleofluidization events in the western sector of the outcrop is indicated by A (older)and B (younger), respectively. (c) Detail of a body of intruded and faulted gravel located in proximity of themaster slip surface. (d) Detail of a fault zone characterized by a well-localized fault core that laterally passesto the almost undeformed sand without any evident damage zone. (e) Detail of the scalloped boundary of anintrusive dike at the southwestern end of the outcrop. Clay chips and larger clasts are well evident. (f) Detailof large clay clasts in sandy matrix within an intruded and faulted breccia body. The position of details inFigures 5c to 5f is indicated in Figure 5a.

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within a plastic clay horizon at the top of the younger lignitelayer. They have NW-SE-bearing near-horizontal axes and anortheastward facing (Figures 10c and 10f).

5. Overconsolidated Quaternary Sediments

5.1. X-ray Diffraction (XRD) Analysis

[13] Thirty-nine samples from the paleofluidization-bearingCalabrian sedimentary infill of the L'Aquila basin were ana-lyzed. They were collected from the Sassa and Roio shallowdrill holes and from the Ponte Peschio and Campo di Pile fieldsites (Figure 1). The 50m deep Roio drill hole sampled a ho-mogeneous sequence of greenish clays with sandy interbeds.Nineteen samples were collected every ~3m and X-rayed forquantitative analyses of the whole-rock composition and claymineral content (Figure 11a) (see Appendix A3). Most of thesamples in the 10–35m depth interval show phyllosilicateamounts in the range of 35%–58% (average 50%) with subor-dinate calcite (17%), albite (14%), quartz (11%), dolomite(6%), and k-feldspar (1%). Occasionally, pyrite and gypsumare present. From 35 to 50m depth, calcite doubles its percent-age with average values of 39%; phyllosilicates have a contentof 51%, and quartz (4%), albite (4%), and dolomite (1%)decrease in abundance with respect to shallower samples(<8m). These samples are characterized by the presence ofchabazite and iron phosphates (sample S3-83) and lack of car-bonate minerals (sample S3-88). Traces of halite have beenidentified in sample S3-20. The <2μm grain-size fractionindicates clay mineral assemblages dominated by illite withan average value of 48%. Chlorite, mixed layer I-S, andkaolinite account for 18%, 21%, and 13%, respectively(Figure 11a). The shallower part of the sedimentary sequenceshows an increase in mixed layer I-S and a decrease in chloriteamount. Mixed layers I-S are randomly ordered and highexpandable structures (R0) with no preferred sequence in

Figure 6. Stereographic projection of structural data col-lected in the Pagliare di Sassa field site. The thick line indi-cates the attitude of the master slip surface; thinner onesrefer to subsidiary faults in the rollover anticline. Black smallcircles provide the orientation of slickenlines. Dotted linesindicate the attitude of bedding. Equal area projection, lowerhemisphere.

Figure 7. Fabric description of clay clasts in intruded andfaulted breccia from analysis of field images. (a) Clast sizedistribution; (b) clast cross-sectional aspect ratio versus size;(c) angle to the horizontal of clast long axes within intrusivebodies; (d) angle to the horizontal of clast long axes withindikes. See text for details.

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stacking of illite and smectite layers. Their composition isuniform throughout the well with an illitic content rangingbetween 25% and 30%.[14] The 30m deep Sassa drill hole intersected a Calabrian

sandy-clayey succession with intercalations of centimetric todecimetric thick peat horizons. X-ray data of the whole-rockcomposition and <2μm grain-size fraction for 13 samplesare shown in Figure 11. Samples with prevailing clayey com-ponent (S1-29, S1-32, and S1-39) display great amounts ofphyllosilicate minerals (illite, mixed layers illite-smectite, ka-olinite, and chlorite) and subordinate amounts of quartz(average 14%), albite (~5%), and k-feldspar (~1%). Locally,calcite and gypsum crystals (<1%) occur. X-ray patterns aredominated by low signal-to-noise ratio as a result of organicmatter presence. In their <2μm grain-size fractions, samplesS1-29 and S1-32 are composed of mixed layer illite-smectitewhich constitutes the 62% of the overall composition and loweramounts of kaolinite (21%), illite (12%), and chlorite (5%).Sandy mud samples mostly contain calcite and phyllosilicates(average 45% and 44%, respectively) and small amounts ofquartz (5%) and albite (3%). K-feldspar, dolomite, ankerite,and pyrite do not exceed 2%. The finer fraction reveals claymineral assemblages dominated by illite (46%), mixed layerillite-smectite (22%), kaolinite, and chlorite (both 16%). Theobserved mixed-layered minerals in both lithologies are ran-domly ordered illite-smectite (R0) with high content of expand-able layers in the range of 60%–75%. Occasionally, twodistinct subpopulations of illitic assemblages have been identi-fied. They correspond to R1 and R0 I-S with high (80%) andlow illitic content (35%), respectively.[15] At Ponte Peschio, samples AQ18 and AQ21 derive

from a decimeter thick layer of grey shale occurring betweentwo thick layers of peat (AQ17) and from a folded ash layer(AQ20) located at the top of the younger peat horizon(Figure 10f). Grey shale mostly contains phyllosilicates (illite,mixed layer I-S, kaolinite, and chlorite) from about 63% up to90% and subordinate amounts of quartz, albite, k-feldspar,dolomite, and calcite. In their <2μm grain-size fractions,mixed layer I-S with an illite content ranging from 20% to30%, chlorite, kaolinite, and illite have been identified. Theash layer is mainly composed of volcanic glass (70%),sanidine (10%), quartz (9%), and clinopyroxene (11%). Anoriented specimen of this ash reveals small amounts of smec-tite suggesting incipient volcanic glass weathering. The peathorizon displays mineral contamination of phyllosilicates(smectite, illite, kaolinite, and chlorite) and quartz.[16] At the Campo di Pile site (Figure 1), the clayey sam-

ple AQ27 contains a mineral assemblage composed ofphyllosilicates (81%), quartz (13%), albite (5%), and k-feldspar(1%). Among phyllosilicates, in the <2 μm grain-size frac-tion, high expandable mixed layer I-S is the most abundantmineral (90%) followed by chlorite (8%) and illite (2%).The illite content in mixed layer I-S is about 20%.

5.2. Thermal Maturity and Density of Organic MatterFrom Optical, Fourier Transform Infrared Spectroscopy(FTIR) and Helium Pycnometer Analyses

[17] Organic matter in the studied sections is representedby peat/lignite composed dominantly of huminite maceralswith a various degree of preservation of cell structures. Onthe collected organic matter samples, we performed reflectedlight microscopy, FTIR, and density analyses according to

Figure 8. Grain size data. (a) Grain size distribution of arepresentative sample of the host sand; (b–j) grain size distri-butions of samples of matrix material in intrusions and dikes.Sample location is provided in Figure 4. See text for details.

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standardized procedures (see Appendices A4, A5, and A6).The first two techniques provided paleothermal (thermal ma-turity) indicators (e.g., vitrinite reflectance VRo and CH2/CH3 ratio) of organic matter [e.g., Lis et al., 2005]. In turn,density analyses provided pieces of information on the levelof compaction of the organic matter. Results are illustrated inFigure 11. Samples AQ1, AQ2, and AQ3 derive from thePagliare di Sassa drill hole and were collected at depths of15.0, 24.0, and 26.6m, respectively (Figure 11). Density

values range between 2.1 g/cm3 in the younger (AQ1) and2.7 g/cm3 in the older sample (AQ3), VRo ranges from0.22% and 0.27%, whereas CH2/CH3 ratio ranges from 3.2to 7.4, increasing with depth. Samples AQ13, AQ14,AQ15, and AQ16 were collected from the Sassa drill holeat depths of 4.8, 5.0, 9.7, and 9.9m, respectively, from centi-meter thick peat horizons. They show density values between1.5 and 1.8 g/cm3, whereas VRo ranges between 0.22% and0.29%, and the CH2/CH3 ratio between 2.4 and 3.6. AtPonte Peschio, samples AQ17, AQ18, and AQ22 derive fromthe two well-exposed peat horizons (Figure 10). They showdensity values between 1.5 and 1.9 g/cm3, VRo between0.14% and 0.21%, and the CH2/CH3 ratio between 2.6 and6.2. At the Campo di Pile site, sample AQ27 was collectedin an outcropping thin peat layer, whereas samples AQ7,AQ8, and AQ9 were collected from the Campo di Pile drillhole at depths of 7.5, 19.9, and 20.2m, respectively. Theyshow density values between 1.8 and 2.7 g/cm3, whereasVRo ranges between 0.19% and 0.24% and CH2/CH3 ratiobetween 3.17 and 8.39. In the AQ27 sample, VRo showshigher and scattered values measured on rare and probablyrecycled or oxidized organic particles. In summary, all or-ganic matter-rich samples from both drill holes and surfaceoutcrops show a very low thermal maturation level, and interms of coal rank, they represent peat to lignite stage.However, they have a wide variability in density values.

5.3. Flat Dilatometer Tests (DMT) in Silty Sediments

[18] In situ flat dilatometer tests (DMT) [Marchetti, 1980](Appendix A7) were performed at four sites located mostlyin the area of Campo di Pile (Figure 1). The sites are locatedwithin a relatively short distance, at a similar elevation of theground surface (generally ~ 630m above sea level). The sed-iments in the four examined sites belong to the same lacus-trine deposits. At all the four sites, the sediments appearoverconsolidated, and the overconsolidation ratio (OCR)values estimated from DMTs are rather variable, e.g., OCRvaries from about 8 to 14 at sites 1–3 (Figures 12a–12c)and between 4 and 5 at site 4 (Figure 12d). The shape ofthe profiles of the horizontal stress index (KD) and OCRshows a general trend to decrease with depth, but with vari-able rates. The maximum past elevation of the ground surfaceHm estimated from OCR, using the assumption of simplemechanical unloading, varies in the range of 724m at site 3(Figure 12c) and 819m at site 2 (Figure 12b).

6. Discussion

6.1. Soft Sediment Deformations asPaleofluidization Structures

[19] Several lines of evidence support the interpretation ofthe soft sediment deformation features in lacustrine deposits de-scribed at Pagliare and Ponte Peschio sites as paleofluidizationstructures. The mushroom geometry and convex-up scallops,combined with preferential alignment of mudstone clasts andquartz-feldspar grains parallel to subvertical dyke margins, atboth mesoscales and microscales, are indicative of the upwardinjection of sands and progressive scouring of the host mud-stone [e.g., Obermeier, 1996; Hibsch et al., 1997; Hurst et al.,2011]. The occurrence of large volumes of multiple clasticintrusions with chaotic mesotextures and microtextures, com-bined with pebble to cobble mudstone angular clasts, suggests

Figure 9. Scanning electronic micrographs of fluidized ma-terials. (a) Sandy matrix within an intrusive body, showing nopreferential clast orientation (sample 2 in Figure 4). (b) Sandymatrix fabric at a dike boundary: clasts are preferentiallyaligned parallel to it (sample 5 in Figure 4). (c) Preferential ori-entation of clasts in a fault core rock (sample 4 in Figure 4).Many clasts are affected by intragranular fracturing in all thethree images.

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Figure 10. (a) Overview of the Ponte Peschio site: large fractures in the peat-lignite-bearing beds are wellevident. (b) Detail of a large fracture and its infilling by very coarse sediments. (c) Stereographic projectionof structural data: contouring of pole to joints in peat-lignite layers indicates the presence of two main setsstriking parallel and perpendicular to the Pettino and Pagliare di Sassa fault zones; dotted lines indicate theattitude of bedding; broken lines indicate the orientation of large fractures in peat-lignite-bearing beds; redlines indicate the attitude of faults and small red circles provide the orientation of the related slickenlines.Equal area projection, lower hemisphere. (d) Detail of a peat-lignite layer showing the occurrence of twomutually perpendicular joint sets. (e) Detail of a fault surface affecting a peat-lignite layer. (f) Detail ofthe folded thin ash layer at the top of the upper peat-lignite layer. The position of samples collected at thissite is indicated in Figures (b) and (f).

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Figure 11. (a) Mineralogy of the clay-size fraction (<2μm) and vitrinite reflectance data for the studiedwells and field sites; the original data are provided in Tables S1 and S2 of the supplementary information.(b) Review of coal density versus coal rank from data available in the literature and comparison with ourdata. See text for details.

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Figure 12. Results of flat dilatometer tests. (a) Site 1, (b) site 2, (c) site 3, and (d) site 4; the original dataare provided in Table S3 of the supporting information. Different colors refer to different DMT tests in thesame site. See Figure 1 for location.

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that such features can be related to sediment fluidization duringepisodes of elevated pore fluid pressure [e.g.,Obermeier, 1996;Montenat et al., 2007]. Furthermore, the variable mean grainsize and bimodal grain size distributions of sandy matrix withinthe intrusions and dikes are consistent with the grain-scalemixing of unconsolidated sandy layers during particulate flowin a water saturated environment [e.g., Obermeier, 1996]. Thelack of grading in the fluidized sediments suggests that fluidshad a particle concentration higher than about 60% [Hurstet al., 2011]. At Pagliare di Sassa, the largest amount of the flu-idized sediments can be defined as intruded and faultedbreccia due to the abundance of very coarse angular clayclasts and blocks in a sandy matrix and the lack of roundedpebbles. Intruded and faulted gravel [Rodriguez-Pascuaet al., 2000] also occurs, particularly adjacent to the masterslip surface (Figure 5).[20] The presence of the thick layer of uppermost Calabrian

clays suggests that they may have acted as the top seal of

the hydrogeological system suitable of liquefaction [e.g.,Obermeier, 1996, 2009]. The lack of lignite fragments in thePagliare di Sassa sedimentary intrusions and dikes rules outa deeper source region for the liquefactions. If this was thecase, sediment liquefaction and fluidization at the Pagliare diSassa site occurred with an overburden of about 25m, after de-position of the clay-rich unit (0.78 Myr) and before erosionand deposition of the overlying alluvial fan sands (0.6 Myr),i.e., in the early middle Pleistocene.

6.2. Coseismic Origin of Paleofluidization Structures

[21] Key features of the paleofluidization structures associ-ated with the Pagliare fault zone include: (i) their very largewidth; (ii) their crosscutting the entire fault hanging wallexposure; (iii) the abundance of very coarse clay clasts in asandy matrix; (iv) the foliated fabric at different scales ofthe chaotic material; and (v) diffuse grain fragmentation inthe matrix material. They all imply deformation at high en-ergy conditions. Earthquake-induced shear is the most likelysource of the required high stress in a high seismicity region,such as the L'Aquila basin. Accordingly, we interpret thesepaleofluidization structures as seismites [e.g., Rodriguez-Pascua et al., 2000; Montenat et al., 2007]. Examples ofcoseismically induced fluidization structures that can becompared to those exposed in the Pagliare di Sassa outcropare provided, among others, by Hibsch et al. [1997] for theHolocene fluvial-lacustrine sediments of the northern Quitobasin (Ecuador). Further support to this inference is providedby the presence in the pristine loose sands of structurallyoversimplified fault zones that lack any deformation structurein damage zones despite their metric displacement magni-tude. This feature has recently been associated with acoseismic origin at shallow depth [Balsamo and Storti,2011]. On the other hand, the lack of detectable geomorphicevidence (Figure 4c), the well-preserved geometric and sedi-mentary correlations among nonfluidized sediments, and thevery good fit of structural data into a fault zone synthetic tothe Pettino one testify against landsliding as a possible mech-anism of paleofluidizations at the Pagliare di Sassa site. Thesame reasoning applies to the Ponte Peschio site, where theobserved open fractures are interpreted to have formed byseismically induced lateral spreading [Obermeier, 1996].[22] At Pagliare di Sassa, the preferential locations of in-

truded breccia and gravel along the ramp segment of the mas-ter fault zone and some antithetic fault zones, and in the hingezone of the rollover anticline, can be explained by the em-placement of fluidized materials in the maximum dilation sites(Figure 13). Moreover, clay-rich sediment shearing along themaster slip surface dramatically reduces across-fault perme-ability [e.g., Fisher and Knipe, 2001]. This feature may havecontributed to the lateral compartmentalization and upwardmigration of fluidized materials.

6.3. Coseismic Origin of Sediments Overconsolidation

[23] The relations between thermal maturity and densityvalues of peat, lignite, and coals from sub-bituminous to an-thracite rank proposed by various authors [Gan et al., 1972;Robbins, 1979; Walker et al., 1988; Vassallo et al., 1991;Huang et al., 1995; Orem et al., 1996; Clarkson and Bustin,1999; Gurdal and Yalcin, 2001; Radlinski et al., 2004;Alvarez and Borrego, 2005; Iordanidis and Van Doesburg,2006; Goudoulas et al., 2010] indicate that density values

Figure 13. (a) Conceptual cartoon showing the formationof a rollover anticline by hanging wall folding above a listricextensional fault zone, with the related dilational sector adja-cent to the master slip surface. (b) Additional sites for pre-ferred dilation in the hanging wall are the fold hinge zone,where a crestal graben can form by tangential longitudinalstrain, and the antithetic fault zones. (c) In all these dilationalsites, sediment fluidization can be triggered by coseismicshaking if appropriate conditions for liquefaction occur inthe faulted sediments.

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of organic matter are generally lower than 1.53 g/cm3 andincrease only slightly with coal rank (Figure 11b). There is,however, a direct dependency between density and the mineralcontent of coal, with more mineral matter causing higherdensity values. Our organic matter-rich samples have ananomalously wide range of density values, spanning from1.5 to 2.7 g/cm3, that is not due to minerals contamination.FTIR-derived CH2/CH3 ratios consistently higher than 2.6,and VRo values lower than 0.35% both indicate low thermalmaturity equivalent to the peat-lignite coalification ranks[e.g., Lis et al., 2005]. Excluding differences in coal rankand mineral matter content as the reasons for generally high-density and wide-density variations in organic matter-richhorizons, we suggest that overconsolidation of sedimentsexerted the dominant control over organic matter density.[24] Validation of thermal maturity data is provided by

XRDmineralogical data of clay-rich sediments, where mixedlayers I-S are R0 structures with no preferred sequence instacking of illite and smectite layers along the c axis and withan illite content of 20%–35%. These features suggest verylow levels of thermal maturity consistent with early diage-netic conditions [Merriman and Frey, 1999]. Thus, sedi-ments filling the L'Aquila basin did not experience deepsedimentary burial. This inference is also supported by themineralogical assemblage of the volcanoclastic ash layer em-bedded in clay-rich horizons at Ponte Peschio site. In thislayer, volcanic glass, which is generally more reactive duringdiagenesis and/or weathering than clays or other minerals[Leggo et al., 2001; Aldega et al., 2009], is almost totally pre-served, and just a little amount has been transformed intoauthigenic smectite particles, indicating that diagenesis is ina very early stage. In addition, smectite becomes unstableas burial proceeds and temperature increases, and it convertsinto mixed layers illite-smectite [Środoń, 1999]. Both the ab-sence of smectite interstratified with illite layers and the highamount of glass in the volcanoclastic layer point for shallowburial and temperatures lower than 60°C [Merriman andFrey, 1999]. Thus, both XRD analyses on the inorganic frac-tion and optical investigations on the organic fraction of sed-iments suggest shallow burial (less than 2 km) and extremelylow thermal maturity (peat and lignite coal ranks and earlydiagenetic zone) for sediments filling the L'Aquila basin.[25] Robust and precise constraints to burial are provided

by the results from flat dilatometer tests, indicating that themaximum thickness of the eroded material originally overly-ing the lower Pliocene sediments west of L'Aquila town wasless than 200m. These data are supported by geological in-ferences indicating for the basin infill a maximum thicknessof about 300m [Ge.Mi.Na., 1963; Lorè and Tallini, 1997;Tallini et al., 2012]. Accordingly, from all informationdiscussed above, we can deduce that burial was not the mainfactor causing the high-density values obtained from thepeat-lignite beds in the L'Aquila basin. This is also supportedby the large variability of these data (Figure 11b). A possibleexplanation to this conundrum could be coseismic shaking ofthe sedimentary basin infill that might have provided the driv-ing mechanism for strong compaction and overconsolidationof the peat-lignite beds. Additional indirect support to thisinference comes from the widespread presence in the sedimentsfrom the Roio Piano and Sassa wells of finely crushed thin-shelled ostracods (E. Gliozzi, 2009, personal communication).It is worth noting that large impulsive dewatering associated

with coseismic overconsolidation can significantly enhancesediment liquefaction and fluidization. If we accept a geneticlink between coseismic sediment dewatering in the westernsector of the L'Aquila basin and the formation of seismites atPagliare di Sassa, then the latter should predate developmentof the lateral spreading structures at Ponte Peschio that postdatepeat-lignite overconsolidation and jointing.

6.4. Inferences on Paleoearthquake Magnitude

[26] Interpretations of the strength of paleoearthquakesare typically made when the regional extent of liquefaction-related features is available [e.g., Obermeier, 1996, 2009;Galli, 2000]. Despite, in our case, data from only thePagliare di Sassa outcrop are available, an attempt to specu-late on paleoearthquake magnitude can be made by usingthe empirical correlation with seismite type proposed byRodriguez-Pascua et al. [2000]. In particular, by assuminga genetic correspondence between intruded breccia andgravel, paleoearthquake magnitudes higher than about 7.5can be obtained. The simultaneous activation of the Pettino,Paganica, and Marine fault zones is expected to generateearthquakes with magnitudes approaching 7.0 [Galli et al.,2010; Moro et al., 2013], and this may suggest that theBarisciano fault zone to the southeast should be also involved(i.e., the entire L'Aquila basin-boundary fault system) inorder to further increase the causative fault length necessaryto generate paleoearthquakes with Mw ≥ 7.5. Actually, fault-trapped wave analysis indicates continuity at depth betweenthe Paganica and Barisciano fault zones [Calderoni et al.,2012]. However, the lack of paleoearthquake timing correla-tion between the Marine-Pettino-Paganica fault system andthe Barisciano one [Moro et al., 2013] does not support the si-multaneous activation of the entire L'Aquila basin-boundaryfault system.[27] A major uncertainty factor in our inference on

paleoearthquake magnitude lies in the empirical relationbetween seismite type, size, and seismic energy [Rodriguez-Pascua et al., 2000]. The capability of loose sediments to be-come coseismically fluidized depends not only on shear stressintensity but also on the total amount of interstitial water [e.g.,Obermeier, 1996]. In our case, the abundance of peat-bearing,initially unconsolidated lacustrine sediments interbedded withgravels may have provided large volumes of coseismicallyoverpressured water able to significantly enhance liquefactionand fluidization of the overlying sediments during seismicshaking. Accordingly, analysis of seismite type and abun-dance can lead to overestimated paleoearthquake magnitudes.[28] In summary, the paleofluidization structures associated

with the Pagliare fault zone imply the probably simultaneousactivation of the Marine-Pettino-Paganica fault system, as pro-posed for the 1703 earthquake [Galli et al., 2010], but possiblynot of the Barisciano fault zone to the SE, even though theseismite type and abundance might suggest it.

7. Conclusions

[29] In this paper, we describe large paleofluidization struc-tures and organic matter-rich sediment overconsolidationaffecting the Quaternary infill of the western L'Aquila basinand discuss their significance for the seismotectonic frame-work of the region. The following main conclusive pointscan be drawn from our study.

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[30] 1. Paleofluidization structures are clastic intrusions anddikes, some meters thick, which mainly consist of angular clayblocks and clasts embedded in a sandy matrix that we define asintruded and faulted breccia. Paleofluidization structures areemplaced along the master slip surface and in the crestal grabenof the rollover anticline in the hanging wall of the newly docu-mented Pagliare fault. According to the mesoscopic and micro-scopic features of the liquefied material, paleofluidizationstructures can be defined as seismites, produced by the seismicactivity of the L'Aquila extensional fault system during theearly middle Pleistocene.[31] 2. At the Ponte Peschio site, large fractures occur

in peat-lignite-bearing sediments by dilating previouslyformed joints that strike parallel to the basin-boundary faultsystem. According to their mesoscopic features, we inter-pret these fractures as being produced by coseismic lateralspreading caused by early middle Pleistocene earthquakesnucleated along the western sector of the L'Aquila exten-sional fault system.[32] 3. Flat dilatometer tests, XRD claymineralogy, and ther-

mal maturity data of organic matter from the paleoliquefaction-bearing sediments and underlying clay-rich and silty sedimentsindicate very shallow burial for the Gelasian-Calabrian infillof the western L'Aquila basin. Conversely, density datafrom organic matter show anomalously high values. Suchapparently contrasting pieces of evidence from differentdata sets can be resolved by hypothesizing that peat-bearingsediment overconsolidation was produced by coseismicshaking rather than progressive burial. Episodic and impul-sive dewatering triggered by strong earthquakes fits verywell with the presence of large paleofluidization structures.[33] 4. The mesoscopic and microscopic features of lique-

fied and fluidized sediments along the Pagliare fault and, inparticular, the presence of intruded and faulted breccia andgravel would empirically correlate with paleoearthquakemagnitude greater than 7.5. However, this might be the caseonly if the entire L'Aquila basin-boundary extensional faultsystem was contemporary activated. The lack of historicaland paleoseismological evidence for such large magnitudeevents and the first-order segmentation of the fault systeminto the Barisciano and Marine-Paganica fault zones do notfavor this hypothesis. Conversely, they support the seismicactivation or either the former or the latter segments of theL'Aquila fault system to generate earthquakes with magni-tude approaching or slightly exceeding 7.0. The discrepancywith inferences from paleofluidization structures can beexplained by assuming a significant role of coseismically in-duced dewatering and water overpressuring that significantlyfacilitated sediment liquefaction and mobilization.[34] 5. Occurrence of large paleofluidization structures in

the overstep between the Paganica and Pettino fault zonesmay indicate that their overlap region is located on a strongcrustal asperity that is seismically active since at least earlymiddle Pleistocene times.

Appendix A: Methods

A1. Laser Diffraction Particle Size Distribution Analysis

[35] We used a Mastersizer 2000 laser diffraction particlesize analyzer manufactured by Malvern Instruments, withan analytical size range between 0.2 and 2000 μm. Allsamples were sieved at 1700 μm to account for shape

anisotropy of coarser particles. Representative subsampleswere obtained by a Quantachrome sieving riffler and rotarysample splitter. Analyses were performed using the HydroMU wet dispersion unit, decalcified water as dispersant liq-uid (refraction index 1.33) and 1.544 and 0.01 as refractionindex and adsorbent coefficient of quartz grains, respec-tively. Details of preliminary sample testing procedurepropedeutic to the selection of analytical parameters aredescribed in Storti and Balsamo [2010a, 2010b] andBalsamo and Storti [2011]. Following this procedure, thepump speed was set at 2100 rpm, sampling time was set at5 s, and no ultrasonication was used. For each sample, 25measurement runs at the same analytical conditions wererepeated on five subsample aliquots and then averaged toprovide the mean curves in Figure 8.

A2. Particle Shape Analysis

[36] To obtain the cross-sectional aspect ratio of clayclasts, color images of the outcrop were converted to greytone images after manually separating touching particles[Rawling and Goodwin, 2003]. Images were smoothed toavoid bias on perimeter and area computations caused byhigh-frequency low-amplitude undulations of clast bound-aries produced by the nonplanar outcrop cliff. Smoothingwas achieved by filtering the original images with a 3 × 3convolution mask designed to remove noise by slightlyblurring the images, especially the edges. Magnificationwas properly set for each size class to ensure comparableresolution, i.e., comparable relative dimensions of clasts inthe acquired images [e.g., Storti et al., 2007]. Aspect ratiomeasurements between clast longer and shorter axes wereautomatically obtained with OPTIMAS 6.51TM imageanalysis software [e.g., Storti et al., 2007]. The high numberof analyzed clasts should ensure statistical validity of the re-sults [Panozzo, 1982].

A3. XRD Analysis of Clay-Rich Sediments

[37] Pelitic samples have been analyzed byX-ray diffraction(CuKα radiation) at 40 kV and 45mA. Randomly orientedwhole-rock powders were run in the 2°–70° 2θ interval witha step size of 0.05° 2θ and a counting time of 3 s per step.Oriented air-dried and ethylene-glycol solvated samples wereX-rayed from 1° to 48° 2θ and from 1° to 30° 2θ, respectively,with a step size of 0.05° 2θ and a count time of 4 s per step.The illite content in mixed layer I-S was determined in the<2μm grain-size fraction according to Moore and Reynolds[1997], using the Δ°2θ method after decomposing thecomposite peaks between 9°–10° 2θ and 16°–17° 2θ. TheI-S ordering type (Reichweite parameter) [Jagodzinski,1949], which describes the stacking of illite and smectitelayers along the c axis, was determined by the position ofthe I001-S001 reflection between 5° and 8.58° 2θ [Mooreand Reynolds, 1997]. Peaks in relative close position wereselected for mineral quantitative analysis in order to mini-mize the angle-dependent intensity effect. Composite peakswere decomposed using Pearson VII functions and theDMSNT Scintag associated program. Integrated peak areaswere transformed into mineral concentrations by using min-eral intensity factors as a calibration constant [see Mooreand Reynolds, 1997, for a review]. Nonclay minerals, suchas quartz, calcite, and gypsum, detected in the <2 μmgrain-size fraction were not included in the quantitative

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analysis of the oriented aggregates, as data refer to thephyllosilicate group only. Illite content in mixed layer I-Sand I-S ordering type were used as temperature-dependantparameters according to Hoffman and Hower [1979] andMerriman and Frey [1999].

A4. Organic Matter Optical Analyses

[38] Samples were prepared according to the standardizedprocedures described in Bustin et al. [1990] at Roma Tre lab-oratories. Crushed material and extracted kerogen particleswere embedded into epoxy resin blocks. The samples werepolished using an automated polishing system, 250, 500,1000 carborundum papers and isopropanol as lubricant.After washing the samples in order to remove debris, threepolishing laps were loaded with alumina powders of decreas-ing grain size (1.0, 0.3, and 0.01μm), and the samples werepolished. Organic matter optical analysis were performed atRoma Tre and Bloomington (Indiana, USA) laboratoriesusing Zeiss Axioplan microscopes, under oil immersion(n = 1.518, 23°C) in reflected monochromatic nonpolarizedlight (λ= 546 nm). On each sample, a minimum of 20 mea-surements was performed on well-polished fragments ofmacerals belonging to the humite-vitrinite group rarelyslightly fractured. Mean reflectance values and related stan-dard deviations were calculated from the arithmetic meanof measurements.

A5. Organic Matter FTIR Analyses

[39] FTIR experiments were carried out at the IndianaGeological Survey laboratories using a Nicolet 6700spectrometer equipped with a DTGS detector. Ground ker-ogen was mixed with potassium bromide in the proportionof 1mg/100mg and was analyzed as KBr pellets. Twohundred scans per sample were recorded in absorptionmode from 500 to 4000 cm�1 with a 4 cm�1 resolution.Bands were identified by comparison with publishedspectra [Painter et al., 1981; Wang and Griffiths, 1985;Sobkowiak and Painter, 1992] and their assignmentsfollowed those by Drobniak and Mastalerz [2006].Integrated band areas were measured using Omnic spec-tral analysis software. The aliphatic C-H stretching region(2800–3000 cm�1) were analyzed and resolved using Fourierdeconvolution [Lin and Ritz, 1993] into five spectral bands at2955 (asymmetrical CH3 stretching), 2925 (asymmetrical CH2

stretching), 2890 (CH stretching), 2865 (symmetrical CH3

stretching), and 2850 cm�1 (symmetrical CH2 stretching). TheCH2/CH3 ratio was calculated using asymmetrical stretchingof these bands (2925/2955) according to Lin and Ritz [1993].Identified band areas were used to calculate the CH2/CH3 ratio(in aliphatic stretching modes) that was used as an index oforganic thermal maturity.

A6. Helium Density on Organic Matter

[40] Helium densities were determined at the IndianaGeological Survey laboratories using a helium pycnometer(Ultrapyc 1200e). The measurements were carried out onoven dried 1.5–7.5 g subsamples. Prior to the analyses, sam-ples were purged with high-purity helium to ensure completeremoval of adsorbed gases. The helium density for each sam-ple was calculated from an average of five measurements.

A7. Overconsolidation Ratio From FlatDilatometer Tests

[41] The overconsolidation ratio (OCR) was estimatedfrom DMT results using the following equation [Marchetti,1980]:

OCRDMT ¼ 0:5KDð Þ1:56 (A1)

where KD is the horizontal stress index derived from the usualDMT interpretation. Equation (A1) has built in the correspon-dence KD=2 for OCR=1. This correspondence has been con-firmed in many normally consolidated (NC) clay deposits (nocementation, aging, and structure). Thus, KD=2 is the lowerboundary value for NC clays. Any excess ofKD above 2 in geo-logically nonconsolidated clay indicates the likely existence ofaging, structure, cementation, and/or other effects [seeMarchetti et al., 2001 for details]. The diagrams shown inFigure 12 were obtained from the interpretation of the resultsof flat dilatometer tests. For each site, the graphs show the pro-files with depth of: (i) the horizontal stress index KD obtainedfrom DMT interpretation; (ii) the OCR ratio estimated fromKD by equation (A1); (iii) the present-day in situ vertical effec-tive stress σ′v0, and the maximum past vertical effective stress(preconsolidation stress) σ′vm, estimated from OCR asσ′vm =OCR σ′v0; the distance between these two diagramsprovides an indication of the maximum overburden q thatwas removed from the present ground surface, evaluated asq= σ′vm� σ′v0; (iv) the maximum past elevation of the groundsurfaceHm compared to its present-day elevationH, the differ-ence Hm�H, corresponding to the presumed thickness of theeroded overburden, was estimated assuming q= γ′ (Hm�H),where γ' is an average value of the submerged unit weight ofthe sediment.

[42] Acknowledgments. We are grateful to the Italian Civil Protectionand particularly to G. Naso, for the availability of the core material drilled inthe western L'Aquila basin and for the support provided for this work. Wewarmly acknowledge E. Peronace and B. Quadrio for inviting us tovisit the outcrop of Pagliara di Sassa. S. Agostini, D. Cosentino, P. Galli,E. Gliozzi, P. Messina, and M.A. Rossi are kindly acknowledged for helpfuldiscussions on the seismotectonic setting of the L'Aquila region.Stereographic projections were made using the Daisy software [Salvini,2013]. Funding for this study was provided by the Italian MIUR (PRIN2009, prot. 2009N73SR4_003, grants to F. Storti).

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