Ital. J. Geosci., Vol. 135, No. 2 (2016), pp. 350-362, 8 figs. (doi: 10.3301/IJG.2015.37) © Società Geologica Italiana, Roma 2016 Geological reconstruction in the area of maximum co-seismic subsidence during the 2009 Mw=6.1 L’Aquila earthquake using geophysical and borehole data MASSIMILIANO PORRECA (1), ALESSANDRA SMEDILE (2), FABIO SPERANZA (2), TANIA MOCHALES (2), FRANCESCA D’AJELLO CARACCIOLO (2), GIUSEPPE DI GIULIO (2), MAURIZIO VASSALLO (2), FABIO VILLANI (2), IACOPO NICOLOSI (2), ROBERTO CARLUCCIO (2), SARA AMOROSO (2), PATRIZIA MACRÌ (2), NICOLETTA BURATTI (3), FEDERICA DURANTE (4), MARCO TALLINI (4) & LEONARDO SAGNOTTI (2) ABSTRACT InSAR images showed that the 2009 Mw=6.1 normal faulting L’Aquila earthquake (Abruzzi region, central Italy) produced a maximum co-seismic subsidence of ca. 24 cm in the epicentral area. We report new results about the stratigraphic architecture of this area by means of the integration of geophysical and stratigraphic data from a new 151 m deep borehole. According to the indication of preliminary geophysical (electrical resistivity tomography and seismic noise) surveys, the borehole was drilled where maximum thicknesses of fine-grained sediments were expected. The geophysical results were also useful to estimate the basin substrate depth and to define the geometry of the continental deposits, successively constrained by the core stratigraphy. The core is characterized by two sequences separated by an erosional discontinuity. The upper sequence is composed by silty, sandy and gravelly deposits, mainly characterized by high values of electrical resistivity. The lower sequence is characterized by prevalence of grey clayey silt and sandy sediments, with low values of resistivity. Based on correlations among the stratigraphic core and outcrop data of the Aterno Valley, we interpret the upper sequence as related to fluvial-alluvial depositional environment during Middle Pleistocene-Holocene, whereas the lower sequence is related to deposition in a prevalent marshy floodplain environment during Early Pleistocene. KEY WORDS: Aterno Valley, L’Aquila earthquake, LAqui-core, Quaternary, continental basin. INTRODUCTION On April 6, 2009 a Mw 6.1 earthquake struck the historical city of L’Aquila (central Italy) and surrounding villages located in the Aterno Basin (BOSI, 1975) after a long seismic sequence. The earthquake was generated by the Paganica normal fault at 8 km depth (CHIARALUCE et alii, 2011; VALOROSO et alii, 2013). Despite many seismological (ATZORI et alii, 2009; CHIARABBA et alii, 2009; Gruppo di Lavoro MS-AQ, 2010; MILANA et alii, 2011) and geological (BOSI & BERTINI, 1970; BAGNAIA et alii, 1992; BERTINI & BOSI, 1993; FALCUCCI et alii, 2009; BONCIO et alii, 2010, 2011; GALLI et alii, 2010; PUCCI et alii, 2015; CIVICO et alii, 2015) studies (1) Università di Perugia, Dipartimento di Fisica e Geologia. Corresponding author e-mail: massimiliano.porreca@unipg.it (2) Istituto Nazionale di Geofisica e Vulcanologia. (3) Total SA, CSTJF. (4) Università de L’Aquila, Dipartimento di Ingegneria. carried out in the epicentral area of the 2009 L’Aquila earthquake, the lack of deep boreholes, associated with subsurface geophysical data, has so far hampered a proper definition of the deep geological structure of the Aterno Valley. The Aterno continental Basin is one of the several fault-controlled Quaternary extensional basins of the central Apennines and its sedimentation history is poorly known due to the scarcity of outcrops in the weakly incised infilling cover (BERTINI & BOSI, 1993; GALLI et alii, 2010). In particular, the Middle Aterno Basin (sensu PUCCI et alii, 2015), which lies in the hanging-wall of the Paganica fault (fig. 1a), has been subjected to several geophysical studies soon after the 2009 earthquake. Magnetotelluric studies (BALASCO et alii, 2011), deep electrical resistivity data (BALASCO et alii, 2011; BONCIO et alii, 2011), highresolution seismic tomography investigations (IMPROTA et alii, 2012), seismic noise measurements (BONCIO et alii, 2011; DI GIULIO et alii, 2011) and gravimetric analysis (Gruppo di Lavoro MS-AQ, 2010) were recently performed to investigate the near-surface structure of the epicentral area. These analyses were mainly focused in the northeastern region of the basin, close to the Paganica fault, where the majority of the co-seismic breaks were recognized (Emergeo Working Group, 2009; GALLI et alii, 2010; BONCIO et alii, 2010; ROBERTS et alii, 2010; GORI et alii, 2012). In this area, the shallow basin stratigraphy has been well constrained thanks to lithological data obtained from shallow boreholes and water wells (BONCIO et alii, 2011). The results of such different approaches have allowed defining the geometry of this part of the Aterno basin and its relationships with active fault systems. In contrast, a detailed study of the south-western part of the basin, hereafter Bazzano sub-basin (IMPROTA et alii, 2012; fig. 1a) has been lacking so far. Only IMPROTA et alii (2012) and BALASCO et alii (2011) documented that this area is characterized by important thicknesses of low-velocity and low-resistivity sediments, respectively. This is furthermore the area that recorded the maximum co-seismic subsidence during the 6 April 2009 earthquake (about 24 cm), as highlighted by DInsar (ATZORI et alii, 2009) measurements (fig. 1b). In the same area, D’AGOSTINO et alii (2012) measured a maximum co-seismic displacement of 15 cm using synthetic aperture radar (DInSAR) and GPS observations. GEOLOGICAL RECONSTRUCTION IN THE AREA OF MAXIMUM CO-SEISMIC SUBSIDENCE DURING THE 2009 351 Fig. 1 - a) Simplified geological map of the Middle Aterno Basin (modified after IMPROTA et alii, 2012) with location of the B1 seismic line and 2009 surface faulting ruptures. The white inset refers to the fig. 2; b) Shaded relief topography of the Middle Aterno Basin superimposed on the differential interferogram from the COSMO-SkyMed ascending orbit (pre-event 4th April, and post event 12th April 2009) (STRAMONDO et alii, 2011): one full-color cycle represents 1.5 cm of ground shift away from satellite (modified after IMPROTA et alii, 2012). This part of the basin can therefore represent the area where the sedimentation record was strongly influenced by the repeated activation of the Paganica fault and should correspond to the actual depocenter of the Middle Aterno Basin. Furthermore, subsidence of the Middle Aterno Valley is particularly active as suggested by frequent floodings in recent times as reported by historical reviews (LEOPARDI & RIMEDIA, 2002) and archeological excavations (COSENTINO et alii, 2001). The aim of our work is to investigate the stratigraphy and geometry of the Bazzano sub-basin subjected to the maximum 2009 co-seismic subsidence, i.e. where highest sedimentation rate and a more complete and continuous stratigraphic record are expected. We present the results of detailed geophysical investigations, such as electrical resistivity tomography (ERT) and seismic noise measurements performed in the Bazzano sub-basin. Afterwards, we described the stratigraphy derived from the 151 m deep core, called LAqui-core, drilled in the point where the maximum thickness of fine-grained continental sediments was expected. Two undisturbed samples were also recovered from LAquicore, in order to perform geotechnical laboratory testing, including grain size distribution. Fine-grained deposits were targeted as they are expected to yield a high-resolution record of the depositional history, in turn controlled by the activity of the seismogenic Paganica fault. GEOLOGICAL SETTING The study area is located in the Quaternary WNW-ESE oriented Middle Aterno intermontane Basin. It is situated between the Aterno River and Ocre Mts (fig. 1a). Orogenic shortening in this sector of the Apennines thrust-fold belt occurred during the Upper Miocene-Lower Pliocene (CAVINATO & DE CELLES, 1999; CIPOLLARI et alii, 1999). Since the Late Pliocene, the area experienced WSW-ENE directed extensional regime, well testified by the presence of mostly S-SW-dipping and NW-SE or W-E trending normal faults, which conditioned the sedimentary basin filling (BONCIO & LAVECCHIA, 2000; GALADINI & MESSINA, 2004; STORTI et alii, 2013). Currently, the whole area is subjected to NE-SW extensional tectonics, mainly focused along the axis of the chain, as shown by the GPS velocities that approach 3 mm/yr (D’AGOSTINO et alii, 2012; D’AGOSTINO et alii, 2014; DEVOTI et alii, 2011). Here, the active faults are arranged in NW-SE trending systems that generally do not exceed 30 km in length. These active faults are characterized by typical recurrence times of 1-2 ka (e.g., MICHETTI et alii, 1996; PANTOSTI et alii, 1996; see GALLI et alii, 2008 for an overall review), or even less (GALLI & NASO, 2009; GALLI et alii, 2011). QUATERNARY GEOLOGY The Plio-Quaternary extensional tectonic regime in the Aterno Valley produced typical morpho-tectonic features, characterized by elongated pre-Quaternary carbonate and siliciclastic ridges, surrounded by Quaternary depressions filled with continental deposits (see geological map by PUCCI et alii, 2015 and fig. 1a). Some of these Quaternary deposits are uplifted and eroded, whereas some others are still in subsidence and buried by Holocene continental deposits. This particular setting is controlled by faults activity as the uplifted areas are generally located in the footwall, whereas the hanging-wall is continuously in subsidence (GIACCIO et alii, 2012 and references therein). The stratigraphic architecture of the Quaternary sequences is therefore well known in the SE part of the area (GIACCIO et alii, 2012; GALLI et alii, 2010; SPADI et alii, 2015), where continental sequences are well exposed. In contrast, the 352 M. PORRECA ET ALII Fig. 2 - Airbone LiDAR (Light Detection And Ranging) image of the studied area (LiDAR image by Civico, 2012) with B1 seismic line (IMPROTA et alii, 2012), ERT lines and seismic noise sites. flat and subsided zones of the NW sector of the Aterno Valley can be only studied by geophysical investigations (BONCIO et alii, 2011) and drilled wells (DEL MONACO et alii, 2013; GE.MI.NA., 1963; Gruppo di Lavoro MS-AQ, 2010; TALLINI et alii, 2012; MANCINI et alii, 2012). Continental deposits exposed in the Aterno valley can be referred to at least two main cycles (BOSI & BERTINI, 1970; BERTINI & BOSI, 1993; MANCINI et alii, 2012; AGOSTINI et alii, 2012): i) a lower (Pliocene-Early Pleistocene) >200 m thick fluvio-lacustrine cycle (Late Piacenzian-Gelasian S. Nicandro Fm. in the SE part of the Aterno Basin; SPADI et alii, 2015), including conglomerates (Vall’Orsa Fm. and Valle Inferno Fm.) and lacustrine silts (Calabrian Madonna della Strada Fm. in the NW part of the Aterno Basin); ii) an upper fluvial-lacustrine cycle (Middle Pleistocene), carved in the former one and consisting of sands rich of volcanic ashes and gravels (S. Mauro Fm. and Civitatomassa Fm. in the SE and NW sector of the Aterno Basin, respectively). These deposits are covered by Late PleistoceneHolocene fluvial sediments, mainly related to the Aterno River, and by slope debris (BERTINI & BOSI, 1993). This schematic subdivision is not always applicable in each single portion of the Aterno Basin, because a complete stratigraphic sequence is restricted only to specific zones (see for example GIACCIO et alii, 2012). In particular, no subsurface geological data are available for the Bazzano sub-basin, whereas a detailed picture of the Quaternary stratigraphy in the Paganica sub-basin is provided by GALLI et alii (2010) and BONCIO et alii (2011). The Paganica sub-basin, located to S of the Paganica fault, was intensively investigated within the activities of the recent seismic microzonation (Gruppo di Lavoro MS-AQ, 2010) in order to construct a geological model. The results of the geological and geophysical investigations are summarized by BONCIO et alii (2011). They found that this subbasin is filled by a thick pile of Quaternary deposits syn-tectonically accumulated on pre-Quaternary bedrock, in turn cut by normal faulting. The thickness of the continental deposits increases from the fault to the inner part of the basin, from a few meters to more than 190 m. The stratigra- phy is subdivided by BONCIO et alii (2011) in three main units: an old (Early Pleistocene?) fine grained lacustrine unit overlaying by gravels and conglomerates; a Middle-Upper Pleistocene sequence of coarse grained alluvial fan and fluvial-lacustrine deposits; thin covers of Late PleistoceneHolocene fine to coarse grained fluvial-alluvial deposits. GEOPHYSICAL INVESTIGATIONS AND BOREHOLE We performed a set of geophysical investigations, in order to characterize the subsurface geology of the Bazzano sub-basin, with particular attention to the continental deposits, and to select the site location for drilling a deep borehole. We acquired 2 electrical resistivity tomography (ERT) profiles and 5 measurements of ambient seismic noise (fig. 2). After these geophysical data acquisition and interpretation, we drilled a 151 m deep borehole (LAqui-core) in the point where significant thicknesses of fine grain sediments were expected according to geophysical surveys. The location of ERT profiles, ambient noise measurements and the borehole are shown in fig. 2. METHODS Electrical resistivity tomography (ERT) The ERT was used to define thickness and geometry of the subsurface depositional units in function of their electrical features. We carried out an electrical resistivity tomography survey using an IRIS Syscal Pro resistivity meter applying different spacing of steel electrodes according to the aim of the survey. The instrument was configured to inject a square waved signal for 250 ms, and the resulting resistivity ground contact was of the order of about 1k Ω·m. We acquired 2 electrical resistivity profiles adopting different quadrupole arrays and electrodes spacing. ERT1 GEOLOGICAL RECONSTRUCTION IN THE AREA OF MAXIMUM CO-SEISMIC SUBSIDENCE DURING THE profile (fig. 3) was acquired using 72 electrodes 5 meters apart for a total of 355 m, with pole-dipole (PD) array in order to have good sensitivities at higher depth (PARK & VAN, 1991; DAHLIN & ZHOU, 2004; SAMOUËLIAN et alii, 2005). ERT2 profile (fig. 4) was generated combining two adjacent profiles by roll along procedure overlapped by 24 electrodes acquired using 72 electrodes 10 meters apart. The total length of this profile was 950 m. This last profile was acquired using both Wenner-Schlumberger (WS) and dipole-dipole (DD) electrode arrays, in order to 2009 353 have accurate vertical and horizontal resolution (LOKE & BARKER, 1995). The north-eastern part of the ERT2 profile is partially in overlapping with the south-western part of the ERT1 profile. To obtain true resistivity values from measured apparent resistivity data, we used the smoothness-constrained least squares method considering models with an infinite perpendicular extension along the profile strike (CONSTABLE et alii, 1987; DEGROOT-HEADLIN & CONSTABLE, 1990; MORELLI & LABRECQUE, 1996). Fig. 3 - Interpreted electrical image and seismic noise measurements across the ERT1 profile. In the seismic noise measurements, the resonance frequency (f0) is highlighted in grey. Fig. 4 - Interpreted electrical image and seismic noise measurement (Bazz5) across the ERT2 profile. 354 M. PORRECA ET ALII Ambient seismic noise Ambient seismic noise measurements were performed in order to estimate the pre-Quaternary substrate depth. The aim is to investigate the resonance frequency in the area, and then to identify the main seismic discontinuities that separate deposits characterized by strong impedance contrast, e.g between infilling sediments and substrate (seismic bedrock). Ambient seismic noises were measured on 5 sites distributed along the ERT1 and ERT2 profiles. 4 of them were distributed along the ERT1 profile (fig. 3). The last measurement was performed at the SW end of the ERT2 profile (fig. 4) in order to check if any difference in the substrate depth occurred. H/V method computes the spectral ratios (HVNSR; NOGOSHI & IGARASHI, 1971; NAKAMURA, 1989) between the horizontal components and the vertical component of ambient vibration recordings. The seismic stations were equipped with Lennartz-3D sensors (0.2 Hz natural frequency), and each noise measurement was characterized by a duration of some hours. We calculated HVNSR dividing the hourly recordings in 60 s running time-windows, removing the time windows contaminated by transients using an anti-trigger algorithm as implemented in the Geopsy software (www.geopsy.org). For each selected time window, the Fourier amplitude spectra are smoothed following KONNO & OHMACHI (1998) filter, with a coefficient equals to 40 for the bandwidth. The mean horizontal amplitude spectra is then computed combining the spectra relative to the two horizontal components. The final HVNSR and the associated standard deviations are obtained by geometrical average of the individual H/V spectral ratios from all the selected time-windows. Cored borehole drilling The borehole LAqui-core (Lat. 42°19’7.7”N; Long. 13°26’22.5”E) was drilled by a rotating drilling machine mounted on a truck, from the 3rd June to the 19th June 2013. A 151 m thick core was recovered during this period. The type of perforation was the Diamond bit coring. In this method a coring bit (101 mm in diameter) connected to the core barrel is used for drilling. The cores were collected in the barrel which was recovered after completion of the run. In the first meters the drilling was performed without the use of water (dry drilling) trying to collect undisturbed poorly consolidated sediments. When coarse sediments were encountered (115.40 m depth), a water drilling and protective hole tubes were adopted in order to get a rapid drilling and avoid hole wall collapses. The protective metal tubes, with a 140 mm diameter, were installed up to 123 m depth. Each single 1 m thick fine-medium grain size sediment core was then stored into a PVC (plastic) sample holder to protect the sediment and keep it in wet original conditions, until the successive cut and sampling for laboratory analyses. Each single core was successively opened and cut in two symmetrical half pieces. Every single piece was described using standard stratigraphic procedure. Two undisturbed samples were also collected from LAqui-core at 3 m and 75 m depth, representative of the fine grain layers in the upper and lower sequences respectively, in order to perform geotechnical laboratory tests and grain size analyses. The study was carried out to investigate the physical and mechanical properties of the sediments at 3.00-3.56 m depth (sample C1), and at 75.00-75.60 m depth (sample C2). Laboratory tests were conducted to determine several geotechnical parameters, particle size distribution curves and classification. In this work we present only results of the grain size distribution as they may provide useful indication for interpretation of the depositional environments. RESULTS Electrical resistivity tomography (ERT) The ERT models are based on the joint inversion of dataset of apparent resistivity data recorded using two or three different electrode arrays. The results of this inversion are presented in figs. 3 and 4 related to the ERT1 and ERT2 profiles respectively. In general, the resistivity sections show two main regions with different resistivity values. Low-medium values, ranging from 15 to less than 70 Ω·m, represent the geophysical signature of the fine-grained sediments. Higher values, ranging from 150 up to 700 Ω·m are typical of coarse sand to gravel sediments. Both the electrical profiles did not reach the pre-Quaternary substrate as the maximum depth of ERT investigation is around 150-170 m, whereas the substrate is probably located at 250 m depth (see interpreted seismic line by IMPROTA et alii, 2012). Both profiles show a similar geometrical organization of the electrical facies. Geological discontinuities or facies variations hamper the horizontal continuity of the recovered resistivity layers in the surveyed area. In particular, it is possible to distinguish, from the top to the bottom, an uppermost electrical facies, ca. 2-5 meters thick, characterized by very low resistivity (<30 Ω·m). This layer is continuous in the ERT1 profile (fig. 3), whereas it thins from the valley axis toward the mountain flank along the ERT2 profile (fig. 4). Below this conductive layer, a high resistivity layer (>100 Ω·m) is present. This layer is not continuous and shows a variable thickness. A maximum thickness of 60 m is estimated in the northern end of the ERT1, i.e. close to the actual Aterno River. Here, strong and evident lateral discontinuity affects this conductive layer, producing a sharp increase of thickness associated to the maximum resistivity values (>600 Ω·m) recorded in the area (ERT1 in fig. 3). In the ERT2, this layer is continuous in the northern part and tends to disappear towards to the border of the valley. Here, another less thick resistive layer occurs with a wedge geometry. Moreover, the contact between these resistive layers and the underlying conductive one is generally irregular and not well defined. The underlying conductive layer has an upper boundary at depths of 25 to 40 m and its thickness is estimated higher than 60-80 m. For this layer, the resistivity ranges from 10 to 50 Ω·m with the lowest values measured in the middle part and southern end of the ERT2 profile (fig. 4). Ambient noise measurements The data show well defined peaks in the H/V spectral ratios (figs. 3 and 4). Two spectral peaks at about 0.7 and 10 Hz are evident in all the H/V curves along the profile ERT1 (fig. 3). The low-frequency peak at 0.7 Hz indicates the fundamental resonance (f0) of the basin (DE LUCA et alii, 2005; Gruppo di Lavoro MS-AQ, 2010), which reflects GEOLOGICAL RECONSTRUCTION IN THE AREA OF MAXIMUM CO-SEISMIC SUBSIDENCE DURING THE the occurrence of a deep seismic basement. Both the low- and high-frequency peaks are not clearly polarized (not shown here), suggesting a regular and sub-horizontal layered geometry in this part of the basin. The spectral peaks in the south-western end of the profile ERT2 (Bazz5) are less defined than those along the ERT1 profile (fig. 4), and the f0 at 1 Hz indicates a thinning of the seismic interface with respect to the ERT1 profile. The resonance peak at low frequency (f0=0.7 Hz) is related to a sharp underground velocity contrast, and is used in this work to infer indication on the seismic bedrock in the area of the borehole. The secondary peak at 10 Hz is related to a velocity contrast in the very shallow part of the profile (<5 m), and it is likely related to the two electrical layers observed in the upper part of the ERT profiles. IMPLICATIONS ON SELECTING LAQUI-CORE DRILLING SITE The Bazzano sub-basin is characterized by low Vp (1000-2000 m/s; IMPROTA et alii, 2012) and low resistivity values (BALASCO et alii, 2011), interpreted as evidence of high thickness (ca. 200 m) of fine-grained sediments deposited during the Quaternary (IMPROTA et alii, 2012). We suspected that this area could be the locus of the maximum and continuous sedimentation of fine grained deposits, in which the sedimentation rate was controlled by the repeated activity of the Paganica fault during Quaternary and Holocene. The finest grain size allows performing several analyses on the sediments (e.g. sedimentological, micropaleontological, paleomagnetic, rock magnetism analyses and radiometric dating) in order to define a high-resolution age model of the core. In turn, the age model could have implications on the repeated activity timing of the Paganica fault, which controls the depositional history of the basin. These analyses are still in progress and the results will be presented in forthcoming papers. Here, we discuss how the results of the geophysical analyses provided indications for selecting the drilling site of LAqui-core. The ERT1 and ERT2 profile demonstrate the occurrence of both resistivity and conductivity changes along depth (figs. 3 and 4). We recognized at least three main facies with well-defined resistivity intervals. In particular, a few meters thick (less than 10 m), sub-horizontal conductive (<30 Ω·m) facies was identified. Its thickness tends to thin south-westward toward the Ocre Mts. flank and it disappears in correspondence of the slope break. After this break, another wedge-shaped resistivity facies is present. The shallow conductive body, consistently present in the whole Aterno plain, is interpreted as silty and clayey sediments probably deposited during repeated floods of the Aterno River. Conversely, the wedge-shaped resistive facies is probably due to the coarse grained alluvial fan fed by the Ocre Mts. flank located to the south with respect to the LAqui-core. Successively, a high resistivity facies (>150 Ω·m) with variable thicknesses (15 to 60 m) characterizes the continental sequence. Its thickness is higher toward the Aterno River and its lower contact shows an irregular pattern (fig. 3) which does not permit an easy interpretation. Furthermore, the highest thickness of this layer is recorded in proximity of the Aterno River (NE part of the profile 2009 355 ERT1, fig. 3). These features seem to suggest that this part of the sequence is characterized by coarse grained resistive deposits probably related to the paleo-Aterno fluvial evolution (see BONCIO et alii, 2011). The deepest part of the investigated sequence is dominated by conductive layers (<50 Ω·m), whose lateral continuity is not always verified. In particular, the central part of the profile ERT2 shows the presence of a resistive body in the middle of this conductive layer (fig. 4). We interpreted this conductive layer as due to fine grained (silt and clay) sediments deposited in lacustrine or marshy depositional environments, as proposed by BONCIO et alii (2011) for the Paganica sub-basin. It is also likely that the continuity of this fine-grained sequence was interrupted by deposition of coarse grained deposits, represented by high resistive layers (south-western part of the profile ERT2). The seismic noise measurements show the presence of two important discontinuities corresponding to a low frequency (0.7-1 Hz) and high frequency (10 Hz) amplitude peaks. There are no particular differences in amplitude and frequency for the first 4 measurements along the profile ERT1, suggesting a regular and sub-horizontal geometry of the layers in this part of the basin (fig. 3). The resonance peak at 0.7 Hz (f0) is generally found in the whole L’Aquila area (see MILANA et alii, 2011; DI GIULIO et alii, 2014). It is generated by the impedance contrast between the clastic infilling and the pre-Quaternary substrate. A clear peak at low frequency is typical in alluvial basin (such as Aterno Valley) when a strong seismic impedance contrast occurs between soft filling and seismic bedrock. The main parameters that affect the H/V peak are the average shear-wave velocity (Vs) and the sediment thickness (H) of the infilling soft soil. Moreover, under the assumption of 1D wave propagation, the resonance frequency f0 is linked through the simple relation f0=Vs/4H. A previous paper based on surface-wave analysis (DI GIULIO et alii, 2014) shows that the average Vs of the sedimentary material in the L’Aquila area is of the order of 700 m/s. Assuming such value, we can estimate a thickness of the seismic bedrock 250 m for the infilling deposits. This is in good agreement with the substrate depth inferred by seismic tomography by IMPROTA et alii (2012), estimated between ca. 240-270 m depth in this part of the Aterno Basin. The fundamental peak at 1 Hz (site Bazz5) indicates that the seismic basement tends to rise in the western part of the ERT2 profile (fig. 4). On the other hand, the high frequency peak gives indication of a shallower discontinuity, probably related to the passage from “soft” clayey-silty deposits to “stiff” coarse gravel deposits. Therefore, we think that this discontinuity is referred to the contact at about 5-10 m depth between the first fine and conductive sediments and coarse gravel represented by high resistivity layer. After the interpretation of geophysical data (ERT and H/V curves), we selected the site for drilling LAqui-core along the ERT2 profile characterized by the highest thickness of conductive layers, possibly far from resistive layers characterized by complicated geometry. For these reasons we avoided to locate the drilling well along the profile ERT1, where a strong sub-vertical discontinuity was identified in the first 50 m of depth. Site selection was focused in the middle part of the profile ERT2 (fig. 4), where the upper resistivity layers have thicknesses less than 30 m, whereas the deep conductive layer has the highest thickness (more than 80 m). We avoided to locate the well 356 M. PORRECA ET ALII Fig. 5 - Schematic LAqui-core stratigraphic log and interpretation of the depositional environments. toward the south-western part of this sector, which may be affected by geological complications due to the decrease of the infilling clastic sequence on the substrate. LAQUI-CORE STRATIGRAPHY The core collected has given valuable information about stratigraphy. The LAqui-core stratigraphy can be subdivided in two main depositional sequences (fig. 5). The upper sequence is composed of 41 meters of brownish oxidized silt and sand deposits, interbedded with m-thick rounded carbonate gravel intervals in the uppermost part. The lower sequence is dominated by grey clayey silt and sandy layers interrupted by 30 m thick deposit of coarse poorly sorted carbonate gravel. The upper and lower sequences are separated by an irregular and strongly oxidized contact, interpreted as an erosional discontinuity. In particular, the upper sequence of LAqui-core starts with 4.6 m-thick brown oxidized clayey silt sediments with poorly organic matter content and poorly defined sedimentary structure (fig. 6a). Below this clayey silt sequence, a coarse gravel deposit is present down to 10.7 m of depth. The gravels are composed by poorly sorted, sub-rounded carbonate clasts in a sandy matrix (fig. 6b). The core stratigraphy continues with fining downward cycling deposits composed by gravels alternated to silty layers down to the depth of 19.9 m. The gravel deposits are predominant with respect to the fine grained layers. They are characterized by well sorted and rounded carbonate clasts (fig. 6c). The silty layers are 0.5 to 1.5 m thick and characterized by oxidized organic matter rich sediments (fig. 6c). From 19.9 to 41 m of depth, the core consists of brown silty sands with clay intervals (fig. 6d). Parallel or poorly undulate laminations frequently occurred in the fine grained layers. Two significant grey volcanic ash layers are present at ca. 29 and 35 m of depth (fig. 5), 25 cm and 10 cm thick respectively (fig. 6d). An erosional discontinuity at 41 m of depth (fig. 6e) separates this upper sequence from the lower greyish clayey silt and sandy sequence (fig. 5). At depths from 41 to 84 m the sequence is characterized by prevalent grey silt and sandy sediments. The silty layers show frequent parallel laminations and organic rich content (fig. 6f). They are alternated with well sorted and clean sands with parallel or cross laminations. The presence of dispersed organic material and some few cm thick peaty layers characterize this part of the stratigraphy. At least two tephra layers were found in this part of the sequence, located at depths of ca. 41 (a few cm below the erosional discontinuity) and 83 m (fig. 5), with thicknesses of a few cm. This grey clayey silt and sandy sequence is interrupted by a 30 m thick deposit of coarse grained and poorly sorted gravel, from 84 to 115.40 m (fig. 6g). Below the coarse gravel deposit, from 115.40 m to the end of the LAqui-core (151 m), the stratigraphy is again characterized by a prevalence of grey clayey silt sequence. Compared to the upper silty sequence, this is generally deformed with frequent convolute laminations and slumping structures (fig. 6h), which are characterized by heavy oxidation phenomena and an increase in the sandy component. The silts are often over-consolidated and a few, 10 to 30 cm thick, peaty layers are present (fig. 6h). No evidence of tephra levels was identified in this lower part of the sequence. The grain size curves of the two samples collected in the fine grain deposits of the upper and lower sequences are very similar (fig. 7). Sedimentological analyses show the percentage of sand (S), silt (SI) and clay (C) is identical for the two samples (fig. 7): 61% (SI), 32% (C), 7% (S) for the upper sequence deposits (grey curve), and 61% (SI), 28% (C), 11% (S) for the lower sequence deposits (black curve). The grain size distribution of the samples fits range of the fluvial-lacustrine and lacustrine deposits (shaded areas) as collected and interpreted by BERTINI et alii (1992) for the western sector of L’Aquila Basin. GEOLOGICAL RECONSTRUCTION IN THE AREA OF MAXIMUM CO-SEISMIC SUBSIDENCE DURING THE 2009 357 Fig. 6 - Core photos of the main lithofacies of the LAqui-core: a) brown massive clayey silt deposits (0-4.6 m). The lower part is characterized by strong oxidation, bioturbation and charcoal fragments; b) coarse gravel deposits (4.6-10.7 m). Poorly sorted, sub-rounded to rounded, not cemented carbonate gravels. The matrix is prevalently composed by coarse carbonate sands; c) alternation of gravel and silty layers (10.7-19.9 m). The upper core is composed by well-sorted, rounded carbonated gravels with scarce or absent matrix; the lower core is composed by massive brown clayey silt with some oxidations levels; d) silty sands and clayey layers (19.9-40.97 m). The upper core is composed by massive brown silt interbedded by a 25 cm thick gravel layer; the intermediate core is composed by prevalent organic matter rich and oxidized silty sediments; the lower core is characterized by silty and sandy layers with 10 cm thick tephra layer; e) erosional discontinuity between fluvial-alluvial and palustrine sequences. The erosion surface is irregular and truncates a reddish coarse sand (top) and grey clayey silt (bottom) deposits; f) grey silt deposits (41.87-84.00). The two upper cores are characterized by grey silt and fine grained sand deposits. Plane-parallel to low-angle cross, alternated light and dark laminations are present in both silt and sand layers. The third core is composed by massive clayey silt with a 20 cm thick black peaty layer; g) coarse gravel deposit (84.00-115.40). Poorly sorted, coarse sub-rounded to rounded clasts. Note the presence of only carbonate clasts; h) grey clayey silt deposits (115.40-151.00 m). The upper core shows disturbed clayey and sandy layers with a charcoal undefined layers and a strong oxidation in the lower part of it. The intermediated core is characterized by 10-15 cm thick deformed silty and sandy layers separated by horizontal thin clay laminations. The lower core is strongly deformed with irregular sedimentary structures, more evident in the lower part. 358 M. PORRECA ET ALII Fig. 7 - Comparison between the two grain size curves of samples collected from LAqui-core and the typical range of the fluvial-lacustrine deposits in the western sector of L’Aquila Basin inferred by BERTINI et alii (1992). DISCUSSION GEOLOGICAL RECONSTRUCTION OF THE BAZZANO SUB-BASIN The LAqui-core stratigraphy was used to calibrate the electrical stratigraphy and to make interpretations on the related depositional environments of the Bazzano sub-basin. The stratigraphic sequence of the LAqui-core is subdivided in two main continental sequences separated by an evident erosional contact at –41 m (fig. 5). The upper part of the sequence is characterized by alternation of brown silty sands and gravels, associated to fluvial-alluvial deposits. The lower part is dominated by grey clayey silt and sandy layers related to a fluvial-palustrine environment. This lower sequence includes a 30 m thick interval of coarse poorly sorted carbonate gravel characteristic of alluvial fan deposits. The core did not reach the basin substrate. The two identified sequences may be related to two main sedimentary phases as also recognized in the Scoppito Basin (north-west of L’Aquila) by MANCINI et alii (2012). These authors correlate the older phase of sedimentation to the deposition of the basal alluvial fan (Basal conglomerates), fluvial sands and overlaying prevalently lacustrine-palustrine unit. This unit, called Scoppito-Madonna della Strada (MDS, MANCINI et alii, 2012), is tentatively attributed to the Late Pliocene?-Early Pleistocene. The younger phase of infilling is essentially dominated by fluvial deposits and comprises the Civitatomassa Unit (CVM, MANCINI et alii, 2012) and Recent alluvial deposits. This unit spans from Middle Pleistocene to Holocene. A similar stratigraphic architecture is also reported by BONCIO et alii (2011) for the Paganica sub-basin, which is strictly connected with the Bazzano sub-basin (see fig. 1). In this case, the authors identify an old (Early Pleistocene) finegrained lacustrine deposits capped by gravels and conglomerates and a Middle-Late Pleistocene sequence of coarse-grained alluvial fan, fluvial and fluvial-lacustrine (Flu-Lac) deposits. The sequence terminates with a thin cover of Late Pleistocene-Holocene fine to coarse-grained alluvium and eluvial-colluvial accumulations. Moreover, recent paleontological investigations have provided new age constraints to the S. Nicandro Fm., representing the oldest outcropping lacustrine deposits. This Formation is now referred to the Late Piacenzian-Gelasian interval (SPADI et alii, 2015). The stratigraphic analysis of the LAqui-core shows a good agreement with the ERT models, allowing to determine a detailed reconstruction of the depositional architecture of the Bazzano sub-basin as shown in fig. 8. In particular, in the upper sequence of the core we recognized an uppermost 4.6 m thick deposit of brown oxidized clayey silt sediments (figs. 5 and 6a). On this youngest deposit the grain-size curve demonstrates the prevalence of silt size (61%). The lithological and granulometric features allow to interpret this interval as related to oxidized alluvial sediments deposited during repeated flooding of the Aterno River during historical and Holocene times. This interpretation agrees with results inferred from archeological and paleoseismological excavations, whichsuggest a Holocene age for the uppermost 5-6 m of sediments of the middle Aterno Valley (COSENTINO et alii, 2001; DE MARTINI et alii, 2012). This thin silty layer is responsible for the lowest resistivity values recorded in the first few meters of the ERT profiles (fig. 8) and for the resonance peak at 10 Hz. These conductive deposits are continuous westward at least up to the slope break of the Mt. Ocre flank (fig. 4). Downward, the LAqui-core stratigraphy continues with gravel deposits alternated to silty layers up to –19.9 m. The well rounded carbonate gravel deposits are predominant with respect to the fine grained layers. All these features suggest that the sediments of this part of the sequence were deposited in a prevalent fluvial environment. The electrical profiles also indicate a prevalence of high resistivity sediments, typical of coarse grained (e.g. gravel and sands) deposits. We correlate this coarse grained deposit with the 10-50 m thick resistivity body underlying the first conductive layer (fig. 8). This first depositional cycle could correspond to the thin cover of alluvium (<20 m thick), which is generally attributed to the Late GEOLOGICAL RECONSTRUCTION IN THE AREA OF MAXIMUM CO-SEISMIC SUBSIDENCE DURING THE 2009 359 Fig. 8 - Schematic geological reconstruction of the Bazzano sub-basin along the ERT2 profile. The palette and scale of the ERT2 profile are the same of the fig. 3. Pleistocene-Holocene (COLTORTI & DRAMIS, 2006; MANCINI et alii, 2012). The resistivity body shows also an irregular geometry with a strong increase of thickness close to the Aterno River (fig. 3), where it presents an U-shaped geometry. The irregular geometry may be related to a stack of fluvial channel bodies with a complex lateral and vertical accretion. Moreover, the strong and sub-vertical discontinuity of this body highlighted in the middle part of the profile ERT1 can be interpreted as a paleo-channel of the Aterno river filled by fluvial and alluvial mainly coarse grain deposits, rather than a tectonic discontinuity. These geometries and corresponding electrical facies are found in similar environmental conditions, such as the case of the anastomosing Columbia River, Canada (BAINES et alii, 2002). From –19.9 to –41 m of depth, we recognized a prevalence of sandy silt deposits with frequent sandy layers showing parallel or cross laminations, distinctive of fluvial and alluvial depositional environments. The paleoenvironment tends to be dominated by alluvial over wash bank deposits toward the bottom as testified by the presence of planar laminations. This part of the upper sequence is characterized by low values of resistivity in the electrical profiles, probably due to the prevalence of silty sediments, undistinguishable from the fine grained lower sequence. At –41 m a strong erosional contact (fig. 6e) separates the upper and lower sequence (fig. 5). From –41 to –84 m the lower sequence is characterized by prevalent grey clayey silt and sandy layers. The silty layers have frequent parallel laminations and organic rich layers (fig. 6f). They are alternated with well sorted and clean sands with parallel or cross laminations. No important thicknesses of clay are recognized in this part of the core, whereas the occurrence of peaty layers suggest a deposition in shallow water with accumulation of organic matter. The grainsize results of a sample collected at –75 m are similar to the sample collected in the first meters of the sequence, with a similar prevalence of silt (61%). The two samples suggest therefore similar depositional conditions. We interpret this part of the sequence as related to a prevalent palustrine depositional environment, with evidence of meandering of the paleo-river that deposited silt or sand in function of the river evolution. This grey silty and sandy sequence is interrupted, at a depth of ca. 84 m, by a 30 m thick layer of coarse grained and poorly sorted carbonate gravels (fig. 5). This deposit is typical of an alluvial fan environment that probably was fed by the river flowing from the upper part of the Mt. Ocre slope, dominated by platform carbonate rocks. This alluvial fan deposit can be tentatively correlated with the alluvial fan identified in the lower sequence of the Paganica sub-basin by BONCIO et alii (2011). Below the alluvial fan, from a depth of 115 m to the base of the LAqui-core (–151 m) we found again a prevalence of grey clayey silt deposits, that appear intensively deformed with frequent convolute laminations and slumping structures. Organic rich layers are also present, indicating accumulation of organic matter in a palustrine and poorly washed environment. All this part of the cored sequence, from –41 to –151 m, is characterized by electrical facies with prevalent low resistivity values (fig. 8), even if some sedimentological and lithogical features are different above and below the alluvial fan deposit. The occurrence of deformational structures is typical of the lowest part of the sequence, below the coarse gravel deposits. Conversely, the clayey silty sediments above the gravel deposits are not deformed and characterized by the presence of thin tephra layers. These differences suggest a significant time gap in the deposition between the two grey silt deposits. The fine-grained and low resistivity facies is laterally continuous and dominates the deepest part of the ERT2 profile. The fine grained deposits of the lower sequence of LAqui-core correspond to the low-velocity and lowresistivity facies identified by IMPROTA et alii (2012) and BALASCO et alii (2011), respectively. In particular, IMPROTA et alii (2012) identified a first shallow very low Vp (<1500 m/s) layer, with an estimated thickness of 30-50 m corresponding to the most recent alluvial-fluvial sequence intercepted by LAqui-core. The authors identified another lens-shaped zone displaying a clear vertical Vp inversion between ~90-200 m depth in the central part of the line B1. They interpret this low-velocity body as equivalent to carbonate lacustrine silts of the PlioceneEarly Pleistocene S. Nicandro Fm. (BERTINI & BOSI, 1993; SPADI et alii, 2015), as also suggested by BONCIO et alii (2011) for the Paganica sub-basin. At this depth, LAqui-core recovered a sequence of fine grey silt and sand, that is not comparable with whitish carbonate silts of the S. Nicandro Fm. We believe that these two sequences are not correlated and they probably were deposited at different times of the Aterno Basin evolution. The presence of the 30 m thick gravel layer is a new outcome of our investigation, because geophysical data, like traveltime tomography (IMPROTA et alii, 2012) and the ERT2 profile do not show this layer. This can be due 360 M. PORRECA ET ALII to a combination of factors: the limited extension and thickness, together with 3-D geometrical irregularity of this alluvial fan body; the limited vertical resolution of traveltime tomography in the case of strong vertical velocity inversions; the poor current coverage in the deeper portions of the resistivity sections. CONCLUSIONS The results of this work provide new constraints on the architecture of the subsurface Quaternary continental deposits of the Bazzano sub-basin, constrained by geophysical and stratigraphic data of the LAqui-core borehole. The main results obtained by the geophysical and geological analyses are here reported: – the electrical resistivity tomography profiles have allowed to recognize fine grained and coarse grained sediments, their thicknesses and geometrical relationships. The fine grained deposits are characterized by low resistivity values (<40 ohm m), whereas the coarse grained by higher resistivity values (>100 ohm m); – ambient seismic noise measurements indicate the occurrence of two main seismic discontinuities at two well defined frequencies (0.7 and 10 Hz). The first fundamental frequency corresponds to the main seismic discontinuity between continental infilling and pre-infilling substrate (seismic bedrock), and allowed to estimate the depth of the basin substrate in accordance to previous seismic investigations (IMPROTA et alii, 2012). The high frequency peak corresponds to a very shallow and thin discontinuity (in the first 5 m) related to the contact between recent alluvial and first coarse grained deposits, as also well resolved by electrical data; – these geophysical results match well with the stratigraphic data obtained by the 151 m thick LAqui-core. The stratigraphy is characterized by alternation of fine and coarse grained sediments, with a prevalence of fine-grained sediments in the deepest part of the core; – correlations between geophysical and detailed stratigraphic data have allowed to reconstruct the subsurface geology of the Quaternary Bazzano sub-basin. The basin is characterized by fluvial-alluvial deposits (upper sequence) with variable thicknesses (20 to 50 m). An erosional discontinuity separates these deposits from fine-grained palustrine deposits (lower sequence). This latter sequence, which constitutes the deepest part of the investigated basin, is further subdivided in two distinct parts by occurrence of alluvial fan deposits. ACKNOWLEDGMENTS The authors wish to thank C. Hernandez, A. Di Chiara, R. Civico, M. Nocentini and M.P. Spigonardi for their support in the stratigraphic logging and geophysical survey. We are grateful to the Grotte di Stiffe (L’Aquila, Abruzzo) for their kindly availability to storage in the beautiful caves. This work has been funded by FIRB-Abruzzo project (RBAP10ZC8K_002). REFERENCES AGOSTINI S., DI CANZIO E., PALOMBO M.R., ROSSI A.A. & TALLINI M. (2012) - Mammuthus meridionalis (NESTI, 1825) from Campo di Pile (L’Aquila, Abruzzo, Central Italy). 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