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High-resolution seismic reflection exploration for evaluating the seismic hazard in a
Plio-Quaternary intermontane basin (L'Aquila downtown, central Italy)
Marco Tallini, Marco Spadi, Domenico Cosentino, Marco Nocentini, Giuseppe
Cavuoto, Vincenzo Di Fiore
PII: S1040-6182(19)30752-9
DOI: https://doi.org/10.1016/j.quaint.2019.09.016
Reference: JQI 7982
To appear in: Quaternary International
Received Date: 7 June 2019
Revised Date: 16 September 2019
Accepted Date: 17 September 2019
Please cite this article as: Tallini, M., Spadi, M., Cosentino, D., Nocentini, M., Cavuoto, G., Di Fiore, V.,
High-resolution seismic reflection exploration for evaluating the seismic hazard in a Plio-Quaternary
intermontane basin (L'Aquila downtown, central Italy), Quaternary International, https://doi.org/10.1016/
j.quaint.2019.09.016.
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© 2019 Elsevier Ltd and INQUA. All rights reserved.
High-resolution seismic reflection exploration for evaluating the seismic hazard
in a Plio-Quaternary intermontane basin (L'Aquila downtown, central Italy)
Marco Tallini
1
, Marco Spadi
1
, Domenico Cosentino
2
, Marco Nocentini
3
, Giuseppe Cavuoto
4
,
Vincenzo Di Fiore
4
1 Dipartimento di Ingegneria Civile, Edile-Architettura, Ambientale, Università degli Studi dell’Aquila, Via Giovanni
Gronchi, 18, 67100 L’Aquila, Italy
2 Dipartimento di Scienze, Università degli Studi Roma Tre, Largo San Leonardo Murialdo, 1, 00146 Roma, Italy
3 Istituto di Geologia Ambientale e Geoingegneria - CNR, Via Salaria km 29.300, 00015 Montelibretti, Roma, Italy
4 Istituto di Scienze Marine-CNR, Calata Porta di Massa, 80133 Napoli, Italy
Correspondence to: Marco Tallini (email: marco.tallini@univaq.it)
Abstract. On April 6, 2009, a Mw 6.1 earthquake struck the Plio-Quaternary intermontane
L’Aquila-Scoppito Basin in central Italy, causing severe damages to L’Aquila historical downtown
and surroundings, which were affected by notable site effects. Previous work has suggested that
different site effects may be related to the complex subsurface geologic architecture, given the
variable thickness and lithology of L’Aquila-Scoppito Basin filling deposits, on top of which the
city was built. To improve the 3D geological model of L’Aquila downtown for seismic site
response evaluation and to estimate the seismic hazard of possible buried active normal faults, a
multitask project was carried out consisting mainly of the integration of surface geology, geological
subsurface datasets and geophysical surveys. Data were interpreted with the aim of creating and
building a detailed stratigraphic and tectonic model for the Plio-Quaternary cover of the continental
basin and the buried morphology of the Meso-Cenozoic bedrock. We discuss and interpret the
results concerning a 1 km-long high-resolution seismic reflection profile and refraction tomography
integrated with stratigraphy from deep and shallow boreholes. The results allowed the
reconstruction of the Plio-Quaternary succession below L’Aquila downtown. The Plio-Quaternary
basin depocentre is located in a minor NNW-SSE graben, an extensional structure within the main
regional graben that borders L’Aquila-Scoppito Basin. Finally, data interpretation allowed to define
the subsoil geological model of the study area, to evidence the recent activity of several faults and
to reconstruct the Plio-Quaternary tectono-stratigraphic evolution of the basin. Together, the data
are useful to evaluate the seismic hazard of cities with great cultural heritage of central Italy, such
as the case study of L’Aquila downtown.
Keywords: intermontane basin; Plio-Quaternary continental deposits; seismic reflection profile;
borehole stratigraphy; L’Aquila-Scoppito Basin; seismic hazard.
1. Introduction
Developing 3D geological models is primary to estimating the seismic hazard of cities with
rich historic heritage in central Italy. Many of these are located in Plio-Quaternary intermontane
basins characterised by high seismicity, as demonstrated by peak ground acceleration (Fig. 1A)
(Meletti and Montaldo, 2007) and by recent earthquakes (Fig. 1B) (i.e. Mw 6.1 L’Aquila event of
April 6, 2009 and Mw 6.0 Amatrice event of August 24, 2016) (Chiarabba et al., 2009; Rossi et al.,
2019). Amplification effects related to seismic wave propagation are mainly due to vertical and
lateral changes of thickness and geological-technical characteristics of subsoil units and/or abrupt
variation in topography (Bard and Gariel, 1986; Lee et al., 2009; Marzorati et al., 2011).
Considering the regional geology of central Italy, the boundary of the Plio-Quaternary cover units
on the Meso-Cenozoic bedrock, generally corresponding to the seismic bedrock, must be taken into
account in evaluating the amplification effect (Lanzo et al., 2011; Martelli et al., 2012; Bordoni et
al., 2014; Gaudiosi et al., 2014; Pagliaroli ert al., 2019). High-resolution surface and subsurface
data, in terms of both geology (i.e. field work on stratigraphy and tectonics, biostratigraphy,
geochronology, boreholes stratigraphy, etc.) and geophysics (i.e. high-resolution seismic reflection
profiles, electrical resistivity tomography, gravimetry, etc.) are needed to create a reliable 3D model
(Di Giulio et al., 2014).
-------------------------------------------------------Fig. 1----------------------------------------------------------
Studies of site effects in urban areas were increased after the April 6, 2009 Mw 6.1
earthquake that struck L’Aquila-Scoppito Basin, causing severe damages in the L'Aquila downtown
(i.e. the cultural heritage urban area encircled inside the medieval walls: termed hereafter AD) (Fig.
2) (Chiarabba et al., 2009; Moro et al., 2017; MS-AQ, 2010). Previous works suggested that
different amplification effects could be related to the complex subsurface geologic architecture,
mainly due to variability in thickness and lithology of continental deposits filling the Plio-
Quaternary graben, on which AD was built (Del Monaco et al., 2013; Durante et al., 2017;
Nocentini et al., 2017; Amoroso et al., 2018; Macerola et al., 2019; Mannella et al., 2019).
This paper provides a new 3D geological model of AD through the integrated study of a high-
resolution seismic reflection profile, combined with seismic refraction tomography, along a ca.
1000 m-long N-S oriented section (termed hereafter Corso section). The Corso section was
interpreted geologically after cross-checking with data from several deep and shallow boreholes, as
well as detailed geological field data obtained in recent years by different authors (Tallini et al.,
2012; Cosentino et al., 2017; Nocentini et al., 2017, 2018). The main result of this work is the
reconstruction of the buried morphology of the top of the Meso-Cenozoic bedrock, which allowed
to better define the Plio-Quaternary tectono-stratigraphic evolution of the L’Aquila-Scoppito Basin
and to furnish a 3D model pivotal to evaluating the seismic site effects via numerical modelling. In
addition, this work contributes to define seismic site effects (amplification and surface-active
faulting) at an urban scale and to refine the geometry and activity of the fault system beneath AD,
key to improve the earthquake-resistant restoration on cultural heritage buildings.
-------------------------------------------------------Fig. 2----------------------------------------------------------
2. Geological setting
L’Aquila downtown (AD) is located in the L’Aquila-Scoppito Basin (ASB), which
corresponds to the western part of the large L’Aquila Basin, a NW-SE oriented intermontane basin
of central Italy. ASB is an E-W–trending half graben, bordered to the north by both the active
south-dipping Scoppito-Preturo normal fault (SPF) and the southwest-dipping Pettino normal fault
(PF) (Tallini et al., 2012; Storti et al., 2013) (Fig. 1).
In the central Apennines fold-and-thrust belt, post-orogenic extensional deformation is
documented from late Pliocene to Present, with the development of normal fault systems, mostly
trending NW-SE to N-S. These fault systems were responsible for the onset and subsequent
evolution of post-orogenic intermontane basins (Giaccio et al., 2012; Porreca et al., 2016; Cosentino
et al., 2017, with references therein) (Fig. 1). In the ASB, the Meso-Cenozoic carbonate units (SLB
and CRP) and the Upper Miocene synorogenic terrigenous deposits (SYN) are unconformably
overlaid by a thick succession of Plio-Quaternary continental clastic deposits (Fig. 3) (Cosentino et
al., 2010).
The continental sedimentation within the L’Aquila-Scoppito Basin started in the late
Piacenzian-Gelasian with the deposition of the San Demetrio-Colle Cantaro Synthem (Fig. 3)
(Spadi et al., 2016; Cosentino et al., 2017; Nocentini et al., 2017, 2018). In the ASB, this synthem is
represented by the presence of the Colle Cantaro Fm. (CCF), which consists of heterometric
breccias, in clayey-silty matrix, deposited in slope and debris flow environment. The Madonna della
Strada Fm. (MDS) unconformably overlays both CCF and the Meso-Cenozoic bedrock. MDS
consists of silt, sand beds, and clayey silts, containing lignite beds. In the lower and middle part of
MDS, coarse- to medium-grained, well-rounded sandy gravel beds are present, showing variable
thickness (10 m–400 m) and channelized geometries with planar and through crossbedding. The
facies association of MDS clearly point to a deposition in a meandering fluvial system, which
developed within the ASB with a wide floodplain and swampy areas during the Calabrian (Early
Pleistocene) (Cosentino et al., 2017). Along the border of the MDS alluvial plain, coarse-grained
slope deposits sedimented, interfingering the fine-grained MDS sediment, called San Marco Fm.
(SMF) (Nocentini et al., 2017). The Meso-Cenozoic bedrock and the previously described
continental units are locally covered by the Fosso di Genzano Synthem (FGS). FGS is composed of
coarse- to medium-grained clast-supported gravel beds, with general horizontal bedding and fining-
upward gradation with yellowish sands or silty layers. The facies are typical of a braided or
wandering fluvial system with lateral alluvial fans. Based on tephra layers (520 ±5 ka, Centamore
and Dramis, 2010; 367 ±2 ka, Giaccio et al., 2012) and large mammal remains, FGS can be referred
to the Middle Pleistocene (Cosentino et al., 2017). Most of AD was built over the Colle Macchione-
L’Aquila Synthem (CMA), which unconformably overlies either the older synthems and the Meso-
Cenozoic bedrock through a highly irregular and erosive basal surface. CMA mainly consists of
highly heterometric breccias with angular carbonate clasts supported by whitish calcareous silty
matrix. The calcareous breccias of CMA are interbedded with carbonate silt deposits (eastern
margin of AD) and with siliciclastic sand and gravels (southern margin of AD). The facies of these
carbonate breccias point to point to huge events of detrital slope deposition through debris flow and
rock avalanche with debris produced mainly by the erosion of the northern margin of ASB (Gran
Sasso chain) (Centamore et al., 2006; Esposito et al., 2014). The deposition of CMA is referable to
cold phases of late Middle Pleistocene (Cosentino et al., 2017; Nocentini et al., 2017). The top of
L’Aquila hill, on which AD is placed, is covered by Collemaggio Synthem (COM), which mainly
consists of reddish to dark brown clayey silts sediments with rare subangular calcareous clasts.
COM is interpreted as epikarst fill and reworked paleosols, probably formed under interglacial
conditions, which suggests a correlation with the Eemian interglacial stage (MIS 5e) (Magaldi and
Tallini, 2000; Nocentini et al., 2017). Finally, the younger continental units are alluvial, colluvial,
slope, and anthropic deposits of Late Pleistocene and Holocene age, which were grouped in the
same supersynthem (ALL).
-------------------------------------------------------Fig. 3----------------------------------------------------------
3. Material and Methods
The subsurface geology of AD was reconstructed both the stratigraphy of several deep and
shallow boreholes (depth 30-440 m b.g.l.) (Fig. 4) and geophysical data (seismic reflection profile,
named Corso section, and seismic refraction tomography, gravimetric) acquired just after the main
shock that struck AD on April 6, 2009. Subsurface data were integrated with outcrop data of AD.
Standard vertical profiling of well logs was used to interpret of the Corso section. In addition,
structural analysis of the seismic reflection profile followed the typical seismic interpretation
methodology in defining reflector terminations (Posamentier and Allen, 1993). Different seismic
facies were recognized on the Corso section according to differences in the amplitude, frequency,
and continuity of seismic reflectors, as well as their terminations and lateral distribution.
The contour line map of the top of the Meso-Cenozoic bedrock in m a.s.l. is also reported
because of its importance in evaluating the seismic response. The elevation (m a.s.l.) of the bedrock
top was obtained for single points based on the outcropping boundaries of geological units, the
geological interpretation of the seismic reflection profile (Corso section), the borehole logs, and the
two orthogonal geological sections. Furthermore, the contour line map was elaborated using the
trend of residual gravimetric anomalies isolines in mGal modified from Blumetti et al. (2002).
-------------------------------------------------------Fig. 4----------------------------------------------------------
3.1. Seismic data acquisition
The Corso section has a NNE-SSW strike and a length of 960 meters. The survey was
conducted with an IVI-MINIVIB shaker truck as vibratory seismic source. Using a 168 kg mass,
this source produces harmonic vibrations (sweep) with a maximum peak force of 27600 N. The
geometry consists of a dense (5 m spacing) 192-channels 10-Hz vertical geophone array. The
source move-up was 10 m; at each of the 91 vibration points, three 15 s long, 10-200 Hz sweeps
were performed. Correlated data were stacked to improve the signal-to-noise ratio. This multi-fold
wide-angle geometry allowed the detection of highly redundant turning waves and deep-penetrating
refracted waves, which contain information on velocity distribution in depth. The acquisition
parameters permitted a maximum common midpoint (CMP) fold of 48-traces (Tab. 1).
Data were acquired using eight 24-bit, 24-channel GEODE Geometrics seismographs and
were recorded on a PC running an applicative code to set field acquisition parameters and to store
the seismic data in seg-2 format.
Several conventional processing steps were applied to produce common depth point (CDP)
stacked sections (Steeples and Miller, 1988; Yilmaz, 2001). In seismic exploration, CDP
corresponds to the halfway point in the travel of a wave from a source to a reflector and then to a
receiver, which roughly refers to the horizontal distance in the seismic image. The processing
sequence is summarized as follows: (i) pre-processing phase: import seg-2 data, cross-correlation,
geometry setup, trace editing (Kill bad traces, Top Muting, Bottom muting); (ii) processing phase:
amplitude correction (Geometrical spreading and AGC – 150 ms), static correction, predictive
deconvolution (operator length 115, predictive dist. 15 ms), Bandpass filter (25-50-110-150),
velocity analysis (semblance - Stretch 90%), NMO, stack, Kirchhoff time Migration.
Seismic tomography was used both to apply static corrections to reflection data
appropriately, and to obtain complementary information on P wave velocity field. In fact,
reflection imaging is difficult in shallow subsoil because only a small number of short-offset
traces for any shot, gather reflection signal. Furthermore, reflections are usually masked by
severe coherent noise which must be strongly attenuated before imaging the reflections.
-------------------------------------------------------Tab. 1---------------------------------------------------------
-
4. Results
4.1. Boreholes stratigraphy
Hundreds of boreholes were drilled in the whole ASB to investigate the subsoil deposits
following the April 6, 2009 L’Aquila earthquake (Nocentini et al., 2017). We show the stratigraphy
of several deep boreholes drilled in the vicinity of the Corso section, which were useful to constrain
the interpretation of the deep subsurface geology of AD as imaged by the seismic reflection profile
(Fig. 4). In addition, the stratigraphy of several shallow boreholes drilled along the Corso section
were used for constraining the uppermost part of the seismic refraction tomography of AD.
Well logs analysis showed rapid changes of thickness and facies of the continental deposits,
which reflect the extensional tectonic deformation starting from ca. 3 Ma (Cosentino et al., 2017).
The relationships between the boreholes S12 and S10 highlighted the presence of MDS lying above
an irregular surface carved both in the Meso-Cenozoic bedrock and the CCF (Fig. 4). The borehole
S10, which drilled 400 m of fine-grained siliciclastic deposits referable to alluvial plain
environments (well log database available at DICEAA’s Applied Geology laboratory of Università
degli Studi dell’Aquila) showed the existence of a possible local depocentre of L’Aquila-Scoppito
Basin considering the shorter MDS thickness in the near areas of AD (Tallini et al., 2012; Cosentino
et al., 2017; Nocentini et al., 2017). Looking at the borehole stratigraphy, CMA, FGS and MDS,
show a variable thickness and latero-vertical changes in lithofacies all over the ASB.
The FGS has been found mainly in the boreholes drilled in the southern sector of AD. In
contrast, the coarse-grained deposits of CMA are almost always present in the boreholes, even
though they show variable thickness between 15 m and 70 m.
The 140 m-deep borehole S5 represents the exemplificative sedimentological and
stratigraphic succession of AD units (Fig. 5). Below CMA, yellowish sub-horizontal laminated fine-
grained sands and silty sands are present and are pertained to FGS. In the borehole S5, a
pedogenetic horizon (oxidized surface), corresponding to a probable stratigraphic unconformity,
distinguishes the FGS from the underlying fine-grained deposits referable to MDS. MDS consists of
massive to laminated sandy silts and sands that pass to alternations of silts, clayey silts, clays, and
fine sands with lignite levels toward the base of the borehole. Within the silty sandy layers of the
upper part of MDS, structures related to soft-sediment deformation, such as highly deformed
laminations showing convolute-like or deformed/broken laminae, are visible in the sedimentary
cores of borehole S5. They are possibly referable to due to fluid expulsion possibly during seismic
wave propagation (seismites) (Fig. 5). In addition, in the borehole S5, the sediments pertaining to
MDS show a relatively high sand content (Fig. 5). These sandy layers show high trough and/or
planar cross-bedding, sometimes coupled with ripple cross laminations with an erosive basis and
scour-and-fill structures.
-------------------------------------------------------Fig. 5----------------------------------------------------------
4.2. Seismic facies recognition
Five seismic facies were identified in the Corso section based on the parameters of the seismic
reflectors, such as geometry, seismic reflection amplitude, continuity, and qualitative reflection
frequency, (Fig. 6; Tab. 2). The five seismic facies were further checked by comparing them with
the lithological information coming from the boreholes stratigraphy. Cosentino et al. (2017)
interpreted the Pettino I seismic reflection profile, which is very close to AD and shows the same
seismic facies of the Corso section. Therefore, the same nomenclature of the seismic facies adopted
by Cosentino et al. (2017) was used for the Corso section.
-------------------------------------------------------Tab. 2---------------------------------------------------------
-
Several discontinuities in the semi-continuous reflectors were interpreted as faults, the main
one is the normal fault located at CDP 98 (240 m) that displaced most of AD formations. It is a SW-
dipping plane with a sub-planar shape and is located beneath Piazza Duomo (the main square of
AD) and is named Piazza Duomo Fault (PDF).
Seismic facies S. Seismic facies S is composed by chaotic seismic facies, with low-amplitude
dipping reflective pattern, and discontinuous reflectors. It is located on the deepest part of the basin
fill. Low-resolution signal of seismic facies S is probably due to the penetration problem of the
seismic pulse. The weak diffusion connected with low resolution signal and discontinuity of
reflectors points to interpret this seismic facies as the acoustic bedrock of ASB.
Seismic facies R. Seismic facies R is typically characterized by high reflectivity and amplitude,
with disturbed or semi-continuous reflectors. It always characterizes the lower part of the Corso
section, showing a maximum thickness of ~200 ms. The faintly continuous reflectors show a
divergent internal configuration, typical of wedge-shaped geometries. The boundary between this
seismic facies and the seismic facies S is irregular, probably due to the extremely disturbed signal.
The irregularly-fringed and high-amplitude reflectors of seismic facies R possibly can be referable
to clastic coarse-grained deposits.
Seismic facies L. Seismic facies L is composed by continuous and parallel reflectors with medium
to high amplitude. This facies has variable thickness between ~300 ms in the northern part and ~450
ms in the central part. Seismic facies L shows horizontal parallel and semi-parallel reflectors, that
are tilted and dragged along the extensional faults affecting the Corso section. The seismic facies L
corresponds to more sub-parallel and laterally extensive reflectors (HARP’s: High Amplitude
Reflection packages, or HAC: High Amplitude Continuous) (Posamentier, 2002). The lower
boundary of seismic facies L is developed on an unconformable and extremely irregular surface, on
top the seismic facies R. Based on boreholes stratigraphy, the seismic facies L consists of fine-
grained deposits.
Seismic facies Ls. Seismic facies Ls is evidenced by inclined oblique reflectors, characterized by
discontinuous to chaotic low-amplitude reflections, which can terminate against each other. This
reflectors configuration could resemble the typical pattern of Lateral Accretion Package (LAP)
(Posamentier, 2002). High-amplitude reflections mark the bounding surfaces of these features
giving to this facies a “U-shaped” geometry. This facies is scattered and is always contained within
the seismic facies L. Concave upward reflectors are confined in the central part of the basin, which
probably are referable to channelized bodies whose location slightly shifted laterally (Schwab et al.,
2007; Deptuck et al., 2008).
Seismic facies BC. Seismic facies BC is typically characterized by a chaotic pattern, with
continuous, low-amplitude irregular reflectors. This facies is distributed in the upper part of the
basin fill. Its basal boundary is highly irregular, and it was recognized down to ~80 ms.
-------------------------------------------------------Fig. 6----------------------------------------------------------
4.3. Tomography
The refraction tomography of the seismic section, integrated with many borehole logs, was
analysed to investigate the upper part of the Corso section.
The refraction velocity field (Fig. 7) involved the first 100 m depth. More precisely, in the
first 20 m depth very heterogeneous units are present and consequently there is a severe scattering;
from 20 up to 80 m, the Vp value increases slowly from 1600-2000. The first clear impedance
contrast is detected at about 80 m depth where Vp reaches 2600 m/s. In the reflection processing,
from 0 up to 80 m of depth, the velocity analysis does not detect Vp variation. This is due to the
severe scattering and to the low Vp gradient.
The calculated Vp values are showed in Table 2 and are compatible with the Vp values
estimated by Improta et al. (2012). Then, for the geological interpretation of the tomography, the
trend of the Vp contour lines and the relative change of Vp were used.
-------------------------------------------------------Fig. 7----------------------------------------------------------
5. Discussion
5.1. Geological interpretation of the Corso section seismic reflection profile
The recognition of seismic facies along the Corso section, combined with previous studies on
ASB geology and the stratigraphy of the many boreholes, allowed the reconstruction of a detailed
geological model for AD (Fig. 8). The semi- ubiquitous presence in the examined boreholes (all
except S12 and S13) of breccia deposits pertaining to the CMA (Fig. 4), which show different
thickness, combined to the highly chaotic seismic facies BC permitted the recognition of this late
Middle Pleistocene unit for the upper part of the Corso section. In the high-resolution seismic
profile, the younger covers (i.e. anthropic, alluvial, and colluvial deposits of Upper Pleistocene and
Holocene), which are characterized by a few meters of thickness (Fig. 4), are indistinguishable from
the underlying CMA, because of the limit of resolution of the used seismic method.
The interpretation of seismic tomography allowed a better definition of the unconformable
boundary between the anthropic cover (AC) and the underlying Quaternary units, as well as the
boundaries among COM, CMA, and MDS. Moreover, it cleared the occurrence of the tectonic
boundary due to the PDF and the presence of alluvial deposits within the calcareous breccias
(CMA). The CMA-MDS boundary was confirmed by the stratigraphy of boreholes S2, S4, S11,
(Fig. 7).
The seismic facies L is mainly represented by continuous and parallel reflectors specific for
fine-grained deposits. Based on this assumption and on borehole stratigraphy, seismic facies L
corresponds mainly to MDS (Lower Pleistocene, Calabrian). The overlaying FGS (lower Middle
Pleistocene), composed largely of silty sands, is possibly comprised in this seismic facies L,
because it is not lithologically distinguishable from MDS. Conversely, the borehole logs show
different facies between FGS and MDS.
Seismic Facies Ls probably corresponds to channelized bodies of coarse-grained deposits
within the generally finer MDS. Their presence is testified by sandy levels in the middle part of well
log S5 (Fig. 5). These channels, defined as channel-point bars, are formed by dunes and ripple
migration lateral to the currents, and are encased in floodplain pelite. The cross-bedded sands
constitute unidirectional flow bedload of a fluvial-channel fill. A similar channelized facies is also
recorded in the borehole LAQUI-CORE, showing 31.4 m of gravel with sandy matrix (Porreca et
al., 2016).
The seismic facies R is characterized by weakly continuous and divergent reflectors
correlated to clastic coarse-grained deposits. In agreement with the interpretation of the close
Pettino I seismic reflection profile (Cosentino et al., 2017), this seismic facies could be linked to the
CCF.
Finally, seismic facies S, although poorly definable, is correlated to the Meso-Cenozoic
bedrock that, below AD, could be represented by Miocene carbonate-ramp as testified by the
borehole S12 (Fig. 4).
-------------------------------------------------------Fig. 8----------------------------------------------------------
5.2. Tectonic features
In the Corso section, several normal faults can be detected (Fig. 8). They mainly cut the MDS
and the underlying units (CCF and the Meso-Cenozoic bedrock). PDF is a SW-dipping normal fault
characterized by a sub-planar shape and different displacements. The resolution of the Corso section
does not evince the displacement of the COM/CMA boundary (Fig. 8), although the COM thickness
in the PDF hangingwall is higher than in the footwall, pointing to a possible tectonic activity of
PDF during the Late Pleistocene (Fig. 6). Moreover, minor faults related to PDF do not displace the
upper part of MDS testifying the syn-sedimentary faulting of MDS.
The syn-tectonic evolution of MDS is also suggested by the highly deformed laminations
recognizable at different depths in borehole S5 (Fig. 5), which could represent soft-sediment
deformations in response to palaeoearthquakes (paleoseismites) (Alfaro et al., 1997; Owen et al.,
2011). This observation indicates strong seismic activity during the Calabrian within the ASB, as
already suggested, on the base of soft-sediment deformations in the outcrops located nearby west of
AD by Storti et al. (2013), and in a borehole placed SE of AD by Porreca et. (2016). Some
empirical correlation between seismite type and paleoearthquake magnitude were proposed by
Rodrıguez-Pascua et al. (2000), but it requires the knowledge of the principal seismogenetic
structures of the area. The simulated activation of the Pettino fault (PF) and its relative fault zones is
expected to generate earthquakes with maximum magnitudes approaching Mw 6.7 (Galli et al.,
2010; Moro et al., 2013). The soft-sediment deformations found in the borehole S5 are assimilable
to “mushroom-shaped structure” due to silts protruding into laminites. These structures are possibly
caused by paleoearthquakes that range from Mw 5 to 7 (Rodrıguez-Pascua et al., 2000), which are
in accordance with paleomagnitudes revealed from the same formation (Storti et al., 2013).
Using a rough correlation between the faulted interval of the bottom CMA boundary (about
40 m) (Fig. 8) and the age of deposition of CMA (~300 ka) (Cosentino et al., 2017) a ~0.14 mm/yr
slip rate is inferred for PDF, which represents a similar value to that calculated for the single splay
of the close active Paganica Fault responsible for the April 6, 2009 L’Aquila earthquake (Galli et
al., 2010).
5.3. New subsurface data for a 3D model
In Fig. 9, the geological sections A-A’ and B-B’-B’’ represent the subsoil model of AD,
which has been updated with the new geological data from the Corso section and the stratigraphy of
the deep boreholes (Fig. 4). More precisely, section A-A’ has been drawn by integrating the section
in Nocentini et al. (2017) with data from the Corso section. The Meso-Cenozoic bedrock is located
at most at 600 m b.g.l., and though it is the deepest value for the bedrock depth in ASB, it is in
accordance with Meso-Cenozoic bedrock depth of other neighboring intermontane basins of central
Italy, such as the Fucino Basin (Cavinato et al., 2002) and the Paganica-San Nicandro-Castelnuovo
Basin (Civico et al., 2017).
The Colle Cantaro-Cave Formation (CCF) shows variable thicknesses along the geological
sections (Fig. 9), and assumes an onlap configuration over the Meso-Cenozoic bedrock due to the
tectonic activity of the normal faults bounding the ASB. The sub-horizontal deposits of MDS testify
a sedimentation occurred in a more stable tectonic environment (Fig. 9). The FGS is scarcely
represented along the geological sections where it is only a few meters thick or completely missing
(Fig. 9). FGS crops out mainly on the southern slope of AD and was sometimes recognized in some
boreholes. All the previous stratigraphic units are unconformably overlain by CMA. CMA is
covered by the Collemaggio Synthem (COM) interpreted as colluviated paleosols draping the CMA
epikarst (Nocentini et al., 2018). COM is barely visible in the Corso section, but is well
recognizable in the refraction seismic tomography (Fig. 7) with maximum thickness of 20 meters.
Finally, alluvial, colluvial and anthropic filling (ALL) are imaged only in the seismic tomography
profile and represent the present-day deposition of sediments in the current geological setting.
-------------------------------------------------------Fig. 9----------------------------------------------------------
The highest thickness of ASB Plio-Quaternary deposits is placed in correspondence with the
borehole S10, in the southern part of AD that defines the ASB depocentre that is located within a
minor NNW-SSE oriented graben, which, in turn, is contained in the main regional graben of the
Pettino Fault system (PF) (Fig. 10).
Fig. 10 shows the main faults affecting the AD subsoil (modified from Nocentini et al., 2017)
and the contour line of the gravimetric anomalies (modified from Blumetti et al., 2002). The
reported faults are mainly extensional and/or transtensive, showing the main activity during the
Quaternary. The faults that define the geometry of the Plio-Quaternary basin are oriented NW-SE,
E-W, and NNE-SSW. The faults trend follows gravimetric contour lines. Positive gravimetric
anomalies (red lines) correspond to the Meso-Cenozoic carbonate and Upper Miocene terrigenous
reliefs, while the negative ones (blue line) represent the Plio-Quaternary basin-fill. The NW-SE and
E-W striking extensional faults (PF, CF, and VMF) bound the main graben, corresponding to the
current Aterno River Valley. Plio-Quaternary continental deposits accumulated here as a result of
the increased accommodation space caused by the activity of the fault system associated with the
master SW-dipping Pettino Fault (PF). The principal local negative gravimetric anomaly (<-3.2
mGal) is exactly in AD and it is bounded by NW-SE normal faults (PF, CF, and VMF) and two
NNE-SSW striking transtensive faults (SGF and SEF), which developed as transfer faults. These
faults generate, within the main graben, a deep lowered sub-rectangular area located within in AD,
characterized by a minor NNW-SSE graben bounded by TF and PDF faults. This subsurface
tectonic setting is well represented by the two orthogonal geological section of Fig. 9. The
extensional fault system of Fig. 10 follows a fractal-type and scale-independent arrangement with
the normal faults, as a key structural element, repeating at different scale (Ackermann et al., 1986;
Turcotte, 1997).
-------------------------------------------------------Fig. 10---------------------------------------------------------
The contour lines of the top of the Meso-Cenozoic bedrock highlight the tectonic morphology
of the lowered sub-rectangular area (Fig. 11). The contour lines show mainly a NW-SE oriented
enclosed area, with the maximum depth located in the southern part of AD. In terms of local
seismic response, the downthrown sub-rectangular block of bedrock highlights a deep basin
geometry in NNW-SSE and E-W directions. This arrangement evidences a bi-dimensional or
possibly three-dimensional seismic effect that must be considered for the seismic hazard evaluation
of L’Aquila downtown.
-------------------------------------------------------Fig. 11---------------------------------------------------------
The Plio-Quaternary geological evolution of ASB can be summarised in five main steps as reported
in the synoptic sections of Fig. 12:
Step A (Piacenzian-Gelasian age): horst and graben geometries involving the Meso-Cenozoic
bedrock were forming and, within the graben structures, CCF, characterised by slope and alluvial
fan facies, sedimented;
Step B (Calabrian age): a large meandering and fluvial plain deposits (MDS), characterised by
floodplain and swampy facies, sedimented into the graben structure (ASB area); along the graben
boundaries-valley slopes, talus and scree deposits (SMF) sedimented;
Step C (early Middle Pleistocene age): gravel-bed fluvial and alluvial fan deposits (FGS)
sedimented into a smaller valley area within the ASB;
Step D (late Middle Pleistocene age): debris flow and rock avalanche deposits (CMA), producing
by the erosion of the northern margin of ASB, sedimented in the AD area as distal zone.
Step E (Upper Pleistocene-Holocene): reddish reworked paleosols (COM) sedimented into the
epikarst of CMA in AD area; alluvial, colluvial, slope, and anthropic Holocene deposits (ALL)
sedimented unconformably onto the older units.
-------------------------------------------------------Fig. 12---------------------------------------------------------
6. Conclusion
The geological interpretation of the ca. 1000 m-long high-resolution seismic reflection profile
(Corso section), supported by deep boreholes and refraction tomography, allowed us to update the
subsoil model of L’Aquila downtown (AD). This model is necessary to evaluate the seismic site
response and the seismic hazard of possible buried active normal faults and to investigate the Plio-
Quaternary tectono-stratigraphic evolution of the studied area (AD and larger L’Aquila-Scoppito
Basin: ASB).
The main results are as follows:
1. the available seismic and borehole data permitted the reconstruction of the AD
subsoil model in two orthogonal directions highlighting a tri-dimensional model (Figs. 9 and 11).
2. the major thickness of the Plio-Quaternary succession of the basin fill below AD was
recognized with a maximum thickness of 600 m. This corresponds to the main depocentre of ASB
located in the southern part of the Corso section (Figs. 6, 9, and 11). More precisely, the borehole
S10 and the Corso section testify a thickness of at least 400 m for the Madonna della Strada Fm.
(MDS) which, as a flood plain deposit, also shows coarse-grained channelized bodies within fine-
grained layers (Fig. 6).
3. the ASB depocentre is tectonically located within a minor NNW-SSE oriented
graben, in turn, contained within a main regional graben related to the tectonic activity of the
Pettino Fault (PF). This observation is compatible with the active tectonics and seismicity of
intermontane basins of central Italy, characterized by complex graben structure from a regional to a
local scale (Fig. 10).
4. the Corso section evidences possible post-CMA (i.e., late Middle Pleistocene)
activity of the Piazza Duomo Fault (PDF).
The results, within the scope of seismic microzoning activity, were useful to reconstruct a
detailed geological subsoil model at an urban scale, and thus to provide evidence of both seismic
site effects and the activity of buried faults. This information was essential to designing an urban
masterplan able to mitigate the seismic hazard of cities with rich cultural heritage of central Italy,
such as the case study site of L’Aquila downtown.
Acknowledgements
We would like to thank the CARISPAQ institution to grant funds for the seismic reflection
survey investigation. Thanks, are also extended to all the IAMC-CNR people who helped us during
the geophysical acquisition phase in the field. The work strongly benefited from preliminary
discussion on the seismic profile interpretation with Massimiliano Rinaldo Barchi and Massimiliano
Porreca (Perugia University) who are warmly thanked. Thanks to an anonymous reviewer, and to
Pierluigi Pieruccini for the valuable comments which improved the final version of the paper, and to
Kerry Jane Rhoden for the careful revision of the English language. This work was carried out
within the general agreement between Università degli Studi dell’Aquila (Dipartimento di
Ingegneria Civile, Edile-Architettura e Ambientale) and Università degli Studi Roma Tre
(Dipartimento di Scienze). The grant to Dipartimento di Scienze, Università degli Studi di Roma
Tre (MIUR-Italy Dipartimenti di Eccellenza, articolo 1, commi 314 – 337 legge 232/2016) is
gratefully acknowledged.
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Figure Captions
Figure 1: Simplified structural sketch of the central Apennines. A: map of the Seismic Hazard in
Italy, showing peak ground accelerations (g) that have a 10% chance of being exceeded in 50 yr
(Meletti and Montaldo, 2007). B: main Pliocene–Quaternary intermontane basins of the central
Apennines. 1- Quaternary volcanic rocks; 2- Plio–Quaternary marine and continental deposits; 3-
Neogene foredeep turbiditic deposits; 4- Meso-Cenozoic carbonate (platform, slope and basin)
deposits; 5- inactive major thrust; 6- active normal fault; 7- inactive normal fault; 78- April 6, 2009
Mw 6.1 L’Aquila earthquake; 89- August 24, 2016 Mw6.0 Amatrice earthquake; PF: active Pettino
Fault; SPF: Scoppito-Preturo Fault; VMF: Via Mausonia Fault; PAF: active Paganica Fault; ASB:
L’Aquila-Scoppito Basin. Modified from Cosentino et al. (2017).
Figure 2: a: L’Aquila view from Google Earth; the red line corresponds to the high-resolution
reflection seismic profile (Corso Section); the yellow dotted line corresponds to medieval wall; AD:
L’Aquila downtown. b: L’Aquila city view from the West.
Figure 3: A: Geologic map of the studied area (modified from Nocentini et al., 2017). PF: Pettino
Fault; CF: Castello Fault, SGF: San Giuliano Fault; TF: Tribunale Fault; PDF: Piazza Duomo Fault;
SEF: Sant’Elia Fault; VMF: Via Mausonia Fault. B: Stratigraphic scheme of L’Aquila-Scoppito
Basin (ASB) (modified from Nocentini et al., 2017).
Figure 4: Correlation panel of selected borehole stratigraphy with location of boreholes. Geological
legend from Fig. 3.
Figure 5: Detailed borehole stratigraphy of S5. A: detail of soft sediments deformation (possibly
seismite) recognized at ~77 m of depth; B: lignite level in the lower part of borehole S5.
Figure 6: A) 2D depth-migrated reflection of the Corso section (horizontal scale= common deep
point, vertical scale= two-way time); B) line drawing of seismic facies (following tab. 4), horizons,
and faults (horizontal scale= linear meters, vertical scale= two-way time); C) line drawing of the
seismic profile with recognised seismo-stratigraphic units (horizontal scale= linear meters, vertical
scale= two-way time). Legend: BC = fan deposits and slope breccias; 1 = channelized bodies in
seismic facies BC; L = alluvial plain deposits; Ls = channelized deposits; R = fan deposits and slope
breccias; S = meso-cenozoic bedrock; 2= fault; 3= channelized bodies; 4= unconformity; 5= top of
Meso-Cenozoic bedrock.; PDF: Piazza Duomo Fault. S2, S4, and S10: boreholes from Fig. 3.
Figure 7: Seismic tomography of the Corso section interpreted through borehole stratigraphies. A:
interpreted seismic refraction tomography with the shallow borehole stratigraphies. B: geological
interpretation of the seismic refraction tomography AC: anthropic cover (buried foundation and
anthropic fill characterized by high and low Vp, respectively); COM: reddish colluviated fine-
grained paleosols (Upper Pleistocene); CMA: calcareous breccia and gravel (upper Middle
Pleistocene); CMA-f: alluvial sand, pelite and gravel (upper Middle Pleistocene); MDS: alluvial
pelite and sand (Calabrian p.p.). In the upper part of this unit, the possible presence of thin alluvial
deposits referring to FGS (lower Middle Pleistocene) cannot be ruled out; PDF: Piazza Duomo
Fault.
Figure 8: Geological interpretation of the Corso section seismic profile. Geological legend from
Fig. 3.
Figure 9: Geological sections crossing L’Aquila hill (modified from Nocentini et al., 2017). The
acronyms of the faults are: PF: Pettino Fault; CF: Castello Fault, SGF: San Giuliano Fault; TF:
Tribunale Fault; PDF: Piazza Duomo Fault; SEF: Sant’Elia Fault; VMF: Via Mausonia Fault. For
both the location and legend see Fig. 3.
Figure 10: Tectonic and gravimetric sketch of Quaternary L’Aquila-Scoppito Basin (modified from
Nocentini et al., 2017 and Blumetti et al., 2002); the green square refers to the area of Fig. 11. 1:
main normal and transtensive fault (PF: Pettino Fault, CF: Castello Fault, VMF: Via Mausonia
Fault, SGF: San Giuliano Fault, SEF: Sant’Elia Fault); 2: minor normal fault (PDF: Piazza Duomo
Fault, TF: Tribunale Fault); 3: minor right lateral faults; 4: Quaternary deposit; 5: Meso-Cenozoic
bedrock; 6: contour lines of residual gravimetric anomalies in mGal (+ and - : positive and negative
anomaly, respectively); 7: trace of the geological sections reported in Fig. 9; 8: L’Aquila downtown
(AD).
Figure 11: Contour lines of the top of Meso-Cenozoic bedrock in m a.s.l.;1: main normal and
transtensive fault (PF: Pettino Fault, CF: Castello Fault, VMF: Via Mausonia Fault, SGF: San
Giuliano Fault, SEF: Sant’Elia Fault); 2: minor normal fault (PDF: Piazza Duomo Fault, TF:
Tribunale Fault); 3: Quaternary deposit; 4: Meso-Cenozoic bedrock; 5: contour lines of the top of
Meso-Cenozoic bedrock in m a.s.l.; 6: trace of the geological sections reported in Fig. 9; 7:
L’Aquila downtown (AD).
Figure 12: Synoptic sections summarising the geological evolution of ASB in five main steps.
Table captions
Table 1: Acquisition characteristics of the Corso section.
Table 2: Vp-lithology associations and their possible equivalent formations used for the
interpretation of the Corso section.
High-resolution of the Corso section
Length 960 m
Seismic Source IVI - MINIVIB
Geophone interval 5 m
Shot spacing 10 m
Number of shots 91
Receiver Spread size 192
Recording parameters
Sweep characteristics Linear, 3 x15 sec up-sweep from 10 to 200 Hz
Sampling rate 1 ms
Record Length 15 s
Geophone 10 Hz
Seismic facies Representation Vp range
(m/s) Lithology Geological
interpretation
BC
1800-2100
Sands and
conglomerates,
breccias
Fan deposits
and slope
breccias
Ls
1750-2200
Sands Channelized
deposits
L
1500-2000
Clay, silts and
sands Alluvial plain
deposits
R
2500-3200
Sands and
conglomerates,
breccias
Fan deposits
and slope
breccias
S
> 3500
Slope to basin
carbonate
sequences
(Meso-
Cenozoic
bedrock)
Meso-
Cenozoic
bedrock
