The 1783 Scilla Case-History (Southern Italy)

Missouri University of Science and Technology
Scholars' Mine
International Conferences on Recent Advances in
Geotechnical Earthquake Engineering and Soil
Dynamics
2010 - Fifth International Conference on Recent
Advances in Geotechnical Earthquake Engineering
and Soil Dynamics
May 24th - May 29th
Numerical Modelling of Earthquake-Induced Rock
Landslides: The 1783 Scilla Case-History
(Southern Italy)
Francesca Bozzano
University of Rome “Sapienza”, Italy
Salvatore Martino
University of Rome “Sapienza”, Italy
Eliana Esposito
Istituto per l’Ambiente Marino e Costiero (CNR-IAMC), Italy
Alfredo Montagna
University of Rome “Sapienza”, Italy
Luca Lenti
University Paris East, Laboratoire Central des Ponte set Chaussées (LCPC), France
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Recommended Citation
Francesca Bozzano, Salvatore Martino, Eliana Esposito, Alfredo Montagna, Luca Lenti, Antonella Paciello, and Sabina Porfido,
"Numerical Modelling of Earthquake-Induced Rock Landslides: The 1783 Scilla Case-History (Southern Italy)" (May 24, 2010).
International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. Paper 25.
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Author
Francesca Bozzano, Salvatore Martino, Eliana Esposito, Alfredo Montagna, Luca Lenti, Antonella Paciello,
and Sabina Porfido
This article - conference proceedings is available at Scholars' Mine: http://scholarsmine.mst.edu/icrageesd/05icrageesd/session04b/
25
NUMERICAL MODELLING OF EARTHQUAKE-INDUCED ROCK LANDSLIDES:
THE 1783 SCILLA CASE-HISTORY (SOUTHERN ITALY)
Francesca Bozzano, Associate Professor,
Research Centre for Geological Risks (CE.RI.) – University of
Rome “Sapienza”, P.le A. Moro 5, 00185 Roma (Italy),
[email protected], tel:+390649914924
Salvatore Martino, Researcher,
Research Centre for Geological Risks (CE.RI.) – University of
Rome “Sapienza”, P.le A. Moro 5, 00185 Roma (Italy),
[email protected], tel:+390649914923
Eliana Esposito, Researcher,
Consiglio Nazionale delle Ricerche – Istituto per l’Ambiente
Marino e Costiero (CNR-IAMC), Calata Porta di Massa 80133 Napoli (Italy), [email protected]
Alfredo Montagna, PhD student,
Department of Earth Sciences – University of Rome
“Sapienza”, P.le A. Moro 5, 00185 Roma (Italy),
[email protected]
Luca Lenti, Researcher,
University Paris East, Laboratoire Central des Ponte set
Chaussées (LCPC), 58 Boulevard Lefebvre 75732 Cedex 15
Paris (France), [email protected]
Antonella Paciello, Researcher,
Ente per le Nuove Tecnologie l’Energia e l’Ambiente
(ENEA) – Via Anguillarese 301, 00123 Roma (Italy),
[email protected]
Sabina Porfido, Researcher,
Consiglio Nazionale delle Ricerche – Istituto per l’Ambiente
Marino e Costiero (CNR-IAMC), Calata Porta di Massa 80133 Napoli (Italy), [email protected]
ABSTRACT
The coastal M. Pacì rock-avalanche was triggered by the 6th February 1783 earthquake near the village of Scilla (southern Calabria)
and involved a subaerial volume of about 5·106 m3. This landslide produced a tsunami wave responsible for more than 1500 human
life losses near Marina Grande beach. A geological survey and a geomechanical characterization of both the intact rock and the
jointed rock mass outcropping in the landslide slope were carried out to obtain an engineering-geology model of the landslide
according to an equivalent-continuum approach.
Dynamic numerical modelling by FDM code FLAC 6.0 was performed to back-analyse the landslide occurrence during the 1783
seismic sequence. At this aim reference synthetic accelerometric ground motions were derived from strong motion records, taking
into account both source and energy features of the 5th and 6th February mainshocks and local expected response spectra. In order to
force the numerical model, levelled-energy multifrequencial equivalent signals were obtained from these reference records by
experiencing the new LEMA_DES approach.
The results of modelling show a post-seismic trigger of the rock-avalanche, related to the second mainshock of the 1783 seismic
sequence, and are in good agreement with the present-day field evidences of the landslide scar area. In addition, no landslide results
by assuming the present shape of the M. Pacì slope if an equivalent input generated for the 1908 Reggio and Messina earthquake is
applied.
Paper No. 4.25b
1
INTRODUCTION
Ground effects due to earthquake-induced landslides
affecting both rock and soil slopes (Hutchinson, 1987; Sassa,
1996; Rodriguez et al. 1999; Esposito et al., 2000; Porfido et
al., 2002; Towhata et al., 2008) can be regarded as
responsible for the greatest damages and losses due to
earthquakes (Bird and Bommer 2004). Seismically-induced
landslides are well documented for some historical
earthquakes, such as the 1783 one occurred in Calabria
region, Italy (Sarconi, 1784; De Dolomieeu, 1785; Vivenzio
1788; Cotecchia et al.,1986 ) and the 1786 one occurred in
Kanding-Luding, China (Dai et al., 2005), while a large
database of earthquake-induced landslides for recent
earthquakes is available in the literature (Rodriguez et al.,
1999).
It is only in the past few years that some researches have been
addressed to the mechanisms of seismically-induced
landslides and to the deterministic prediction of earthquakeinduced ground failure scenarios (Wasowsky and Del
Gaudio, 2000; Havenith et al., 2002; Bozzano et al., 2008a;
Martino and Scarascia Mugnozza, 2005; Gerolymos and
Gazetas, 2007). The interactions between seismic input and
geological setting of the slope, including the presence of preexisting landslide masses, have been recently dealt with
(Bozzano et al. 2008c; Del Gaudio and Wasowsky, 2007;
Esposito et al., 2000; Martino and Scarascia Mugnozza,
2005: Martino et al., 2007) in order to point out the possible
role of some seismic input properties (i.e. frequency content,
directivity, peak of ground acceleration) on the triggering and
the mechanism of a landslide.
At this regard, the here discussed case history of the 1783
Scilla rock avalanche proves that the above mentioned
features, commonly taken into account for explaining
different seismic response effects in the frame of seismic
microzonation analyses, can be also responsible for
seismically induced landslides, which occur along fault zones
and involve huge volumes of intensely jointed rock masses.
THE SCILLA ROCK AVALANCHE
The Scilla rock-avalanche is one of the main ground effects
induced by the 1783 “Terremoto delle Calabrie” seismic
sequence and represents one of the most damaging landslides
historically reported in Italy. The landslide produced a
tsunami up to 16 m high, that killed more than1500 people in
the neighbour Marina Grande beach (Gerardi et al, 2008;
Graziani et al, 2006). The landslide occurred on February 6
1783 at 1:45 a.m., in the M. Pacì coastal slope very close to
the village of Scilla, about 30’ after one of the mainshocks
(Hamilton, 1783;Torcia, 1783; Gallo 1784; Sarconi, 1784;
Minasi, 1970, 1971, 1785) with estimated magnitude of 5.96.1. The landslide is witnessed, among other historical
records, by three engravings of A. Minasi, a XVIII century
local painter, illustrating the M.Pacì slope before and after the
landslide (Fig.1).
Paper No. 4.25b
Fig. 1. Engravings showing the M. Pacì slope before and after
the 6 February landslide (from Minasi A., 1783).
The Scilla landslide can be considered as a complex event
(Cruden and Varnes 1996) characterized by an initial sliding
mechanism, along structurally controlled joints, evolving into
a rock avalanche (Fig.2) (Bozzano et al., 2008b). No
reactivations of this landslide are recorded up to the present
time.
ENGINEERING-GEOLOGY MODEL
A detailed engineering-geology survey of the area involved
by the landslide has been carried out (Bozzano et al., 2008b)
and the main results are synthetically reported in this paper.
The submarine part of the slope has been also investigated by
geophysical techniques (Bosman et al., 2006; and Mazzanti,
2008).
The area is mainly characterized by gneiss rock masses,
belonging to the Scilla Metamorphic Unit (Fig.3).
2
Fig. 2. View of the Scilla landslide scar area from the
Tyrrhenian sea.
In the upper part of the scar area white breccias of gneiss
cemented by a calcitic matrix widely outcrop; these breccias
are associated to a normal fault, circa parallel to the coast line
(E-W), partially corresponding to the main scarp of the Mt.
Pacì landslide. Similar calcitic breccias also outcrop
downslope in correspondence to other tectonic joints
belonging to the same set.
The Holocene activity of these faults has been recently
quoted by Ferranti at al. (2007).
Fig. 3. Geological map of the Scilla landslide area; the trace
TM of the geological section of Fig.4 is also shown.
Paper No. 4.25b
3
Fig. 4. Geological section along Mt. Pacì slope and related
engineering-geology model (see Fig. 3 for the legend). The
pre-landslide shape of the slope is also reported (white zones).
The left flank of the landslide is characterised by intensely
jointed to cataclastic gneiss; moreover, intensely foliated
gneiss outcrop, representing the melanocratic facies of the
Scilla Metamorphic Unit. The same flank corresponds to a
main fault, bounding westward the Mt. Pacì-Mt. Bova horststructure (Ghisetti, 1984), with dip direction nearly N50°E.
In order to characterize the geomechanical properties of the
material involved in the Scilla landslide, 16 geomechanical
scanlines (Fig.3) were performed by measuring orientation,
spacing, aperture and type of filling on the main joint sets as
well as JRC and Schmidt hummer values. In 29 additional
geomechanical stations measures of Jv and Ib indexes were
also obtained (ISRM, 1978). Moreover, three boreholes were
drilled: the first one within the landslide deposit (Fig.3) and
the second and third ones 1.5 km far from the Mt. Pacì slope.
Point load and ultrasonic lab-tests were performed on the
sampled cores of gneiss in order to evaluate both strength and
deformational parameters of the intact rock.
Moreover, an attempt was made to evaluate the subaerial
thickness of the landslide debris (Fig.3) through velocimetric
measurements of ambient noise (26 recording stations).
However, no results was obtained since it was not possible to
point out any impedance contrast between debris and bedrock
due to the huge dimension of the blocks (up to 5 m3).
Starting from the in site geomechanical characterization a
mapping of the rock mass geomechanical properties, was
performed in the 3 following steps.
1 - A classification of the rock mass was obtained
experiencing the equivalent continuum approach by
Sridevi and Sitharam, 2000: at this regard, the two
independent rock mass jointing parameters Jf (joint factor
Paper No. 4.25b
by Ramamurthy, 1994) and Jv were considered for the
analysed geomechanical stations. The computed values
were plotted and clustered to derive rock mass classes.
2 - A class distribution for the rock mass in the landslide area
was obtained by interpolating and contouring the Jf and
the Jv values obtained for each geomechanical station.
3 - In order to map the location of each class, an overlapping
of the contour lines for the two indexes was obtained
through an intersection criterion.
The same equivalent continuum approach was also applied to
obtain the distribution of the jointed rock mass stiffness and
to take into account the in site depth-dependent stresses
(Sridevi and Sitharam, 2000; Esposito et al, 2007). In
particular, the values of the Young’s modulus of the intact
rock have been estimated in the range 3000-7210 MPa,
according to the stiffness values measured by ultrasonic labtests as well as the empirical equations which depend on the
confining stresses and which are proposed in the above
mentioned approach (Tab.1).
Based on the resulting stiffness values for the intact rock, the
stiffness values (i.e. shear modulus, G and bulk modulus K)
for the jointed rock masses have been derived taking into
account both the confining stresses (i.e. depth) and the Jf
index (Tab.1); the obtained stiffness values correspond to 9
lythotechnical units (LU) and are clustered in three classes
having the same jointing conditions (i.e. same Jv and Jf).
Moreover, the strength values corresponding to the identified
LU (i.e. cohesion, c; friction angle, φ and tensile strength, σt)
have been derived according to the Hoek and Brown (1980)
criterion (Tab.1).
The highly jointed rock masses (LU: III, VI and IX) strictly
correspond to both the coastward dipping fault zones and to
the eastward dipping fault zone which outcrops along the left
flank (Fig.4); moderate jointed rock masses (remnant LU)
widely outcrop all along the right flank of the landslide.
4
Table 1. Geomechanical parameters
engineering-geology model of Fig.4
referred
to
the
of the rock mass quality, in terms of rock mass jointing, from
the left to the right flank of the landslide.
SEISMICITY OF THE SCILLA AREA
The obtained 3D engineering-geology model of the landslide
proves the significant structural constrain on its kinematism,
also justifying the main geomorphological evidences in the
subaerial scar area (Fig. 4), such as the planar shape of the
left flank and the subvertical cliff of the crown area.
The evidence of a northeastward movement of the landslide
in the detachment area related to the biplanar wedge-like
shape of the detached volume, is consistent with the increase
Paper No. 4.25b
The seismicity of the Scilla area is strictly connected with the
Siculo-Calabrian rift zone, one of the most seismically active
areas of the Italian peninsula, characterised by several
seismogenic sources (CPTI04, http//emidius.mi.ingv.it/CPTI;
ITHACA, http://www.apat.gov.it/site/it-IT/Progetti/ITHACA
_catalogo_delle_faglie_capaci)
capable
of
producing
earthquakes with M≥7 and intensity values I≥ 10 both on the
MCS scale (CPTI04) and on the new ESI 2007 scale (Michetti
et al., 2007, Porfido et al., 2008). Historical seismicity, as
regards large earthquakes, occurred in this area, is fairly well
documented and shows a very high-recurrence of events with
at least 34 earthquakes with 8<I<11 on the MCS scale (Tab.2;
Fig. 5), four of which (1638, 1693, 1905, 1908) are with M≥7
(CPTI04). Among the major earthquakes of the SiculoCalabrian seismic belt, the 1783 multiple earthquake, was
characterised by a three years long sequence and five main
shocks generated by individual segments of regional WNWESE tectonic trends.
According to CPTI04 catalogue, the 1783 seismic sequence
started at the beginning of February and went on until the end
of March, reaching a maximum release of energy on March
28 with assessed macroseismic magnitude M=6.94. More
recent studies on the 1783 sequence (Jacques et al., 2001;
Galli and Bosi, 2002; Ferranti et al. 2008; Catalano et al.,
2008) attribute a value of M≥7 to the main shocks occurred
respectively on February 5 and March 28, 1783. More than
30000 lives were lost and 200 localities were completely
destroyed by the February 5 main shock; the epicentral area
(I0=11 MCS and ESI 2007 scales) was located on the Gioia
Tauro plain, at the western foot of the northern Aspromonte
mountain. The shock produced spectacular ground effects,
both primary and secondary, such as tectonic deformations,
ground fractures, liquefactions phenomena, tsunami,
hydrological changes and diffuse landsliding of large size,
which in most cases dammed the rivers creating more than
200 new temporary lakes. The second shock, occurred on
February 6, struck the coast between Scilla and Palmi (I= 910 MCS) and induced the large the Monte Pacì-Campallà
rock avalanche from the sea-cliff west of Scilla. According to
many historical sources this rock avalanche generated a
disastrous tsunami (max waves heights of 16 m) that affected
the coast for a total length of 40 km, from Bagnara to Villa
San Giovanni and from Torre Faro to Messina, causing more
than 1500 casualties in Scilla (Minasi,1785; Grimaldi, 1784;
Sarconi, 1784; Vivenzio, 1783; Gerardi et al., 2008).
From February 7 to March 28, three main shocks took place
with epicentres migrating northwards from Mesima Valley to
Catanzaro. In particular the last one caused severe damage
along both the Tyrrhenian and Ionian coasts. The cumulative
effects of all these earthquakes was devastating, more than
380 villages were damaged, 180 were totally destroyed.
5
From a seismogenic point of view the 1783 sequence can be
related to the active fault segments presents in southern
Calabria (Cotecchia, 1986; Jacques et al., 2001, Galli and Bosi
2002; Catalano et al., 2008; Ferranti et al., 2008). Several
Authors combining geological, morphological and
seismological data have identified three of the most
significative fault segments generating the 1783 sequence.
Table 2. List of the largest earthquakes occurred in southern
Italy in the last 2000 years with intensity ≥8
I on the MCS
scale (data from CPTI04).
Year Month Day
-91
374
853
8
31
1169
2
4
1184
5
24
1509
2
25
1626
4
4
1638
3
27
1638
6
8
1659
11
5
1693
1
11
1743
2
20
1767
7
14
1783
2
5
1783
2
6
1783
2
7
1783
3
1
1783
3
28
1786
3
10
1791
10
13
1818
2
20
1823
3
5
1832
3
8
1835
10
12
1836
4
25
1854
2
12
1870
10
4
1894
11
16
1905
9
8
1907
10
23
1908
12
28
1909
7
1
1947
5
11
1978
3
11
1978
4
15
Paper No. 4.25b
Epicentral area
Southern Calabria
Southern Calabria
Messina
Eastern Sicily
Crati Valley
Southern Calabria
Girifalco
Calabria
Crotonese
Central Calabria
Eastern Sicily
Southern Ionio
Cosentino
Calabria
Southern Calabria
Calabria
Central Calabria
Calabria
North-East. Sicily
Central Calabria
Catanese
Northern Sicily
Crotonese
Cosentino
Northern Calabria
Cosentino
Cosentino
Southern Calabria
Calabria
Southern Calabria
Southern Calabria
Calabro Messinese
Central Calabria
Southern Calabria
Patti Gulf
Io
9.5
9.5
9.5
10
9
8
9
11
9.5
10
11
9.5
8.5
11
8.5
10.5
9
10
9
9
9
8.5
9.5
9
9
9.5
9.5
8.5
11
8.5
11
8
8
8
9
Maw
6.3
6.3
6.3
6.6
6
5.57
6.08
7
6.6
6.5
7.41
6.9
5.83
6.91
5.94
6.59
5.92
6.94
6.02
5.92
6
5.87
6.48
5.91
6.16
6.15
6.16
6.05
7.06
5.93
7.24
5.55
5.71
5.36
6.06
Fig. 5. Distribution of historical earthquakes occurred in the
last 2000 years in the Siculo-Calabrian rift. Red bold line
indicate fault segment reactivation (data from CPTI04 and
ITHACA database).
De Dolomieu (1784) was the first to observe a fault rupture
associated to the 5th February 1783 main shock, at least 9-10
miles long and several feet wide, with an estimated slip of
about 3 m, localised in the Gioia Tauro Plain, between S.
Giorgio Morgeto and Santa Cristina d’Aspromonte. Recent
studies (Cotecchia, 1986; Jacques et al., 2001; Galli and Bosi
2002) attribute this shock to the reactivation of part of the
Cittanova normal fault for a maximum length of 25 km
(Fig.5). The source responsible for the second event appears
located offshore Scilla and is consistent with the reactivation
of the Scilla fault for an inferred length of 10 km as suggested
by Guarnireri at al. 2004 and Ferranti et al., 2008. The third
event, occurred on February 7, took place at the western
portion of the Serre Mountain, about 40 km northeast of the
first main shock and was related to the Serre fault for a total
reactivation of 24 km (Galli and Bosi 2002; Catalano et al.,
2008).
Table 3. Felt intensity value in MCS scale of the most
important earthquake which hit the Scilla town in the last
three centuries (data from CPTI04 database).
Date
Epicentral zone
19.03.1706
05.02.1783
06.02.1783
07.02.1783
28.03.1783
16.11.1894
08.12.1898
08.09.1905
23.10.1907
28.12.1908
16.01.1975
13.12.1990
Southern Calabria
Calabria
Southern Calabria
Calabria
Calabria
Southern Calabria
Northeaster Sicily
Calabria
Southern Calabria
Southern Calabria
Messina Straits
Southeaster Sicily
I MCS
Scillla
5-6
9
9-10
7-8
7-8
7
4
7-8
7
9
6
3
6
The analysis of the available historical catalogues shows that
Scilla has been heavily hit by numerous earthquakes with
local intensities ranging between 3 and 10 degrees on the
MCS scale (Tab.3). Among these the 28th December 1908,
Southern Calabria-Messina earthquake (I=11MCS, Maw 7.24;
from CPTI04), was the most ruinous, in terms of casualties (at
least 80000) and damages, occurred in the Messina Straits. It
was particularly catastrophic along the Calabrian coast,
between south of Reggio Calabria and south-west of Scilla,
and along the eastern coast of Sicily from its easternmost tip
to south of Messina (Baratta, 1910; Barbano et al., 2005)
(Fig.5). As a consequence Scilla was inundated by the
earthquake-induced destructive tsunami with a locally run-up
exceeding 1 m and observed inundation of 10 m (Baratta
1910). Some important secondary effects such as fractures and
landslides were observed along the main roads and a large
rockfall fell down close to the M. Pacì tunnel.
territory on a 5km wide mesh (INGV-DPC S1-Project, web
site: https:\\esse1.mi.ingv.it ) (Fig.6).
DERIVED DYNAMIC EQUIVALENT INPUTS
Following a methodology recently developed in Italy for
seismic classification (Orazi et al, 2008), two inputs
representative of the 5th and 6th February earthquakes were
obtained. An input representative of the 28th December 1908
Reggio and Messina earthquake was moreover obtained to
back analyse the response of the M. Pacì slope to the last felt
strong ground motion as well as to evaluate the effects of
similar seismic events on the present slope.
First of all, given the historical analysis of the selected
events, the European Strong-Motion Database (Ambraseys et
alii, 2000) and the COSMOS (Consortium of Organizations
for Strong-Motion Observation Systems) web site
(http://db.cosmos-eq.org) were searched according to proper
magnitude, intensity, depth and distance intervals, in order to
find records obtained in similar source and site conditions.
Only time-histories recorded in free-field and in rock sites
were considered. It was not always possible to take into
account the focal mechanism, since normal fault ruptures,
which are the dominant mechanism in the Mediterranean
area, are less represented in COSMOS database, while, on the
other hand, the European database does not collect
magnitudes larger than 7.
The selected records are:
1) Ulcinj station – 15th April, 1979 Montenegro earthquake
(ms 7.04, epicentral distance 21km, fault distance 9km)
for the event of February 5 ;
2) Atina station – 7th May, 1984 Lazio-Abruzzo earthquake
(ms 5.79, epicentral distance 15km, fault distance 12km)
for the event of February 6 ;
3) Gilroy1 station – 18th October, 1989 Loma Prieta
earthquake (ms 7.1 epicentral distance 28km, fault
distance 3km) for the 1908 event.
As a second step, the time-histories were adapted to the local
expected spectral response using the shape of the uniform
hazard spectrum with a 475 years return-period, calculated in
Scilla from the uniform spectra available for the whole Italian
Paper No. 4.25b
Fig. 6. Derived synthetics for the earthquakes of the 5 and 6
February, 1783 and of the 28 December, 1908.
Starting from the obtained strong-motion time-histories,
levelled-energy, multifrequencial, dynamic equivalent signals
were derived according to the LEMA_DES (Levelled-Energy
Multifrequential Analysis for Dynamic Equivalent Signals)
approach by Lenti and Martino (these proceedings), able to
control the frequency as well as the energy content of the
used equivalent inputs (Fig.7).
In particular, the procedure for deriving equivalent signals by
the LEMA_DES approach points out that the 1783
considered earthquakes are characterised by a very lowfrequency content. This feature is shown by the percentage
FFT values (respect to the FFT maximum) which are greater
than 90% at frequencies lower than 2 Hz (Fig.7). On the
contrary, the 1908 Reggio and Messina earthquake is
characterised by percentage FFT values no greater than 70%
in the same frequency range.
For the 1783 earthquakes a high reliability was obtained for
the LEMA_DES signals, since the convergence errors
(CRE%) are less than 10% (Fig.7) while, for the 1908
7
earthquake, a middle reliability was obtained since the
convergence error is in the range 10%-50%.
Fig.7. Multifrequencial equivalent inputs derived by
LEMA_DES procedure for the earthquakes of 5 and 6
February, 1783 and of 28 December, 1908 (down) with
related numerical reports (up).
Paper No. 4.25b
NUMERICAL MODELING
A stress-strain 2D numerical modelling was performed by the
FLAC 6.0 (Itasca, 2006) FDM software in dynamic
configuration to back analyse the seismic trigger of the 1783
Scilla landslide. The 3D shape of the subaerial and submerged
slope topography before the landslide occurrence (Mazzanti,
2008) was considered for modelling both the 5th and the 6th
February, 1783 earthquakes. Moreover, the slope shaking due
to the 28th December 1908 Reggio and Messina earthquake
was modelled, considering the present shape of the slope (i.e.
without the landslide mass).
For the numerical models, a 400 x 200 mesh with a 5 m
square resolution was used, assuming the engineering-geology
model of Fig.4, obtained along section AA’ of Fig.3.
The model has a total length of 2000 m and a maximum
height difference of 800 m (left side); the absolute levels reach
about -300 m b.s.l. and about 500 m a.s.l..
As derived from the above described engineering-geology
model of the slope (cfr. par. 2), this model is composed of 34
zones, whose properties can be referred to the 9 lithotechnical
units (LU) reported in Tab.1.
An elastic perfectly plastic constitutive law was attributed to
all the zones; moreover, both x-displacements and ydisplacements were permitted along the lateral boundaries.
An initial static gravitational equilibrium was reached before
applying the seismic sequence consisting in the three derived
LEMA_DES signals. Since the numerical model resolution is
up to 20 Hz, these signals can be regarded as more adapt than
the synthetic ones to be modelled; moreover, their shortness
(few seconds of duration) guarantees a faster solution of the
numerical dynamic computation, including the damping
effects, even if a very complex geomechanical model was
adopted. The dynamic inputs have been applied as horizontal
accelerations all along the basis of the numerical model.
Mechanical dissipation was computed using a Rayleigh
Damping function.
During the dynamic modelling, where plasticity state (under
both tensile and compressive conditions) was reached, both
stiffness and strength parameters were modified according to
an elastic perfectly plastic behaviour, by instantaneously
zeroing cohesion and reducing the dynamic shear modulus (G)
down to the value recorded for the landslide mass blockdebris by in site down hole investigations.
The results obtained from the performed numerical back
analysis (Fig.8) can be summarised in the following list:
1) after the first earthquake (February 5, 1783) at yielding
zones, due to new jointing of the rock mass, result up to 350
m a.s.l. within a depth varying in the range 50-15 m, on the
contrary no relevant effects result in the submerged portion of
the slope, except for some cracks within the rock mass located
at about 90 m b.s.l.;
8
Fig. 8. Results of the dynamic numerical modeling by the code
FLAC 6.0: 1) sub-vertical cracks; 2) crack propagation
within the submarine slope; 3) opening of cracks along the
crown area; 4) generalized collapse of the slope; 5)
previously existing cracks; 6) falls in correspondence with the
crown area.
2) after the second earthquake (February 6, 1783) at yielding
zones widely result up to 450 m a.s.l., continuously distributed
both along the surface and across the depth of the model, and
well fit the rupture surface of the landslide which can be
actually observed on the slope (Fig.9);
3) after the last modelled earthquake (December 28, 1908)
new superficial at yielding zones (i.e. up to a depth of 10 m)
result especially close to the crown area of the 1783 landslide
and could be related to some rock falls from the already
existing scarp.
During the first and the second earthquake of the modelled
sequence a significant deepening of the at yielding zones can
be observed in the post-seismic phase (i.e. when no equivalent
Paper No. 4.25b
dynamic input is acting in the model); on the contrary, during
the third earthquake rock falls already occur in the co-seismic
phase.
CONCLUSIONS
The results obtained by the dynamic numerical modelling of
the 1783 Scilla landslide are very consistent with the
documented landslide event as well as with the field evidences
collected in the landslide area. In particular, these results
demonstrate that the Scilla rock avalanche resulted from a
cumulated strain effect due the two earthquakes of the 5 and 6
February, 1783. The landslide involved the subaerial slope,
while no relevant effects were induced in the submerged slope
during the shaking. At this regard, since no strong
geomechanical constraints exist for the submerged slope, a
numerical test was performed by adopting the lithotechnical
units with the highest Jf all along the submerged slope. Also
in this case, no significant effects result due to the applied
dynamic inputs.
9
Fig. 9. Result of dynamic numerical modeling by code FLAC
6.0: at yielding state within the rock mass (red zones) before
landslide collapse on February6, 1783 and actual Mt. Pacì
slope shape (white markers).
The landslide crown area well corresponds to the present
scarp, located at about 450 m a.s.l., while a possible break out
plane can be recognised at about 150 m a.s.l. even if no
evidence can be actually collected due to the presence of a
block-debris mass. Significant post-seismic effects due to the
earthquake shaking can be assumed especially in terms of
plasticization deepening and can justify the delayed trigger of
the landslide on 6 February 1783, with respect to the
earthquake mainshock.
Based on the previously discussed results, a possible rupture
model for the earthquake induced Scilla landslide can be
summarised in the following steps:
1) a widespread plasticization (i.e. zones characterised by at
yielding conditions as well as by a relevant stiffness decay) of
the rock mass occurred during the 5th February, 1783
earthquake all along the Mt. Pacì slope and generated a rock
mass volume prone to a coherent sliding;
2) the following mainshock on February 6, acted on the rock
mass volume which was widely at yielding conditions,
accelerated it simultaneously downslope and, as a
consequence, became responsible for the sliding trigger;
3) the sliding break out appeared in correspondence with the
coast line;
4) after the occurrence of the rupture, the rock sliding evolved
in a rock avalanche which ran out to the bottom of the
submerged slope, in correspondence with the Scilla channel
where nowadays its block-type deposit was found at about
300 m b.s.l. (Bozzano et al., 2008b; Mazzanti, 2008).
Paper No. 4.25b
ACKNOWLEDGMENTS
The research was carried on in the frame of the project funded
by the Italian Ministry for Research and University
“PRIN2006: The engineering-geology model as a tool for the
dimensioning of gravity-induced coastal instabilities” (coordinator Prof. F.L. Chiocci; scientific responsible Prof. F.
Bozzano).
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