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37
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
DOI: 10.4408/IJEGE.2016-01.O-04
F
rancesca
BOZZANO
(*)
, L
uca
LENTI
(**)
, F
abrizio
MARRA
(***)
, s
aLvatore
MARTINO
(*)
,
a
ntoneLLa
PACIELLO
(****)
, G
abrieLe
SCARASCIA MUGNOZZA
(*)
& c
hiara
VARONE
(*)
(*)
Sapienza Università di Roma -
Dipartimento di Scienze della Terra - Piazzale Aldo Moro 5 - 00185 Roma, Italy
(**)
University Paris-Est LCPC/The French institute of science and technology for transport, development and networks (IFSTTAR)/Department GERS -
Champs sur Marne, France
(***)
Istituto Nazionale di Geofisica e Vulcanologia (INGV) - Via di Vigna Murata, 605 - 00143 Rome, Italy
(****)
Agenzia Nazionale per le Nuove Tecnologie, l’Energia e lo Sviluppo Economico Sostenibile (ENEA-C.R. Casaccia) -
Via Anguillarese, 301 - 00123 Rome, Italy
Corresponding author: chiara.varone@uniroma1.it
SEISMIC RESPONSE OF THE GEOLOGICALLY COMPLEX ALLUVIAL VALLEY AT
THE “EUROPARCO BUSINESS PARK” (ROME – ITALY) THROUGH INSTRUMENTAL
RECORDS AND NUMERICAL MODELLING
EXTENDED ABSTRACT
Questo studio ha come obiettivo l’analisi di risposta sismica locale della valle del Fosso di Vallerano, un bacino alluvionale situato
a sud del centro storico città di Roma in corrispondenza del quartiere EUR-Torrino. A supporto di questa analisi è stata condotta una
ricostruzione geologica ad alta risoluzione dell’area in esame, che interpreta la complessa combinazione di processi glacio-eustatici,
sedimentari, tettonici e vulcanici che hanno interessato l’area dell’attuale città di Roma. La successione stratigrafica risultante dalla
correlazione di 250 sondaggi geognostici comprende depositi derivanti da condizioni sedimentarie differenziate in relazione alla
evoluzione paleogeografica dell’area romana dal Pliocene fino all’attuale. In particolare, sono state individuate quattro unità geologiche:
• depositi sedimentari marini plio-pleistocenici (ascrivibili alle Formazione delle Marne Vaticane, di Monte Mario e di Monte delle
Piche) che rappresentano il substrato geologico dell’area;
• sedimenti alluvionali pleistocenici depositati dal Paleo Tevere 4 (ascrivibili alla Formazione di Santa Cecilia, 650-600 ka);
• depositi vulcanici eruttati dai Distretti Vulcanici dei Colli Albani e dei Monti Sabatini (561-360 ka);
• depositi alluvionali recenti che hanno riempito le incisioni vallive dalla fine della regressione Würmiana fino all’attuale (18 ka-
Presente).
Le suddette unità litologiche hanno reso possibile la calibrazione di una sismo-stratigrafia tramite modellazioni numeriche 1D che
si sono avvalse di: i) 55 misure di rumore (noise) sismico ambientale; ii) 10 registrazioni di weak motions ottenute tramite una rete
velocimetrica attiva nell’area nel 2009 e corrispondenti alla coda della sequenza sismica de L’Aquila-Gran Sasso, iii) un’indagine
cross-hole.
In base a tale calibrazione, il bedrock sismico è risultato localizzato al tetto delle ghiaie basali della Formazione di Santa Cecilia e
non corrisponde al substrato geologico dell’area (depositi sedimentari marini plio-pleistocenici). La sismo-stratigrafia così ottenuta è
stata estrapolata all’intera valle e sono state ottenute le funzioni di amplificazione 1D, assumendo che sia un comportamento elastico
lineare che uno non lineare per i depositi trattati. A tal fine, sono state simulate tre sezioni geologiche trasversali e caratteristiche
dell’assetto locale della valle in quanto contraddistinguono (spostandosi da SE verso NW): i) un sistema alluvionale caratterizzato da
due valli che corrisponde a due valli dal punto di vista topografico, ii) un sistema alluvionale caratterizzato da due valli che corrisponde
ad un’unica valle dal punto di vista topografico, iii) un sistema alluvionale caratterizzato da una singola valle corrispondente ad un’unica
valle dal punto di vista topografico.
I risultati della modellazione numerica mostrano come il Fosso di Vallerano sia caratterizzato da un primo modo di risonanza (a
circa 0.8 Hz) e da numerosi modi superiori a frequenze localmente dipendenti dell’assetto geologico del corpo alluvionale. Gli effetti
non lineari sono stati modellati applicando, come sollecitazione al modello 1D, i terremoti di riferimento (strong motion) previsti dalla
normativa regionale attualmente vigente (D.G.R. Lazio 387/09).
I risultati numerici mostrano una tendenziale riduzione sia della posizione in frequenza dei modi di risonanza (fino ad un massimo
di 0.5 Hz in meno su ogni valore di picco) che dell’ampiezza dell’amplificazione a frequenze maggiori di 7 Hz.
Considerando i rapporti di forma (sensu b
ard
& b
ouchon
, 1985) ed i valori di contrasto di impedenza tra i depositi alluvionali ed il
bedrock sismico del Fosso di Vallerano, nella valle è attesa una risposta di tipo 1D con generazione di onde laterali dai bordi (1D+lateral
waves
). Inoltre, è ragionevole assumere che la risposta sismica sia influenzata sia da effetti deformativi cosismici dovuti alla natura
eterogenea del corpo alluvionale (M
artino
et alii, 2015) che da effetti legati alla presenza dell’agglomerato urbano che interagisce con
il suolo (K
haM
et alii, 2006; s
eMbLat
et alii, 2009). Alla luce di queste considerazioni, saranno in futuro condotte simulazioni 2D al
fine di valutare il ruolo delle deformazioni cosismiche del terreno e del complesso urbano su esso edificato sulla risposta sismica locale.
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F. BOZZANO, L. LENTI, F. MARRA, S. MARTINO, A. PACIELLO, G. SCARASCIA MUGNOZZA & C. VARONE
38
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
ABSTRACT
The analysis of the local seismic response in the “Europarco
Business Park”, a recently urbanized district of Rome (Italy)
developed over the alluvial valley of the “Fosso di Vallerano”
stream, is here presented. A high-resolution geological model,
reconstructed over 250 borehole log-stratigraphies, shows
a complex and heterogeneous setting of both the local Plio-
Pleistocene substratum and the Holocene alluvia. The local
seismo-stratigraphy is derived by a calibration process performed
through 1D numerical modelling, accounting for: i) 55 noise
measurements, ii) 10 weak motion records obtained through a
temporary velocimetric array during the August 2009 L’Aquila-
Gran Sasso seismic sequence and iii) one cross-hole test available
from technical report. Based on the reconstructed seismo-
stratigraphy, the local seismic bedrock is placed at the top of a
gravel layer that is part of the Pleistocene deposits and it does
not correspond to the local geological bedrock represented by
Plio-Pleistocene marine deposits. 1D amplification functions
were derived via numerical modelling along three representative
sections that show how in the Fosso di Vallerano area two valleys
converge into a single one moving from SE toward NW. The
obtained results reveal a main resonance at low frequency (about
0.8 Hz) and several higher resonance modes, related to the local
geological setting. Nonlinear effects are also modelled by using
strong motion inputs from the official regional dataset and pointed
out a general down-shift (up to 0.5 Hz) of the principal modes of
resonance as well as an amplitude reduction of the amplification
function at frequencies higher than 7 Hz.
K
eywords
: local seismic response, engineering-geological
model, seismic measurements, numerical modelling, recently
urbanised area, Rome
INTRODUCTION
Moderate to severe damages were historically recorded in
Rome urban area (Italy) as a consequence of strong earthquakes
originated in two seismogenetic zones close to the city: the Alban
Hills volcanic area and the Central Apennines orogenic belt (Fig.
1) (M
oLin
et alii, 1986). This was the recent case of the seismic
sequence of L’Aquila (Central Apennine) that started with the 6
th
April 2009 Mw 6.3 main shock, about 150 km NE of Rome (Fig. 1).
Several studies have been focused so far on the local seismic
response in the Rome urban area (r
oveLLi
et alii, 1994, 1995;
b
ozzano
et alii, 2008 ; b
ozzano
et alii, 2012; c
aserta
et alii,
2012; M
artino
et alii, 2015). These studies pointed out that the
heterogeneous composition of the alluvial deposits of the Tiber
River and its tributaries is responsible for both 1D amplification
effects, mainly related to soil stratigraphy, and 2D amplification
effects that can be related to wave refraction at the edge of the
valley as well as the lateral heterogeneities (M
artino
et alii,
2015). The here considered case study is located in the Fosso
di Vallerano valley, about 10 km SE of the historical centre of
Rome that corresponds to a recently urbanized area which hosts
the “Europarco Business Park” with the highest building-towers
in the city (Fig. 2a).
The Fosso di Vallerano valley hosts two left tributaries of
the Tiber River (the Vallerano and the Cecchignola creeks); it
is characterized by a flat portion corresponding to flood plains
(10 m a.s.l.) bordered by hills (35-50 m a.s.l) (Fig. 2b, 2c). The
area shows a complex geomorphological evolution due to the
Würmian glacio-eustatic cycle that led to a series of successive
deviations and rearrangement of the river bed (a
scani
et alii,
2008). A high-resolution geological model was reconstructed
on the basis of several tens of borehole logs, available from site
investigations. Moreover, a temporary seismometric array was
installed during the summer 2009 to record the aftershocks of
the L’Aquila seismic sequence. A convergence process between
data from seismometric records and results of a 1D numerical
modelling was performed to provide the best fit in terms of
amplification functions. Such a process aimed at evaluating
the influence of the subsoil composition on the local seismic
response and to output the 1D seismic response along a cross
section passing from the “Europarco Business Park”. Due to the
scarcity of seismic records in the city of Rome, the here presented
Fig. 1 - Structural map of the Central Apennines including the area of
Rome. a) Main N-S faults and conjugated fault systems; b) bur-
ied faults linked to the extensional tectonic regime c) Seismoge-
netic faults; d) inactive thrust fronts; e) location of the 9
th
April
2009 L’Aquila earthquake (Mw 6.3) epicentre. The individual
seismogenetic sources (white bordered rectangles) and related
labels are also reported from the DISS 3.1.1. catalogue
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SEISMIC RESPONSE OF THE GEOLOGICALLY COMPLEX ALLUVIAL VALLEY AT THE “EUROPARCO BUSINESS PARK” (ROME – ITALY) THROUGH
INSTRUMENTAL RECORDS AND NUMERICAL MODELLING
39
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
experimental study can be regarded as a significant contribution
to the local seismic response. This study lays also the foundations
for future studies focusing on the “Site-City Interaction” (K
haM
et alii, 2006; s
eMbLat
et alii, 2009) to assess the influence of
urban agglomerates on the local seismic response. In this
perspective, the availability of geophysical data recorded before
the strong urban development of the “Europarco Business Park”,
can provide an important contribution to the evaluation of the
progressive influence of the buildings on the seismic response.
GEOLOGICAL SETTING
The geological bedrock of the Rome urban area is constituted
of clay to sand deposits ascribable to three main sedimentary
cycles corresponding to marine transgressions (M
arra
, 1993;
M
arra
& r
osa
, 1995) that follow the continentalization of the
area, during Middle-Upper Pleistocene, and were controlled by
several factors including tectonic (F
accenna
et alii, 1994a, b;
M
arra
, 1999, 2001; h
earty
& D
ai
P
ra
, 1986; K
arner
et alii,
2001a), eustatism and fluvial evolution (e.g.: K
arner
& M
arra
1998; M
arra
et alii, 2008). Moreover, part of the geological
bedrock outcropping in the Fosso di Vallerano area (Fig. 6) is
constituted by volcano deposits ascribable to the activity of the
Volcanic Districts surrounding the city (K
arner
et alii, 2001b;
G
iordano
et alii, 2006; M
arra
et alii, 2009; 2014; s
ottiLi
et
alii, 2010, M
arra
et alii, 2014). As it regards the sedimentary
deposition, a first cycle is associated to the Marne Vaticane
Formation (Pliocene-Early Pleistocene, M
arra
et alii, 1995)
which constitutes the geological bedrock of the Tiber alluvia
body. Nonetheless, previous studies referred to the historical
centre of Rome (b
ozzano
et alii, 2008; c
aserta
et alii, 2013)
demonstrated that it does not generally correspond to the seismic
bedrock, as it results at the top of the coarse grained deposits at
the basis of the most recent Tiber River alluvial deposits.
The second and third marine cycles correspond to the
deposition of the marine to continental sediments outcropping in
the north-western area of Rome (Monte Mario hill) and include
the Limi di Farneto unit, the Monte Mario, Monte delle Piche,
Monte Ciocci Formations (b
onadonna
, 1968; c
osentino
et alii,
2008; M
arra
, 1993; b
erGaMin
et alii, 2000).
More in particular, the Monte Ciocci Formation is ascribable
to paleo-river deposits (Paleo-Tiber 1 in Fig. 3) and it is followed
by three other Paleo-Tiber depositional units (Paleo-Tiber 2-4),
among which the Paleo-Tiber 4 only is recognizable in the
Fosso di Vallerano valley. These deposits are strictly related to
the glacial-eustatic sea oscillations (K
arner
& M
arra
, 1998;
M
arra
et alii, 1998; K
arner
et alii, 2001a; F
Lorindo
et alii,
2007; M
arra
et alii, 2008; M
arra
& F
Lorindo
, 2014; M
arra
et
alii, 2015) as shown by several studies carried out by integrating
stratigraphical, geochronological and paleomagnetical data. These
studies led to the identification of 10 aggradational successions
which correspond to as many glacial terminations, encompassing
Marine Isotopic Stage (MIS) 22/21 through 2/1 (Fig. 3).
These successions are generally fining-upward (K
arner
&
M
arra
, 1998), with coarse-grained gravel and sand, up to 10 m
in thickness, at the base of each section. The basal coarse-grained
deposits are followed by a relatively thin sand horizon, which
grades upward into a several meter thick pack of silt and clay. In
the older deposits, related to the Marine Isotopic Stages (MIS) 21
through 15, clays reached a moderate thickness (<10 m), probably
as a consequence of the smaller sea level oscillations associated
to these early glacial cycles (K
arner
et alii, 2001a). On the
contrary, a significant increase of clay thickness is observed in
the later successions, up to that of the modern Tiber River, which
reaches 70 m within the present-day coastal plain (M
arra
et alii,
2008, 2013).
AVAILABLE DATA
To construct a high resolution engineering-geological model
of the Fosso di Vallerano valley borehole log stratigraphies as well
as data from geophysical investigations were taken into account.
More in particular, 250 boreholes (Fig. 4) distributed over an
area of about 25 km
2
, one cross-hole test, log-stratigraphies
and expeditious geomechanical on-site investigations (Pocket-
Penetrometer and Pocket Vane-test) were available from technical
reports and official archieves of the study area (b
ozzano
et alii,
2000; v
entriGLia
, 2002). Moreover, specific seismometric records
were collected from 2009 until 2014 consisting in seismic noise
Fig. 2 - a) Photo view of the two “Europarco Business Park” towers
during their construction in 2012 from the Laurentina Vetus
archaeological site, located immediately SW of the Fosso di
Vallerano river valley (left) and actual view of the towers from
the terrace of Euroma2 shopping centre (right); b) satellite
views of the “Fosso di Vallerano” area from SE (left) and E
(right); c) panoramic photo-view of the Torrino hill
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F. BOZZANO, L. LENTI, F. MARRA, S. MARTINO, A. PACIELLO, G. SCARASCIA MUGNOZZA & C. VARONE
40
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
measurements. A velocimetric temporary array was also installed
in summer 2009 to record weak-motion events during the tail of
L’Aquila seismic sequence.
Borehole data
The aggradational successions deposited by the Paleo-Tiber
River and its tributaries in the area of Rome result in the filling of
a complex, laterally discontinuous network of paleo-river valleys
incised in the marine deposits. Each succession is substantially
characterized by a lateral homogeneity, with a coarse gravel
horizon at its base, grading upward into sand, silt and clay layers.
However, this upper, finer portion of the sedimentary pack is
characterized by diffused lateral variation of the lithological
features, due to the fluvial sedimentary conditions that cause the
juxtaposition of lenticular bodies of sand, clay and peat.
A high-resolution geological model was requested to
represent such a complex geological framework.
At the time of this study, no more reliable geophysical
investigations or boreholes could be performed except for single-
station noise measurements because of the intense urbanisation of
the area and consequent disturbance.
Seismometric measurements
From 2009 until 2014 five ambient noise surveys were carried
out in the Fosso di Vallerano valley, using three different triaxial
velocimetric stations, for a total of 56 measurements. The first
survey was performed by a 4Hz digital tromometer TROMINO
(Micromed) set to a 128 Hz sampling frequency that acquired
noise samples 20 minutes long in different hours of the day.
Since 2012 four further surveys were realized by a 1.4 Hz SL06
acquisition system (SARA Instruments) set to a 200 Hz sampling
frequency and a LENNARTZ LE3D/5s sensor coupled with
a REFTEK 130 digitizer set to a 250 Hz sampling frequency.
Noise samples from 45 minutes to 2 hours long were acquired
in different hours of the day; given the short distance among the
measure sites (less than 500 m), repeated recordings in the same
site were performed only in case of ambiguous data or results
inconsistent with the one obtained in the neighbouring sites.
The records, sampled with a 40 s moving time window, were
de-trended, tapered, converted to the frequency domain and
smoothed by a Konno-Ohmachi function (b=40) to get average
HVSR (Horizontal to Vertical Spectral Ratio) according to
n
aKaMura
(1989). Especially in 1D condition, HVSR peaks of
significant level (>2 according to SESAME, 2004) can point out
frequencies which are amplified by the local geological condition.
The ambient noise analysis (Fig. 5) shows a homogeneous
Fig. 3 - Left column: δ
18
O‰ vs. time for the depositional units by the Paleo-Tiber River and its tributaries. The Marine Isotopic Stages (MIS) are also
reported (Arabic numbers within the graph); the stars highlight the deposits of the Fosso di Vallerano area. Right column: sketch illustrating the
Fosso di Vallerano valley evolution since 900 ka to present (colours correspond to the graph of the left column).
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SEISMIC RESPONSE OF THE GEOLOGICALLY COMPLEX ALLUVIAL VALLEY AT THE “EUROPARCO BUSINESS PARK” (ROME – ITALY) THROUGH
INSTRUMENTAL RECORDS AND NUMERICAL MODELLING
41
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Fig. 5 - CTR (Regional technical Map) scale 10.000 – Black points indicate the location of the ambient noise recording station (the fundamental reso-
nance frequency is also reported); black triangles indicate the velocimetric array; the black square corresponds to the position of the calibration
Soil Column A. The traces of the three geological cross-sections AA’, BB’ and CC’ are also reported; the outcropping seismic bedrock corresponds
to the screened areas
Fig. 4 - CTR (Regional technical Map) scale 10.000 - Location of the boreholes considered for the reconstruction of the high-resolution geological setting.
Contour (in colours) of the bedrock depth obtained through interpolation of the available borehole data.
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F. BOZZANO, L. LENTI, F. MARRA, S. MARTINO, A. PACIELLO, G. SCARASCIA MUGNOZZA & C. VARONE
42
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
response of the valley with a fundamental resonance frequency of
0.8±0.1 Hz, instead, the surrounding reliefs show no significant
resonance peaks of the HVSR functions.
From June until July 2009 a free-field seismometric array
operated in STA/LTA (Short Time Average to Long Time
Average) acquisition mode in the Fosso di Vallerano valley, in
order to record weak-motion events during the tail of L’Aquila
seismic sequence and therefore compare such results to the ones
obtained from noise analysis. The array (Fig. 5) was composed
of two stations (Fig. 6), whose location was selected taking
into account both the noise survey results and the requirement
of identifying low noise, free-field spots in an urbanized area.
Each station was instrumented by three single components, 1 Hz
velocimeters (SS1 Kinemetrics) triaxially arranged, connected
to a 24 bit data logger (K2 Kinemetrics) and a GPS device for
absolute timing. One station (V) was located on the alluvial
deposits, in the NE sector of the valley, while a reference (R)
Fig. 6 - a) Photo view of the outcropping seismic bedrock composed
of volcanic deposits; b) seismometric array: V station (left), R
station (right)
Fig. 7 - Comparison between the three-component accelerograms recorded in R (left) and V (right) stations for the earthquake EQ-12 of Tab. 1
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SEISMIC RESPONSE OF THE GEOLOGICALLY COMPLEX ALLUVIAL VALLEY AT THE “EUROPARCO BUSINESS PARK” (ROME – ITALY) THROUGH
INSTRUMENTAL RECORDS AND NUMERICAL MODELLING
43
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
station (sensu b
orcherdt
, 1994) was placed on the local seismic
bedrock, corresponding to the volcanic hills that border the
valley (i.e. where no evidence of amplification was pointed out
by noise analysis). It’s worth noticing that this seismic bedrock
does not coincide with the one existing below the alluvial
deposits. The seismometric array recorded overall about 30
earthquakes (an example is shown in Fig. 7) in the magnitude
range 2.6-4.6, from Database of Individual Seismogenic
Sources (DISS 3.1., Working Group 2010 - ITCS028, ITCS013,
ITDS073, ITDS072).
In the present study 10 records only (Tab. 1) were considered
as they do not show disturbances due to human activities. A 5%
cosine-taper window starting 1 s before the P-phase onset was
applied to the earthquake records to obtain signals with duration
of 90 s; the signals were pass-band filtered in the frequency
range 0,2- 20 Hz and converted to the frequency domain. The
smoothed spectra were used to achieve both the average Receiver
Functions (RF) (L
erMo
et alii, 1993) and the average Standard
Spectral Ratio (SSR) at site, computed from the spectral ratios of
the horizontal components recorded for each event in the alluvial
valley to the equivalent components recorded at the reference site
(b
orcherdt
, 1994). The obtained results proved the high quality
of the reference site, since no significant amplification effect was
pointed out by RFs; on the contrary, both the RF and the SSR
function derived for V station show a well-defined peak at 0.8 Hz,
in agreement with noise analysis results.
METHODS
Geological model
Based on the boreholes data, 5 main litho-stratigraphic groups
were distinguished:
• Plio-Pleistocene Marine sediments (Marne Vaticane, Monte
Mario e Monte Ciocci/dellePiche Formations) that represent
the geological bedrock of the area (PP);
• Pleistocene alluvial sediments deposited by the Paleo-Tiber 4
River (Santa Cecilia Formation; MIS 15; 650-600 ka; Marra
and Florindo 2014) (PT);
• Volcanic deposits erupted from the Alban Hills and the Monti
Sabatini Volcanic District (561-365 ka; K
arner
et alii, 2001b)
(VL);
• Pre – würmian fluvio-palustrine deposits (Valle Giulia - San
Paolo – Aurelia and Vitinia Formation; 500-200 ka; K
arner
& M
arra
, 1998) (FP);
• Recent alluvial deposits filling the valley incisions since the
end of the Würmian regression to the present (MIS 1;18ka-
Present; M
arra
et alii, 2013) (AL);
Seven geological sections (three of them reported in Fig.
8) were realized by cross-correlating 250 boreholes and were
taken into account for reconstructing a 3D geological model
of the Fosso di Vallerano area. The main outcome from such a
3D model consists in the evidence of a highly heterogeneous
alluvial filling that is characterized by both vertical and lateral
lithotechnical contacts (Fig. 8-9). This complexity can be ascribed
to the combined effect of both a major Middle Pleistocene sin-
sedimentary tectonic and the intervening erosive processes.
Based on the reconstructed 3D geological model (Fig. 9),
the geological bedrock of the most recent alluvial deposits of
the post-würmian aggradational cycle (AL) is represented by
the fluvial-lacustrine deposits of the Paleo-Tiber 4 unit (PT-
Santa Cecilia Formation; MIS 15), unlike the main Tiber Valley
(b
ozzano
et alii, 2008) and other tributary valleys (c
aserta
et
alii, 2012) where it is represented by the consolidated clays of the
Plio-Pleistocene Monte Vaticano Formation.
More in particular, intense erosive processes occurred during
the last würmian glacial period due to the similar combined effect
of both the regional uplift (h
earty
& d
ai
P
ra
, 1986; K
arner
et alii, 2001a) and the eustatic low-stand; the erosive processes
originated deep fluvial incisions that were filled, during the post-
würmian eustatic rise, by the recent alluvial deposits. These
recent sediments (AL) are characterized by remarkable vertical
and lateral heterogeneities, originated by the coupled processes
of alluvation and colluvation, as well as by possible, secondary
sin-sedimentary tectonic activity (Fig. 8).
A more detailed stratigraphic analysis highlighted the
presence of peaty clay and peat deposits that fill most of the
valley and reach a thickness of 45 m.
Based on the aforementioned 3D model and considering the
vertical stratigraphy as a time depositional sequence, more recent
from the bottom to the top, the peaty and peaty-clay deposits
indicate that, during the Holocene, the alluvial valley was mostly
characterised by a low energy hydrographic regime, characterized
by the presence of stagnant water, causing the emplacement of
abundant organic matter. At the same time, the north western
part of the valley was characterized by low-energy environment
responsible for the deposition of clays without organic matter,
so suggesting a more straightforward lacustrine conditions, that
hindered the formation of peat. Active subsidence in this sector,
linked to weak tectonics that reactivated the faults dislocating
the Paleo-Tiber 4 deposits, may explain the deposition of the
lacustrine clay as well as the capture of the river bed north of the
Montorio Hill, as highlighted by a
scani
et alii (2008).
The more recent clayey-sandy deposits are characterized by
a marked volcanic component originated by erosive processes
that involved the outcropping volcanic deposits. Fans composed
by Paleo-Tiber 4 basal deposits, due to slope denudation
processes involving the flanks of the valley are interlayered by
lateral unconformities with the clays and peaty-clays. The so
resulting alluvial fill is characterised by a significant lithological
heterogeneity due to the presence of lens- to the disc-like
depositional bodies.
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Fig. 8 - Geological cross sections along the AA’, BB’, CC’ traces of Fig. 5 Legend: Recent alluvial deposits (18 ka-present): 1) Anthropic filling material
(AL-AF); 2) Sandy-Clays characterized by a marked volcanic component (AL-VSC); 3) Peaty clays, plastic (AL-PC); 4) Clays and silts, plastic
(AL-CS); 5) Peat (AL-PT); 6) Sands and silty sands, sometimes containing gravel polygenic (AL-SD); 7) Polygenic, loose and heterometric
gravels, with volcanic and sedimentary components (AL-GR). Volcanic deposits from the District of Alban Hills and the M. Sabatini 561-360 ka
( Marra et al.,2009): 8) Undifferentiated pyroclastic material (VC). Pre-wurmian deposits (Valle Giulia – San Paolo – Aurelia – Vitinia Forma-
tions): 9) Fluvio-palustrine deposits composed of loose gravels, sands and silts. Paleo-Tiber 4 deposits (Santa Cecilia Formation) 650-600 ka
(Karner&Marra 1998 - Florindo et al., 2007): 10) Sandy clays and silts, sometimes with freshwater gastropods (PT-SC) 11). Clays and silts
with peaty layers (PT-CL); 12) Sands and silty sands. (PT-SD); 13) Loose gravels with heterometric sedimentary components (PT-GR). Plio-
Pleistocenic bedrock (Marne Vaticane - Monte Mario - Monte delle Piche Formation) (Marra, 1993 - Marra et al., 1995): 14) Marine clays and
silty clays; 15) Marine sands and silty sands. 16) Fault. 17) Borehole
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Sensitivity numerical analysis
To attribute dynamic properties to the subsoils of the Fosso
di Vallerano valley, in order to obtain an engineering-geological
model for local seismic response analysis, punctual data were
considered to derive a seismostratigraphy that was extrapolated
to the area by taking into account the stratigraphic setting that
resulted by the reconstructed geological model.
At this aim, the seismometric records were used to calibrate
the local seismo-stratigraphy based on the reconstructed high-
resolution geological model. Such a calibration was carried out
by direct comparison between the instrumental records and the
outputs from 1D numerical modelling performed through EERA
(Equivalent - linear Earthquake Response Analysis, b
ardet
et alii, 2000) code. The study focused on the analysis of a soil
column obtained in correspondence to the velocimetric station V
(Fig. 10b), located in the Fosso di Vallerano valley.
Three earthquakes, representative of the three seismogenic
areas (including several seismogenic sources according to DISS
3.1. see #EQ-6; EQ-12; EQ-13 in Tab. 1), were selected to be used
as seismic inputs for the numerical modelling. The velocity time
histories were decimated to a 50 Hz sample frequency, corrected
for the instrument response, filtered in the range 0.4-15 Hz and
Fig. 9 - 2D planar restitution of the high resolution 3D geological model of the Fosso di Vallerano valley referred to different depths a.s.l.. See Tab. 2 for
key to legend
Tab. 1 - Recorded seismic events (INGV Source)
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derived to get accelerations.
The obtained acceleration time histories were used to calibrate
the numerical model; each recorded event at the R station was
deconvoluted at the column base and applied as input (incrop)
obtaining, at the surface (outcrop), the signal modified by the
soil column. The spectral ratio between the outcrop signal at the
column surface and the input signal, deconvoluted at the outcrop
once again, represents the seismic local amplification function,
A(f), of the ground column (b
orcherdt
, 1994). This function is
comparable with the SSR function obtained for the same earthquake
by considering the records at the R and at the V station.
Mechanical and dynamic properties were attributed to each
lithotechnical unit according to literature data (b
ozzano
et alii,
2008; c
aserta
et alii, 2012) (Tab. 2).
The calibration procedure was rigorously performed for the
stratigraphic column A (Fig. 10b) as it is representative for the
geological setting of the V station.
Each numerical modelling was carried out by separately
applying the horizontal components (NS and WE) of each
recorded earthquake. An average function with its standard
deviation was then computed to be compared with the average
SSR that represents our experimental A(f).
The here adopted calibration procedure was performed
through a sensitivity analysis, by assuming the stratigraphy
Fig. 10 - Vs profiles considered in the trial and error calibration process. a) T1 b) T2 c) T2b d) T3. See Tab. 2 for key to legend
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average SSR function (2-6 Hz) as well as the peaks in the 7-10 Hz
frequency range are no more visible.
The shape of modelled A(f) in the calibration test T2 was not
yet in a suitable agreement with the experimental one in terms of
both frequency value and amplitude of the first resonance mode
(about 1 Hz) as well as of the higher resonance modes.
Therefore, an additional calibration test (T3) was carried out
by varying the impedance contrasts between each layer from 1.5
up to 3.The best results (Fig. 11d) were obtained considering the
V(s) values shown in Fig. 10d (T3), and assuming impedance
contrasts of 1.8 - 1.4 - 2.5 - 2.9 respectively.
The performed sensitivity analysis allowed to best tune the Vs
profile along the soil column (Tab 2 - Fig. 9); the final resulting
seismo-stratigraphy is in good agreement with other literature
data (b
ozzano
et alii, 2008; c
aserta
et alii, 2012) but points out
a significantly different value for the AL-VSC deposits of the
Fosso di Vallerano valley compared to similar alluvial deposits
by the Tiber River and its tributaries.
Moreover, it is worth noticing that in the Fosso di Vallerano
valley the seismic bedrock is located at the top of the Paleo-Tiber
4 gravel (Pleistocenic) and it does not correspond not to the local
geological bedrock (i.e., the Holocene/Pleistocene discontinuity
contact) neither to the outcropping seismic bedrock, which
consists of volcanic deposits. This represents a relevant difference
if compared to the main Tiber River valley, which hosts the largest
part of the Rome historical centre, where the seismic bedrock is
coincident with the top of the gravel level at the basis of the most
recent alluvial deposits (b
ozzano
et alii, 2008).
The so calibrated seismo-stratigraphy was extrapolated to
the Fosso di Vallerano valley by: i) considering geometry and
juxtaposition of stratigrafic levels as resulting from the high-
resolution geological model; ii) verifying the correspondence
between geological units and the seismic strata; iii) attributing to
derived from the high resolution 3D geological reconstruction
and by varying the shear wave velocity (Vs) values along the
calibration soil column A. A “trial and error” convergence was
performed to best fit the experimental A(f) with the numerical
one. At this aim, the Vs values obtained by extrapolating literature
data (b
ozzano
et alii, 2008; c
aserta
et alii, 2012) as well as
the results of the local cross-hole test were assumed as starting
conditions (Fig. 10a T1) while the thickness of layers and their
juxtaposition were fixed according to the available geological
constrains. A first calibration test (T1) was performed assuming
that the Paleo-Tiber 4 deposits (Santa Cecilia Formation-PT)
could represent the seismic bedrock of the area (Fig. 10a T1).
The obtained A(f) does not fit the experimental one (Fig. 11a) so
demonstrating that, in agreement with the results by c
aserta
et
alii (2012) for the Grottaperfetta alluvial valley in Rome, such a
hypothesis is not suitable for the area.
A second calibration test (T2) was performed, by considering a
Vs value of 350 m/s within the volcaniclastic sandy – clay deposits
(AL-VSC) and a linear increase of the Vs (5 m/s for meter) within
the clay-silty layer of the Paleo-Tiber 4 deposit (PT-CL) from
an initial value of 488 m/s; a velocity contrast of about 500 m/s
with respect to the gravel deposits of the Santa Cecilia Formation
(PT-GR), taken to be the local seismic bedrock, was moreover
set (Fig. 10b T2). The shape of the resulting A(f) (Fig. 11b) is in
good agreement with the experimental one. The significance of
the velocity inversion within the soil columns, i.e. between the
sandy-clays volcaniclastic (AL-VSC) layer and the peaty-clay
layer (AL-PC), was evaluated by assuming in a third test (T2a) that
the aforementioned velocity inversion does not exist, i.e. assuming
the same Vs value (118 m/s) for both the considered lithotechnical
units (Fig. 10c T2a). As it results from the modelling, the A(f) shape
significantly changes and does not fit anymore the experimental
one (Fig. 11c); indeed, the characteristic “trough shape” of the
Tab. 2 - Initial values of the dynamic properties attributed to the litotechnical units and best fit values obtained by the sensitivity analysis performed as-
suming the T3 Vs profile
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Fig. 11 - Comparison between results of the numerical modelling and experimental data for the T1 (a) T2(b) T2a (c) and T3 (d) Vs profiles.
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the seismic strata the dynamic properties used for the calibration
model. A coincidence was found between the Holocene alluvial
units and the seismic strata as they represent visco-elastic deposits,
up to 75 m thick, with Vs varying in the range 118-713 m/s. The
Pleistocene deposits were distinguished in a upper seismic stratum,
from 5 up to 50 m thick, including the geological units PT-CL and
PT-SD, with a visco-elastic behaviour and Vs ranging from 357
m/s up to 607 m/s and in a lower seismic stratum corresponding
to the PT-GR that represents the local seismic bedrock with a
Vs of 1100 m/s. The resulting engineering-geological model is
summarised in the synoptic diagram of Fig. 12.
Numerical modelling along cross sections
The seismo-stratigraphy calibrated at V station was
extrapolated to three geological sections crossing the Fosso
di Vallerano valley (Fig. 8 - AA’- BB’- CC’ cross sections).
At this aim, the three sections were discretized by 50 - 56 - 64
Fig. 12 - Engineering geological model, in terms of rheology and Vs ve-
locity, assumed for the subsoil of the Fosso di Vallerano valley.
The velocity value corresponding to the volumetric threshold is
also indicated
Fig. 13 - Output of the numerical modelling in terms of A(f)x obtained along the cross section BB’ of Fig. 8. a) Linear condition, LC; b) Equivalent-linear
condition (weak motions), ELC1; c) Equivalent-linear condition (strong motions), ELC2. White to green colours indicate the A(f) intensity; d)
Modelled geological cross section. The fundamental resonance frequencies derived by the ambient noise measurements is also reported
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RESULTS
The A(f)x function was obtained along the geological cross
sections by interpolating through a Kriging regression the A(f)
values computed for each soil column. For each section, the
results are summarized in contour maps, corresponding to the
different assumed conditions (Fig. 13-14-15).
The comparison between the results obtained under LC (Fig.
13a) and ELC1 (Fig. 13b) conditions demonstrates that the A(f)x
functions are coincident and therefore the recorded weak motions
are not affected by singularities in their frequency content, that
can alter the seismic response if compared to the theoretical one
(i.e. under LC).
This suggests that, in future studies aiming at the evaluation
of the “Site-City Interaction”, the recorded weak motions could
be used for the 2D modelling in the Fosso di Vallerano valley.
Based on these considerations, the results of the modelling of
soil columns respectively, each one having an average lateral
representativeness of 20 m, in order to obtain through EERA the
spatial distribution of the amplification function (A(f)x) under
free field conditions.
The modelling was performed by assuming three different
conditions:
• Linear condition (LC - viscoelastic rheology; non iterative
approach)
• Equivalent-linear condition 1 (ELC1 - viscoelastic rheology;
iterative approach performed by applying the recorded weak
motions (#EQ-6; EQ-12; EQ-13 in Tab. 1) as seismic inputs)
• Equivalent-linear condition 2 (ELC2 - viscoelastic rheology;
iterative approach performed by using the strong motions
required by the current Italian Regional regulations for the
regulations for the “3rd level of Seismic Microzonation” as
seismic inputs).
Fig. 14 - Output of the numerical modelling in terms of A(f)x obtained along the cross section AA’ of Fig. 8. a) Equivalent-linear condition (weak motions)
ELC1; b) Equivalent-linear condition (strong motions), ELC2. White to green colours indicate the A(f) intensity; c) Modelled geological cross
section. The fundamental resonance frequencies derived by the ambient noise measurements is also reported
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amplifications functions characterizing the eastern portion of the
valley, are characterized by multiple upper modes (amplification
intensity from 3 up to 7) due to the presence of paleo-slope debris
(Vs= 550 m/s) within the upper part of the PT deposits. Where the
alluvia are composed by silty-clay deposits (AL-CL Vs=235 m/s)
the A(f)x shows upper modes characterized by amplification levels
up to 5; on the other hand, where the alluvia are characterized by
lower Vs values (AL-PC Vs=150 m/s; AL-PT Vs=140 m/s), the
upper modes are negligible in the A(f)x function as the computed
intensity results lower than 3. It is worth noting that, when the
upper modes of resonance are not relevant, the amplitude of the
first mode significantly increases.
The results obtained for the cross-section AA’ show a different
amplification level of the upper modes linked to the different
geological setting of the valley:
i) in the eastern part of the valley, mainly composed by AL-
the three cross-sections are discussed for the ELC1 and ELC2
conditions only.
The results obtained by assuming ELC1 (Fig. 13b -15a - 15a)
show a fundamental resonance mode of 1 ± 0.2 Hz within most
of the valley; these values, that are in good agreement with the
HVRS results, are due to the stratigraphic position of the PT-GR,
i.e. the local seismic bedrock and the thickness of the resonant
body (35-65m). However some particular geological-structural
conditions are present in which the first resonance mode increases
to higher frequency values (2.0-4.5 Hz), this is the case of the
middle part of the cross section BB’ (Fig. 14a) where the thickness
of the alluvia significantly decreases (down to 15-20 m).
On the contrary, the secondary resonant modes are influenced
by specific seismo-stratigraphic settings of each cross-section, i.e.
of each modelled soil column.
As it results by analysing the AA’ cross-section, the
Fig. 15 - Output of the numerical modelling in terms of A(f)x obtained along the cross section CC’ of Fig. 8. a) Equivalent-linear condition (weak motions)
ELC1; b) Equivalent-linear condition (strong motions) ELC2. White to green colours indicate the A(f) intensity; c) Modelled geological cross
section. The fundamental resonance frequencies derived by the ambient noise measurements is also reported.
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model and, in particular on the location of the geological bedrock
respect to the alluvial deposits, the Fosso di Vallerano test site
is typified by a complex “valley system” (properly considered
as soft soils within a bedrock container) as it changes from SE
toward NW in: a) a double-valley system that coincides with
a double-valley topography; b) a double-valley system that
corresponds to a single-valley topography; c) a single-valley
system that corresponds to a single-valley topography. Such a
peculiar condition is strictly related to the Holocene depositional
system of the Vallerano River which buried the more ancient
alluvial deposits and part of the geological bedrock, specifically
represented by the Pleistocene Paleo-Tiber 4.
By considering the shape ratio (sensu b
ard
& b
ouchon
1985) of the Fosso di Vallerano valley along the three sections
AA’ (0.18) BB’ (Eastern valley: 0.87 Western valley: 0.20)
CC’ (Eastern valley:0.44 Western valley: 0.19 ) as well as the
impedance contrast resulting from the Pleistocene silty-clayey
deposits and the local seismic bedrock, i.e. the top of the basal
Paleo-Tiber 4 Pleistocene gravels (AA’= 2.8; BB’= eastern
valley: 2.7 western valley: 2.8; CC’= eastern valley: 2.8 western
valley: 2.6), a 1D-plus-lateral-wave local seismic response should
be expected according to b
ard
& b
ouchon
(1985). Nonetheless,
it is reasonable to assume that: i) the local seismic response as
well as the stress-strain effects due to the heterogeneous alluvial
deposits are significantly conditioned by lateral contacts related
to the lithological hetereogeneities (M
artino
et alii, 2015), ii) the
building agglomerate should be regarded as a physical system
interacting with the subsoil (K
haM
et alii, 2006; s
eMbLat
et alii,
2009) and leading to a “urban-field” more than to a “free-field”
local condition. In this regards, future analyses will be focused
on the 2D numerical modelling of the seismic response and the
induced stress-strain effect by assuming both free field conditions
and Site-City Interaction.
CONCLUSIONS
The high-resolution geological model derived for the Fosso di
Vallerano valley revealed a very complex setting for both the alluvia
and the local geological substratum. More in particular, erosive and
slope processes, placed in a context characterized by geodynamic
activities, led to complex stratigraphic relationships among the
deposits that filled the valley from the Pleistocene to the Present.
A local seismo-stratigraphy was calibrated based on earthquake
records collected during the 2009 L’Aquila seismic sequence as
well as noise measurements; such a calibration allowed to model
a 1D seismic response by obtaining amplification functions all
along three selected geological cross sections and put in evidence
the peculiar geological setting of the valley that changes from
a double- to a single-valley system moving from South-East to
North-West. As it resulted from the here performed calibration
process at the soil column A, representative of the V seismic
CL (Vs=235 m/s), the upper modes are characterized by
amplification values ranging from 4 to 7 (Fig. 13b);
ii) in the western part of the valley, characterized by deposits
with lower Vs values (Al-PC Vs= 150), the upper modes show
amplification values < 3. As already observed in the AA’ cross
section, the portion characterized by higher Vs values shows
upper modes characterized by higher amplification level.
Very similar results were also obtained for the cross-section
CC’ (Fig.14).
The comparison between the results obtained by assuming
ELC1(Fig. 13b-14a-15a) and ELC2 (Fig. 13c-14b-15b) shows
that, if non-linear conditions are considered, all the resonance
frequencies visible in the A(f)x functions are characterized by a
not negligible shift (i.e. up to a maximum of 0.5 Hz) towards
lower frequency values, while the amplitude of the A(f)x
functions remains almost constant or decreases in relation to the
presence of softer soil (AL-PC , AL-PT).
DISCUSSION
The numerical modelling performed along three geological
cross sections in the Fosso di Vallerano valley shows that: i) the
first resonance mode generally varies in a close range between
0.8-1.2 Hz and is related to an average soft-soil thickness of about
60 m ascribable to the whole Holocene alluvia and partly to the
Pleistocene deposits; ii) the first resonance mode corresponds
to higher frequency values (2.0-4.5 Hz) where the thickness of
the soft-soils significantly decrease (15-20 m) due to the local
geological setting, i.e. to the horst-type structure of the Plio-
Pleistocene bedrock; iii) the secondary modes become negligible
on behalf of the principal resonance mode where the soil columns
are characterized by lower V(s) values, i.e. high presence of
peaty clays deposits; iv) the nonlinearity due to strong motions
produces a down-shift of the principal modes of resonance as
well as an amplitude reduction of the A(f)x in the portion of the
alluvial body characterized by high concentration of peaty clays
deposits. Nevertheless, the here obtained A(f)x functions only
express the effects of 1D stratigraphic setting, i.e. thickness and
layering of the alluvial deposits; on the contrary they do not take
into account either the lateral heterogeneity of the alluvial filling
or the shape of the valley. Moreover, based on the numerical
model that best fits the empirical A(f)x function for the calibration
soil column, it is worth noticing that in the Fosso di Vallerano
valley the seismic bedrock is located at the top of the Paleo-
Tiber 4 gravel (Pleistocenic) and it does not correspond to the
geological bedrock (i.e. the Holocene/Pleistocene discontinuity
contact) neither to the outcropping one (i.e. volcanic deposits).
This result represents a relevant difference respect to the main
Tiber River valley where the seismic bedrock is coincident with
the Holocene gravel of the G level, according to b
ozzano
et alii
(2008). Based on the high-resolution engineering geological
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layer of Holocene alluvia, a second homogeneous soft-layer of
Pleistocene paleo-river deposits and a seismic bedrock which
does not coincide to the geological one (represented by the Plio-
Pleistocene marine clays) as it corresponds to fluvial gravels, part
of the Pleistocene deposits.
ACKNOWLEDGEMENTS
This research was funded by “Sapienza” University of Rome
in the frame of the project “Analisi di risposta sismica locale in aree
edificate, analisi dell’interazione dell’edificato con il sottosuolo;
valutazione degli effetti del non sincronismo dell’azione sismica
sulle costruzioni; livelli di conoscenza e fattori di confidenza
nella valutazione delle costruzioni esistenti” (Anno 2012 – prot.
C26A12EHRT, P.I Prof. G. Scarascia Mugnozza). The authors
wish to thank A. Grillo and S. Hailemikael for their support
to the geophysical field surveys; José A. Peláez for his useful
suggestions to the manuscript improvement; EUROPARCO
Society for the boreholes log stratigraphies provided.
station: i) the main resonance of the valley is observed in a very
narrow frequency range, between 0.8 and 1.2 Hz; ii) where the
alluvial body is characterized mainly by peaty clays deposits,
i.e. V(s) value about 150 m/s, the secondary modes become
negligible on behalf of the principal resonance mode; iii) the local
seismic bedrock does not correspond to the geological one as it is
represented by a gravel level within the Pleistocene Paleo-Tiber
4 deposits, i.e. it does not corresponds to the bottom of the most
recent alluvia; iv) nonlinear effects are clearly visible by applying
strong motion inputs to derive the 1D A(f)x functions as they
produce a down-shift of the principal modes of resonance as well
as an amplitude reduction of the A(f)x in the portion of the alluvial
body characterized by high concentration of peaty clays deposits.
The resulting engineering-geological model, for local
seismic response analysis, is significantly different respect to
the one obtained by b
ozzano
et alii (2008) and c
aserta
et alii,
(2013) for the historical centre of Rome as it consists in a three-
layer seismo-stratigraphy including a first heterogeneous soft-
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SEISMIC RESPONSE OF THE GEOLOGICALLY COMPLEX ALLUVIAL VALLEY AT THE “EUROPARCO BUSINESS PARK” (ROME – ITALY) THROUGH
INSTRUMENTAL RECORDS AND NUMERICAL MODELLING
55
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Received January 2016 - Accepted April 2016
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