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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B08302, doi:10.1029/2006JB004837, 2007
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Seismotectonics of the Nubia plate compressive margin
in the south Tyrrhenian region, Italy:
Clues for subduction inception
A. Billi,1 D. Presti,2 C. Faccenna,1 G. Neri,2 and B. Orecchio2
Received 6 November 2006; revised 10 April 2007; accepted 30 April 2007; published 1 August 2007.
[1] Seismological and structural data are used to constrain the active tectonics of the
Nubia plate compressive margin in southern Italy. In this region, compressional
displacements have resumed at the rear of the Maghrebian orogenic wedge since about
700–500 ka and are presently accommodated along a seismic belt in the south Tyrrhenian
area, where a passive margin has developed during late Neogene-Quaternary times.
Earthquake data from the south Tyrrhenian belt are analyzed with Bayloc, a probabilistic
nonlinear location method. Results show that the seismic belt is segmented and involves a
series of NE-SW and NW-SE elongated clusters. The NE-SW clusters are interpreted as
high-angle reverse fault zones verging toward the southeast, whereas the perpendicular
clusters are interpreted as possible strike-slip fault zones transferring the contractional
displacements to advanced segments of the belt in the southeast. Brittle deformations
observed in the Quaternary volcanic island of Ustica are consistent with the geometry and
kinematics of the seismic belt deduced from the seismological data. The south Tyrrhenian
active belt may constitute an early stage of subduction of the Tyrrhenian oceanic crust
beneath Sicily. A similar scenario has been hypothesized also for the westward
prolongation of the south Tyrrhenian belt off the Algerian coast. By integrating
instrumental and historical seismic data with tectonic evidences, we infer that the south
Tyrrhenian belt is probably capable of producing earthquakes with a maximum size
close to magnitude 7. The analyzed data suggest a multifault system behavior for the
studied compressional belt. This inference is important to estimate and mitigate the
seismic and tsunamic hazards in such a densely populated region.
Citation: Billi, A., D. Presti, C. Faccenna, G. Neri, and B. Orecchio (2007), Seismotectonics of the Nubia plate compressive margin
in the south Tyrrhenian region, Italy: Clues for subduction inception, J. Geophys. Res., 112, B08302, doi:10.1029/2006JB004837.
1. Introduction
[2] The tectonic mechanisms leading to the conversion of
a passive margin into an active one with subsequent
subduction of oceanic lithosphere have been widely studied
[Dickinson and Seely, 1979; Cloething et al., 1982, 1989;
Muller and Phillips, 1991; Erickson, 1993; Toth and
Gurnis, 1998; Faccenna et al., 1999; House et al., 2002];
however, their knowledge is still substantially limited
because of the absence of obvious, present-day examples
of trench initiation. At convergent margins, in fact, the
accretion of allochthonous terranes usually masks the early
compressional structures including evidences of subduction
inception. On the other hand, examples of compressive
deformations at passive margins are documented only in a
few sites. These include the region off Iberia in the Atlantic
1
Dipartimento di Scienze Geologiche, Università Roma Tre, Rome,
Italy.
2
Dipartimento di Scienze della Terra, Università di Messina, Messina,
Italy.
Copyright 2007 by the American Geophysical Union.
0148-0227/07/2006JB004837$09.00
Ocean [Cloething et al., 1990; Masson et al., 1994, AlvarezMarron et al., 1997], the British Isles [Ziegler et al., 1995],
the Scotian basin in eastern Canada [Erickson and ArkaniHamed, 1993], and the Fiordland Region in New Zealand
[House et al., 2002].
[3] In this paper, we document the geological and seismological features of an incipient compressional belt
developing along a late Neogene-Quaternary passive margin in the central Mediterranean region. In this region
(Figure 1), since the Oligocene time at least, the collision
between Nubia and Eurasia has produced the north-northwestward subduction of the Calabrian slab for about 400 km
and the associated growth of the south verging Maghrebian
orogen [Anderson and Jackson, 1987; Faccenna et al.,
2004; Rosenbaum and Lister, 2004]. Since about late
Miocene time, this process was accompanied by back-arc
extension in the modern Tyrrhenian Sea area [Malinverno
and Ryan, 1986]. Geological evidence has shown that
thrusting at the front of the Maghrebian fold-thrust belt
(i.e., in the southern Sicily and in the Sicily Channel) mostly
ceased by mid-Pleistocene time [Lickorish et al., 1999].
Seismological and geodetic (GPS) evidences have shown,
however, that Nubia and Eurasia are still converging and
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Figure 1. Schematic tectonic map of the central western Mediterranean and European areas. Main
Cenozoic fold-thrust belts are shown. The study area is located in the southern Tyrrhenian Sea off the
northern coast of Sicily (southern Italy). Inset (top left) is a shaded relief map of Sicily and Tyrrhenian
seafloor [Marani et al., 2004]. Islands of the Aeolian Archipelago are indicated as follows: A, Alicudi;
F, Filicudi; L, Lipari; P, Panarea; S, Salina; St, Stromboli; and V, Vulcano. U indicates the island of Ustica
located in the southern Tyrrhenian Sea. The Drepano Thrust in the southern Tyrrhenian Sea is drawn after
Pepe et al. [2005] and marks the tectonic contact between the inner (northern) Kabilian-PeloritanCalabrian and the outer (southern) Sicilian-Maghrebian contractional belts.
that the relative contractional displacements are being
mainly accommodated at the rear of the Maghrebian belt
(Figure 1), namely along an E-W trending compressive belt
in the southern sector of the Tyrrhenian Sea [Goes et al.,
2004; Montone et al., 2004; Pondrelli et al., 2004; Neri et
al., 2005], where the passive margin of Sicily has developed
during late Neogene-Quaternary times. Despite several
studies on the active tectonics of the south Tyrrhenian
compressive belt, the architecture and kinematics of this
belt is still mostly unconstrained. In most studies, for
instance, the south Tyrrhenian belt is considered as dominated by active, hinterland-verging (i.e., northward) thrusts
[e.g., Goes et al., 2004], this tectonic setting implying
underthrusting and, possibly, future subduction of the Tyrrhenian oceanic crust toward the south. In contrast, on the
basis of seismic reflection data, Pepe et al. [2005] suggested
a southward vergence for the south Tyrrhenian active
compressive belt.
[4] The aim of this paper is to constrain the active
tectonics of the Nubia plate compressive margin in the
south Tyrrhenian area. To do so, we analyze a set of
earthquake data with Bayloc [Presti et al., 2004], a probabilistic nonlinear location method. A set of structural data
from the Quaternary volcanic island of Ustica (Figure 1) are
also presented and compared with the seismological data.
The analyzed data bear important insights into the past
evolution and future scenarios of the Nubia plate margin in
the central Mediterranean area. The presented evidences
may be interpreted as clues for a very incipient subduction
of the Tyrrhenian oceanic lithosphere beneath the Maghrebian orogen. A comparison with tectonic studies on the
same compressive margin in a more western location (i.e.,
off the Algerian coast [Déverchère et al., 2005; Domzig et
al., 2006; Stich et al., 2006]) sheds light on the lateral
changes of style and activity of the Mediterranean subduction complex. Moreover, Cinti et al. [2004] indicated the
compressive margin in the southern Tyrrhenian area as a
probable (i.e., probability = 5 – 10%) site for a M 5.5
earthquake by about 2015 [see also Jenny et al., 2006]. In
this framework, detecting the architecture of the seismogenic apparatus is necessary to assess and mitigate the
seismic and tsunamic hazards in this densely populated
region.
2. Geological Setting
[5] The study area is located in the southern sector of the
Tyrrhenian Sea, at the transition between the Maghrebian
fold-thrust belt, in the south, and the Tyrrhenian back-arc
oceanic basin, in the north (Figure 1). The Moho is 25–
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Figure 2. Epicenter map of earthquakes that occurred between January 1994 and April 2005. The
considered earthquakes (500) are shallower than 30 km and greater than 3.0 in magnitude. Figure 2
shows that during the study period the seismic belt in the southern Tyrrhenian Sea was the site for most
earthquakes that occurred in the Tyrrhenian-Sicilian region (except the active volcanic area of Mount
Etna). Note the Sisifo and Tindari faults in the eastern sector of the study area. Triangles in the inset
(bottom left) indicate the seismic stations operating during the study period.
30 km deep in the northern coast of Sicily and shallows
toward the north at the continent-ocean transition, where it
is approximately 10 – 15 km deep [Neri et al., 2002].
Similarly, the base of the lithosphere shallows from a depth
of about 70 km in Sicily to a depth of less than 40 km in the
Tyrrhenian area [Ansorge et al., 1992].
[6] In Sicily, the fold-thrust belt is traditionally divided into
two main belts, namely the inner (northern) Kabilian-PeloritanCalabrian and the outer (southern) Sicilian-Maghrebian belts
[Catalano et al., 1996]. The Sicilian-Maghrebian belt involves
imbricate sheets of Mesozoic – early Tertiary carbonate rocks,
whereas the Kabilian-Peloritan-Calabrian belt includes
imbricate sheets of Paleozoic metamorphic and igneous
rocks, and Mesozoic sedimentary covers. The KabilianPeloritan-Calabrian and the Sicilian-Maghrebian belts form
a south vergent orogenic wedge hereafter named
Maghrebian fold-thrust belt or Maghrebides. The Maghrebian fold-thrust belt grew upon the Meso-Cenozoic sequences of the African margin during Oligocene – middle
Pleistocene times [Lickorish et al., 1999]. Continental collision occurred during Oligocene – early Miocene times and
caused the tectonic superposition of the Kabilian-PeloritanCalabrian belt onto the rock succession of the SicilianMaghrebian belt, which mostly formed during Miocene
time. The tectonic contact between the Kabilian-PeloritanCalabrian belt and the underlying Sicilian-Maghrebian belt
is a N dipping, S vergent, regional thrust (i.e., also known as
the Drepano Thrust [Pepe et al., 2005]), which cuts across
the northeastern Sicily and the southern Tyrrhenian area (see
inset in Figure 1). Off the northern Sicilian coast, the
location of the Drepano Thrust (Figure 1) is uncertain
because drawn by interpolating a set of seismic profiles.
[7] Back-arc extension started during late Tortonian time
and lasted until early Messinian time [Pepe et al., 2000].
Contraction resumed at the rear of the orogenic wedge
during Messinian –early Pliocene times and caused both
thrust reactivation and transverse strike-slip faulting
[Ghisetti, 1979]. In middle-late Pliocene time, back-arc
extension renewed and lithospheric breakup occurred in
late Pliocene time. The extensional tectonics led to the
formation of the oceanic Tyrrhenian basin during Quaternary time [Faccenna et al., 2004, 2005; Rosenbaum and
Lister, 2004; Nicolosi et al., 2006]. Contractional displacements at the front of the Maghrebian fold-thrust belt ceased
by mid-Pleistocene time [Lickorish et al., 1999]. Afterward,
the southward propagation of the thrust front has been
possibly hindered by the thick and buoyant HybleanPelagian continental lithosphere, which has resisted to
subduction (Figure 1). Consequently, contractional displacements have shifted to the rear of the belt in the south
Tyrrhenian region [Hollenstein et al., 2003; Goes et al.,
2004; Pondrelli et al., 2004]. At present, the compressive
belt active in this region is laterally (i.e., eastward) bounded
by the regional, NNW striking Tindari Fault (Figure 1),
along which right-lateral, strike-slip and transtensional displacements are presently accommodated [Billi et al., 2006].
In the east of the Tindari Fault, active NW-SE oriented
extension occurs possibly in connection with the rollback of
the Calabrian slab [Neri et al., 2003; Montuori et al., 2007].
Toward the north-northwest, the Tindari Fault intersects the
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Figure 3. (top) Epicentral density map obtained by the Bayloc location method for the crustal
earthquakes of duration magnitude over 1.5 recorded in the south Tyrrhenian compressive belt between
January 1994 and April 2005. (bottom) Hypocentral cross sections on profiles perpendicular to the main
seismic clusters (labeled a – d) detected in the epicenter map, where the profile tracks are indicated by
arrows. Contour intervals for the probability density level are as follows: 68%, 50%, and 25%.
Percentages of density in the epicentral map (top) refer to the total activity in the belt, whereas
percentages of density in the individual cross sections (bottom) refer to the total activity in their own
sectors. Number of earthquakes for each section is as follows: a 160, b 70, c 460, and d 180.
Location of epicentral map (top) is shown in Figure 2.
Sisifo Fault (Figure 2), a seismic WNW striking structure,
which presently accommodates right-lateral strike-slip displacements [Finetti and Del Ben, 1986; Billi et al., 2006].
[8] The Quaternary Aeolian Islands in the southeastern
Tyrrhenian Sea (Figure 1) form an island arc whose volcanic activity and products are connected with the subduction
of the Calabrian slab [Barberi et al., 1973]. Volcanic
activity of the Aeolian volcanoes started about 1.3 Myr
ago and is still active in at least three of the seven major
islands, namely Lipari (last eruption in 729), Vulcano (last
eruptions in 1888 – 1892), and Stromboli (persistently
active). Toward the west, the Quaternary island of Ustica
(Figure 1) is the summit of a submarine volcano as high as
more than 2 km from the seafloor [Calanchi et al., 1984].
Sodic alkaline basalts are exposed at Ustica, whose magmatism is akin to oceanic island basalts [Trua et al., 2003]. The
magmatism of Ustica is possibly connected with mantle
return flows around the narrow Calabrian slab [Marani and
Trua, 2002; Faccenna et al., 2005; Montuori et al., 2007].
From the age of the exposed rocks, it is inferred that the
volcanic activity of Ustica occurred between about 750 and
130 ka [de Vita et al., 1998]. The volcanic activity of Ustica
ceased during middle – late Pleistocene time, possibly as a
consequence of the onset of the compressional tectonics in
the south Tyrrhenian area.
3. Seismological Data and Analysis
[9] The seismic data set used in the present study has
been extracted from the earthquake catalog of Istituto
Nazionale di Geofisica e Vulcanologia (available at http://
www.ingv.it/banchedati/banche.html) and includes P and S
wave arrival times of seismic events shallower than 30 km
occurred in Sicily during January 1994 to April 2005. We
selected the earthquakes with magnitude Md 3.0 and
with a minimum of eight arrival time readings (about
500 events). For a preliminary earthquake location, we used
the SIMUL linear algorithm by Evans et al. [1994] and the
three-dimensional crustal velocity model proposed for the
study region by Barberi et al. [2004]. The epicenter map of
Figure 2 shows that, in the studied interval of time, most
seismicity of Sicily and surrounding regions occurred in the
Etnean active volcanic area and along the south Tyrrhenian
compressive belt. The location of this belt is shown with a
rectangle in Figure 2.
[10] We focused our attention on the seismic events
occurring in the belt and, for this purpose, we used a
magnitude threshold of 1.5. We located Md 1.5 earthquakes in the belt by the Bayloc nonlinear probabilistic
method by Presti et al. [2004]. Bayloc uses the arrival times
of P and S waves and their velocity models as input data,
performs a grid search in the location parameter space,
considers the nonlinearity of the location problem, and
gives the hypocenter location as a location probability
cloud. When drawing cumulative maps or hypocenter
cross sections, Bayloc weighs the individual earthquakes
inversely to spreading of the respective probability clouds.
With respect to linearized methods based on hypocenter
point representations, Bayloc was shown to be more effective in the detection of hypocenter trends and seismogenic
structures [Presti et al., 2004]. Bayloc uses a Bayesian algorithm to compute the probability density function p(x, y, z)
for the spatial coordinates of a seismic event. The spatial
distribution of probability density relative to a set of earthquakes is computed as the sum of probability densities of
the individual events. Epicentral density maps and hypocentral density cross sections are obtained by integration of
cumulative probability densities [Presti et al., 2004; Presti,
2006].
[11] Figure 3 (top) shows the Bayloc epicentral map
obtained for the earthquakes of magnitude Md 1.5
occurring at depths shallower than 30 km in the south
Tyrrhenian belt between January 1994 and April 2005
(about 1000 events). Earthquake locations have been computed on a regular three-dimensional grid spaced 2.5 km.
In Figure 3, contour levels of probability distribution
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Figure 4. Bayloc relocations of synthetic events generated
over two-dimensional grids in sector a of Figure 3 (see text
for details). The plots show the results obtained considering
synthetic earthquakes spaced 1 km apart (left) on a vertical
grid oriented N-S and (right) on a NW striking grid dipping
toward the southwest. The white line indicates strike and
dip of the grid in map and cross-section views, respectively.
Contour intervals for the probability density level are as
follows: 68%, 50%, and 25%.
correspond to 68%, 50% and 25% of earthquake density.
Figure 3 (top) shows two main epicentral trends oriented
NW-SE and NE-SW. During the studied interval of time, the
most active sectors in the belt were: (a) the sector located in
the southwest of Ustica and corresponding to the area of the
June –July 1998 seismic sequence, which included 80 earthquakes with a maximum magnitude of 5.2; (b) the sector
located in the south-southeast of Ustica affected by relatively small phases of seismic activity during August –
September 1994 (about 20 earthquakes with Mmax = 3.8);
(c) the sector located off the Palermo city and corresponding
to the area of the September – December 2002 seismic
sequence involving earthquakes with a maximum magnitude of 5.9; (d) the sector including the western Aeolian
Islands and corresponding to the Sisifo Fault zone. In the
last decade, this fault zone has been characterized by
frequent low-magnitude seismic swarms. During 1980 and
during the years immediately preceding and following
1980, the Sisifo Fault zone has been moderately active with
seismic sequences characterized by earthquakes with a
maximum magnitude of about 6 [Neri et al., 1996, 2003].
[12] Figure 3 (bottom) displays vertical cross sections
perpendicular to the long axes of individual clusters
detected in Figure 3 (top). For sectors a to d in Figure 3,
the percentage of events falling in the upper 10 km is as
follows: a = 81%, b = 78%, c = 93% and d = 83%. Highangle northward dip of hypocenter distribution is a clear
common feature of vertical sections a to d.
[13] Because permanent ocean bottom seismometers
(OBS) are absent in the study area, the geometry of the
seismic network available for earthquake location is not
optimal and, in some cases, almost critical (Figure 2). A first
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level of check of locations used in drawing maps and cross
sections was guaranteed by the Bayloc feature that weighs
earthquakes according to their resolution [Presti et al.,
2004; Presti, 2006]. An additional, even more conservative
effort to reduce the bias introduced by poorly located events
has been made in the present study by excluding from maps
and cross sections the earthquakes for which the epicentral
and hypocentral errors are found greater than predetermined
values. We made several attempts by varying the error upper
bounds and found that the hypocenter trends are fairly stable
in a relatively wide range of the error upper bound. In
Figure 3, the map and cross sections obtained by excluding
all earthquakes (18% of the total earthquakes) for which the
epicentral and hypocentral errors were found greater than 6
and 10 km, respectively, are displayed. A further check of
hypocenter trends detected in Figure 3 has been done by
synthetic earthquake relocations. We henceforth report the
test performed in sector a (Figure 3), which is the most
critical one having a network azimuthal gap of almost 180°.
The synthetic events were positioned on two-dimensional
grids with spacing of 1 km. For each synthetic event, the
arrival times of P and S waves were estimated at the stations
that recorded a randomly selected event of the real sequence
occurring in the same sector. A Gaussian noise with
standard deviation of 0.1 and 0.4 s was adopted for
perturbation of P and S wave readings, respectively. In
Figure 4, we show the results obtained by relocating the
earthquakes positioned on two grids significantly rotated
with respect to the hypocenter pattern of the real sequence,
i.e., a vertical grid oriented N-S (Figure 4, left) and a NW
striking grid with a dip of 60° toward the southwest
(Figure 4, right). Bayloc relocations of the synthetic events
provided a faithful reproduction of the two grids, both in
strike and in dip (Figure 4). By making several attempts
(i.e., changing dip and strike of the synthetic quake grid),
we obtained compelling evidence of the reliability of the
real hypocenter trend found in sector a (Figure 3); that is,
the trend detected in sector a should not derive from the
activity of faults having strike and dip different, respectively,
from N45° ± 25°W and 70° ± 20°NE. Similar evidences of
reliability of hypocenter trends have been obtained also for
sectors b to d of Figure 3. These evidences are not reported for
conciseness.
[14] Figure 5 shows the focal mechanisms of the earthquakes with magnitude 4.0 available for the south
Tyrrhenian belt since 1978. The chosen magnitude threshold allows us to better focus our attention on regionalscale geodynamic processes. Nineteen out of 21 focal
mechanisms were taken from the literature [Anderson and
Jackson, 1987; Pondrelli et al., 2002, 2004; regional
centroid moment tensors, 2006, available at http://www.ingv.
it/seismoglo/RCMT); Harvard CMT catalog, 2006, available
at http://www.seismology.harvard.edu]. Focal mechanisms
5 and 6 have been estimated in the present study by
inverting the P-onset polarity data (i.e., 17 and 14 data,
respectively) with the FPFIT standard procedure [Reasenberg
and Oppenheimer, 1985] and by using the crustal velocity
model provided by Barberi et al. [2004].
[15] Figure 5 shows that reverse faulting on E to NE
striking planes is dominant within the south Tyrrhenian belt,
whereas a combination of dextral strike-slip and normal
faulting characterizes the eastern boundary of this belt, i.e.,
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Figure 5. Focal mechanisms (lower hemisphere projections) and relative locations for earthquakes with
magnitude 4.0 available in the study area. Reverse faulting is dominant in the central and western
sectors of the south Tyrrhenian seismic belt, whereas normal and dextral strike-slip faulting characterizes
the eastern edge of this belt, i.e., on the Tindari Fault system. Inset shows polar diagrams of P and T axes
(black and white, respectively) relative to the two major domains; that is, diagram A includes data from
the central and western sectors of the south Tyrrhenian seismic belt (focal mechanisms 2, 3, 4, 7, 8, 9, 10,
11, 13, 14, 15, 16, 17, 18, 19, and 20), whereas diagram B includes data from the eastern edge of the belt
(focal mechanisms 1, 5, 6, 12, and 21). Focal data are listed in Table 1.
the Tindari Fault system. The polar plots of P and T axes are
displayed in the inset of Figure 5. These plots show that
seismic activity in the central western sector of the south
Tyrrhenian belt occurred in response to a NNW-SSE oriented compressive stress, whereas the seismicity in the
eastern sector is connected with a more complex and
heterogeneous stress field probably related to a transitional
domain between compressional and extensional environments [Billi et al., 2006].
4. Structural Data
[16] The south Tyrrhenian active contractional margin has
been thus far considered as dominated by a S dipping N
vergent reverse fault system [e.g., Goes et al., 2004;
Serpelloni et al., 2005; Billi et al., 2006]. This interpretation
was apparently supported by the occurrence of a S dipping,
NE striking thrust fault signaled on the island of Ustica
[Bousquet and Lanzafame, 1995], which is the only emergent area along the central western sectors of the studied
compressive margin. In this section of the paper and in the
following ones, the structural evidences from Ustica are
critically reconsidered and properly framed into the active
tectonic context of the south Tyrrhenian margin.
[17] We analyzed the rock deformations exposed on the
volcanic island of Ustica by field surveys and structure
mapping at the 1:5,000 scale. The exposed deformations
consist of open extensional fractures, Neptunian dikes, and
faults (Figure 6a). Extensional fractures and Neptunian
dikes were observed in several exposures (e.g., Figure 6b),
particularly along the coast. Neptunian dikes have a maximum dilational displacement of about 1.5 m and are
usually filled by poorly consolidated marine calcarenites.
Fractures and dikes consist of high-angle (dip 70°)
surfaces striking between N30° and N80°. The average
strike of the analyzed population of dikes and fractures is
N53° (see diagram A in Figure 6a). The relative axis of
maximum extension is inferred as trending about NW-SE.
In site 3 (Figure 6a), a lava flow emplaced over some
Neptunian dikes, this evidence showing that fractures and
dikes are, at least in part, synvolcanic.
[18] A set of faults (Figures 6a, 6c, and 6d) was observed
in the western sector of the island as already signaled by
Bousquet and Lanzafame [1995]. These faults consist of
middle- to low-angle surfaces striking preferentially N60°–
70° (Figure 6a). Over most fault surfaces, a set of lineations
was observed. These structures were dubiously interpreted
as kinematic indicators showing a reverse-to-transpressional
(left lateral) fault kinematics. In particular, the main sense of
tectonic transport for the hanging wall blocks is toward the
north-northwest (Figure 6). The relative axis of maximum
shortening is inferred as trending about NNW-SSE. In site 2
(Figure 6a), some Neptunian dikes are cut and displaced by
the low-angle faults, this evidence showing that the reverse
faults postdate the dikes and the extensional fractures.
5. Discussion
5.1. Tectonic Architecture
[19] The architecture of the south Tyrrhenian seismic belt
is characterized by an overall E-W trend and a curved shape
with a northward convexity. In detail, the belt is split into
seismic segments (i.e., elongated clusters or clouds) trending preferentially NW-SE and NE-SW (Figure 3). We
interpret these elongated clusters as fault zones activated
in different periods between 1994 and 2005. The fault
attitude and orientation inferred from the earthquake focal
mechanisms (Figure 5) support this interpretation.
[20] We interpret the NW-SE and NE-SW fault zone
trends as mostly inherited from previous tectonic processes,
namely the compressive tectonics (Oligocene-Pliocene
time) that led to the development of the Maghrebian foldthrust belt [Pepe et al., 2005] and the extensional tectonics
(Miocene-Pleistocene time) that led to the development of
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Figure 6. (a) Schematic map of the volcanic island of Ustica showing main crater rims and a reverse-totranspressional (left lateral) fault signaled by Bousquet and Lanzafame [1995]. The structural data
collected in the numbered sites (from 1 to 5) are plotted in equal-area Schmidt diagrams (lower
hemisphere projections) shown in Figure 6. Diagrams A and B show the contours of all dike and fracture
poles, respectively, and all faults with relative slickenlines. (b) Photograph showing a set of NE striking
dikes and fractures in site 5. (c) Photograph showing a possible reverse-to-transpressional fault in site 2
(hanging wall block moved toward the north-northwest). (d) Photograph showing a possible reverse fault
in site 1 (hanging wall block moved toward the north-northwest).
the Tyrrhenian back-arc basin [Pepe et al., 2000]. In
particular, the extensional tectonics led to the partial collapse of the inner (northern) portion of the Maghrebian foldthrust belt and to the development of several NE and ENE
trending basins in the coastal area of northern Sicily and in
the southern Tyrrhenian region (e.g., the Cefalù basin in
Figure 7, Pepe et al. [2000]). In the oceanic Tyrrhenian
basin, Nicolosi et al. [2006] have recently shown a set of
NE trending magnetic anomaly stripes, from which a major
NE-SW tectonic trend (i.e., strike of faults) is inferred. The
complex tectonic setting inherited from the PaleogeneQuaternary contractional and extensional tectonic phases
is probably the cause for the observed segmentation and
trends of the south Tyrrhenian seismic belt (Figure 7). This
hypothesis is also consistent with recent seafloor bathymetric maps of the Tyrrhenian Sea (Figure 7) [Marani et al.
[2004]), where a morphologically very heterogeneous passive margin occurs between the continental domain of Sicily
and the oceanic one of the Tyrrhenian region. Along this
margin, alternated basins and ranges are mostly NE-SW
oriented.
[21] In cross-sectional views (Figure 3), the south Tyrrhenian active belt is a zone of seismic deformation inclined
toward the north with dip angles greater than 60°. Such high
angles may be explained by two models. In a first model,
the high-angle seismic zones are interpreted as inherited
thrust zones, which originally developed as low-angle and
then became high-angle zones with the emplacement, at
their footwall, of forelandward piggyback thrust sheets.
Such a thrust imbrication typically produces the progressive
steepening of the inner early thrusts [e.g., Storti et al.,
2000]. In an alternative model, the high-angle seismic zones
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Figure 7. Model for the tectonic architecture and active kinematics of the south Tyrrhenian seismic belt
deduced from the presented data. The belt involves NE and NW trending seismic segments. The NE
trending segments are mostly compressional faults, whereas the NW and WNW trending ones are
interpreted as right-lateral strike-slip zones, which transfer the compressional displacements toward the
southeast [see also Goes et al., 2004]. The Tindari Fault in the eastern sector of the belt is right-lateral
transtensional [Billi et al., 2006]. Most seismic faults cut through the Drepano Thrust (drawn after Pepe et
al. [2005]) whose age is Oligocene. The kinematics of the south Tyrrhenian belt is also consistent with
GPS data [D’Agostino and Selvaggi, 2004], from which N trending and NW trending convergence
vectors are inferred for the eastern and western sectors of the belt, respectively. In particular, two pairs of
converging white arrows with associated numerical values (mm/yr) are representative of the direction and
rate of tectonic convergence computed after the GPS velocity vectors from Fossa, Trapani, and Ustica
stations. A possible future evolution for this belt involves the subduction of the Tyrrhenian oceanic
lithosphere toward the south beneath the Maghrebian fold-thrust belt. The trench may develop at the rear
of the south Tyrrhenian seismic belt as indicated in Figure 7 (future trench). C.B. indicates the Cefalù
basin located along the northern coast of Sicily.
may be interpreted as connected with the inversion of basinbounding high-angle normal faults.
[22] The fault zones imaged through the Bayloc seismological analysis (Figures 3 and 7) apparently do not coincide
with the Drepano Thrust (i.e., as drawn by Pepe et al.
[2005] by interpolating data from seismic profiles), which
marks the tectonic boundary between the inner crystalline
thrust sheets, in the north, and the outer sedimentary ones,
in the south. This suggests that the present seismic activity
occurs along faults younger than the Drepano Thrust (i.e.,
note in Figure 7 that most seismic clusters cut across the
Drepano Thrust), such as basin-bounding normal faults
generated during the late Neogene-Quaternary back-arc
extension or such as newly generated faults. Against this
hypothesis, Pepe et al. [2005] suggested that an inherited
thrust (i.e., Oligocene – early Miocene in age) was the
hypocenter for the compressional seismic sequence of
September 2002. A proper campaign of seismic reflection
profiling in the southern Tyrrhenian Sea may clarify the
origin of the observed high-angle fault zones and their
tectonic relationships with the Drepano Thrust.
5.2. Present Kinematics
[23] Focal mechanisms show that the south Tyrrhenian
seismic belt is mostly accommodating reverse displace-
ments in response to a maximum compression oriented
approximately NNW-SSE (Figure 5). This evidence, combined with the cross-sectional attitude of the seismic segments (i.e., dipping toward the north, Figure 3), shows that
the general vergence of the belt is presently toward the
south.
[24] In previous papers, it was demonstrated that on the
NW striking faults at the eastern edge of the south Tyrrhenian seismic belt (i.e., the Sisifo and Tindari faults
[Finetti and De Ben, 1986; Billi et al., 2006]) right-lateral,
strike-slip displacements are presently accommodated.
These faults presumably transfer the reverse displacements,
which are presently active on the south Tyrrhenian belt,
toward the southeast in the Calabrian arc [Goes et al., 2004;
Pondrelli et al., 2004; Gutscher et al., 2006]. By analogy
with the kinematics of the Sisifo and Tindari faults, the NW
striking seismic zones included in the south Tyrrhenian belt
may act as transfer structures shifting the reverse displacements accommodated on the NE striking compressional
zones. The focal mechanisms available for the study area
(i.e., M 4, Figure 5) provide, however, evidence only for
the reverse displacements accommodated on the NE striking
faults and not for possible strike-slip displacements accommodated on the NW striking faults. This can be explained
by assuming that minor strain release occurs on the transfer
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faults (i.e., M < 4). Compressional earthquakes on the
master reverse structures should be, in fact, greater in
magnitude than the strike-slip earthquakes on the transfer
structures because the amount of slip accommodated on the
transfer structures is only a fraction of that occurring on the
master reverse structures. According to this model, strikeslip earthquakes along NW striking faults should generally
be less than 4 in magnitude and are therefore absent in
Figure 5, where only focal mechanisms of M 4 earthquakes are displayed. In the south Tyrrhenian area, the
impossibility to estimate fault plane solutions of M < 4
earthquakes is connected with the nonhomogeneous geometry of the seismic network (i.e., absence of offshore
stations, Figure 2).
[25] The right-stepping pattern of the NE striking reverse
structures is very similar to that observed along the
same plate margin in Algeria and off the Algerian coast
[Meghraoui et al., 1986], where focal mechanisms from
recent earthquakes are very similar to those presented in this
paper [Stich et al., 2003].
[26] Indirect evidence for strike-slip faulting along the
south Tyrrhenian belt is provided by the occurrence of ENE
striking reverse and thrust faults at Ustica. These faults
imply considerable horizontal oblique-compressional displacements. Reverse and thrust faults, in fact, develop as
low-angle surfaces for the effect of contractional strain,
which is horizontally transmitted through a continuum
medium (i.e., rocks). Because the faults observed at Ustica
are at the summit of a conic volcano as high as 2000 m at
least [Marani et al., 2004], the only way to transmit some
horizontal tectonic contraction in such a location is via a
high-angle transpressional fault cutting across the entire
volcano. In such a model, the observed surficial faults
would represent the expression of reverse-to-transpressional
fault segments occurring within a larger WNW striking
strike-slip-to-transpressive fault system (i.e., positive
flower structure) across the Ustica volcano [Bousquet and
Lanzafame, 1995]. The occurrence of differently oriented
fault segments within a larger strike-slip fault system is
common across orogenic wedges [e.g., Sylvester, 1988;
Holdsworth et al., 1998]. This model may also explain
some of the observed high-angle zones of seismic deformation in the south Tyrrhenian belt (Figure 3).
[27] Recently published GPS data for the SicilianTyrrhenian region [D’Agostino and Selvaggi, 2004] are
used to estimate the present relative motion across the south
Tyrrhenian belt (Figure 7). By combining the velocity
vectors from the Ustica station with those from the Trapani
and Fossa stations along the northern coast of Sicily (note
that Fossa is right on the western block of the Tindari Fault,
Figure 7), we obtained that the two blocks that face along
the south Tyrrhenian seismic belt are horizontally approaching by about 7 mm/yr along the N174° direction, in the
eastern sector (i.e., result obtained by combining the velocity vectors from Ustica and Fossa stations), and by about
3 mm/yr along the N124° direction, in the western sector
(i.e., result obtained by combining the velocity vectors from
Ustica and Trapani stations). These estimated motions
across the south Tyrrhenian belt are consistent with the data
presented in this paper (see diagrams A and B in the inset of
Figure 4 and diagram B in Figure 6a), according to which
the south Tyrrhenian seismic belt mostly involves ENE and
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NE striking reverse faults and NW striking right-lateral
strike- or oblique-slip faults. A greater slip rate in the
eastern sector of the south Tyrrhenian belt is consistent
with the occurrence of a retreating slab beneath the Calabrian arc [Faccenna et al., 2004; Montuori et al., 2007].
[28] By considering about 500 kyr, at least, as the period
of activity for the south Tyrrhenian compressive belt [Goes
et al., 2004] and a reverse heave rate between 3 mm/yr
(western sector) and 7 mm/yr (eastern sector), we obtained
an estimate of the total reverse heave produced since 500 ka
between about 1500 and 3500 m for the western and eastern
sectors, respectively. Because of errors in the original data
[D’Agostino and Selvaggi, 2004], however, the above
estimates based on GPS data should be considered as
moderately accurate.
5.3. Seismic Potential
[29] The south Tyrrhenian compressive belt has been so
far depicted as a continuous structure along the entire
northern coast of Sicily (i.e., an about 250 km long thrust
[e.g., Serpelloni et al., 2005; Billi et al., 2006]). A maximum seismic potential of magnitude between 7.1 and 7.6 or
even M 8 has been recently proposed for this structure
[Jenny et al., 2006]. The segmented geometry of the belt as
shown in Figure 3 gives new insight about its seismic
potential. Previously published tectonic [Giunta et al.,
2004; Pepe et al., 2005] and morphological [Marani et
al., 2004] data in the same area support our results
concerning the segmented geometry of the belt.
[30] By using the standard relationships by Wells and
Coppersmith [1994], a rupture area of about 1800 km2 can
be estimated for an earthquake with reverse mechanism and
magnitude in the 7.1– 7.6 range. Such an earthquake implies
a rupture length on the order of 100 km, at least, if crustal
thickness and fault dip values of 15 km and 60°, respectively,
are assumed. The epicentral map obtained through the
Bayloc method (Figure 3) suggests that no fault segments
with such a length should reasonably occur in the belt.
[31] The maximum magnitude recorded in the belt during
the last decades has been 5.9 (i.e., the 6 September 2002
earthquake; earthquake 14 in Table 1). Also, recent reviews
of historical data [e.g., Guidoboni et al., 2003; Azzaro et al.,
2004; Working Group CPTI, Catalogo parametrico dei
terremoti Italiani, versione 2004 (CPTI04), Istituto Nazionale di Geofisica e Vulcanologia, Bologna, Italy, 2004,
available at http://emidius.mi.ingv.it/CPTI04/] have led to
estimate offshore locations (i.e., within the south Tyrrhenian
belt) and magnitude values of about 6 for the three earthquakes (1726, 1823 and 1940), which, during the last five
centuries, have produced the most severe effects along the
northern coast of Sicily.
[32] From the above considerations, we conclude that all
available data, including the geologic and tectonic information, suggest a maximum seismic potential for the south
Tyrrhenian belt lower than 7.1– 7.6 or even 8. It is evident
that intrinsic approximations of the available relationships
between earthquake magnitude and rupture size [e.g., Field
et al., 1999; Dowrick and Rhoades, 2004], as well as
inaccuracy of historical earthquake catalogs do not allow
us to definitely rule out the existence of a structure in the
study area that may generate an earthquake with magnitude
over 7; however, the presently available information lead us
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Table 1. Order Number, Date, Origin Time, Hypocentral Parameters, Magnitude, P and T Axes Attitude (i.e., azimuth and plunge) for the
Focal Mechanisms Displayed in Figure 5a
N
Dateb
OT
Lon, °E
Lat, °N
Depth, km
Mag
P_az
P_plg
T_az
T_plg
Sourcesc
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
780415
800528
800601
810622
900328
950827
980117
980620
980621
980621
980914
990214
020405
020906
020906
020910
020920
020927
020928
021002
060227
23.33
5.57
5.26
9.36
5.47
19.42
12.32
2.25
8.59
12.59
5.24
11.45
4.52
1.21
1.45
2.32
23.06
6.10
2.46
22.57
4.34
15.07
14.25
14.33
14.09
14.91
15.17
12.75
13.08
12.67
13.10
13.60
15.06
14.74
13.57
13.73
13.70
13.74
13.66
13.71
13.72
15.18
38.39
38.48
38.39
38.49
38.16
38.28
38.45
38.46
38.43
38.50
38.46
38.17
38.48
38.42
38.44
38.47
38.46
38.41
38.47
38.46
38.15
21.0
12.0
10.0
13.0
24.1
8.9
10.0
10.0
10.0
10.0
10.0
17.7
15.0
15.0
15.0
15.0
16.1
15.0
15.0
15.0
15.0
5.7
5.5
4.8
4.8
4.2
4.0
4.8
5.2
4.6
4.6
5.0
4.7
4.4
5.9
4.7
4.4
4.7
5.2
4.6
4.9
4.5
18
159
334
323
195
185
342
349
348
349
349
179
347
329
137
315
325
324
340
325
251
8
5
6
1
20
15
17
23
10
7
15
77
6
6
2
20
12
22
7
7
67
116
249
148
56
280
90
193
184
208
123
187
300
102
231
232
91
177
170
109
215
108
42
5
84
71
0
5
71
66
78
79
74
7
77
53
64
64
76
65
80
69
18
A and J
A and J
POND04
POND04
this study
this study
POND02
RCMT
RCMT
RCMT
RCMT
RCMT
RCMT
CMT
RCMT
RCMT
RCMT
CMT
RCMT
RCMT
QRCMT
a
Abbreviations are N, order number; OT, origin time; and Mag, magnitude. Hypocentral parameters are longitude (Lon), latitude (Lat), and depth.
Date is given in year, month, and day format.
c
Bibliographic sources are abbreviated as follows: A and J, Anderson and Jackson [1987]; POND04, Pondrelli et al. [2004]; POND02, Pondrelli et al.
[2002]; RCMT, regional centroid moment tensor; and QRMCT, quick regional centroid moment tensor the Istituto Nazionale di Geofisica e Vulcanologia
catalog (available at http://www.ingv.it); and CMT, the Harvard CMT catalog [Dziewonski et al., 1981] (available at http://www.seismology.harvard.edu).
b
to consider a maximum magnitude close to 7 as more
realistic for earthquakes within the south Tyrrhenian belt.
5.4. Implications for the Regional Plate Tectonics and
for Future Scenarios
[33] Since about 700 – 500 ka, contractional deformations
across the Maghrebides in Sicily have resumed at the rear of
the orogenic wedge and are presently active across the south
Tyrrhenian belt [Goes et al., 2004]. The causes for this
process are presumably connected with the continental
collision and the associated substantial cessation of subduction across the Maghrebides during Quaternary times
[Lickorish et al., 1999; Faccenna et al., 2004]. Tomographic
images show a large slab window beneath Sicily and the
southern Tyrrhenian Sea, thus supporting the hypothesis of
a substantial cessation of subduction in Sicily [Faccenna et
al., 2005; Montuori et al., 2007]; however, geodetic and
seismic data show that the Eurasia-Nubia convergence in
this region is still ongoing and progresses with high convergence rates [Hollenstein et al., 2003; D’Agostino and
Selvaggi, 2004; Pondrelli et al., 2004]. This explains why
the Maghrebian fold-thrust belt is still active in its south
Tyrrhenian portion, where the back-arc passive margin is
being compressionally inverted (Figures 5 and 7). A similar
tectonic history has been inferred for the northern passive
margin of the Tyrrhenian ocean, (i.e., in the Ligurian Sea,
Figure 1), where compressional inversion started about 5 –
3 Myr ago [Bigot-Cormier et al., 2004; Chardon et al.,
2005], and for the westward prolongation of the south
Tyrrhenian margin in the Alboran Sea and off the Algerian
coast (Figure 1) [Comas et al. [1992]; Stich et al. [2003];
Déverchère et al. [2005]; Domzig et al. [2006]. In the
Alboran-Algerian segments of the Nubia compressive margin, the compressional inversion of the back-arc passive
margin started earlier (i.e., about 7 – 5 Myr ago) than in the
south Tyrrhenian region. The eastward migration of the
inversion tectonics along this margin (i.e., from the
Alboran-Algerian to the Tyrrhenian regions) is probably
connected with a parallel migration of continental collision
and associated cessation of subduction across the west
Mediterranean subduction zone [Faccenna et al., 2004].
The temporal gap between onsets of inversion tectonics in
the Alboran-Algerian margin and in the south Tyrrhenian
one may explain the different, present kinematics of thrusts.
Whereas the south Tyrrhenian active thrusts mostly verge
toward the foreland (i.e., southward), the Alboran-Algerian
active thrusts mostly verge toward the hinterland (i.e.,
northward). This evidence suggests different evolutionary
stages for the Alboran-Algerian and south Tyrrhenian compressional margins. The older Alboran-Algerian margin
may truly represent a case of incipient southward subduction of the back-arc basin as proposed by Déverchère et al.
[2005], whereas the younger south Tyrrhenian margin may
still be in a very early stage of contraction involving the
compressional reactivation or inversion of inherited, north
dipping faults [Pepe et al., 2005]. However, because further
northward subduction of the Hyblean-Pelagian foreland
beneath the Maghrebian fold-thrust belt is highly unlikely
for the continental nature of this foreland lithosphere, with
the progression of the Eurasia-Nubia convergence, a likely
future scenario may involve the subduction of the Tyrrhenian oceanic lithosphere toward the south beneath the
Maghrebian fold-thrust belt. The nucleation of subduction
may occur earlier off the Algerian coast and then propagate
toward the east involving the south Tyrrhenian region.
[34] Models of subduction inception predict a dynamic
uplift for the overriding plate during the early stages of
compression [Cloething et al., 1990; Shemenda, 1992; Toth
and Gurnis, 1998; Faccenna et al., 1999]. The Holocene
tectonic uplift of northern Sicily (highest uplift rate probably
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BILLI ET AL.: SOUTH TYRRHENIAN SEISMOTECTONICS
exceeding 2 mm/yr [Rust and Kershaw, 2000]) may thus be
explained, at least in part, by a process of subduction
inception in the south Tyrrhenian region. Further geophysical studies off the Sicilian coast are necessary to constrain
both temporally and spatially the uplift process. It should be
noted also that both the Ligurian (northern Tyrrhenian Sea)
and the Alboran-Algerian margins are presently undergoing
uplift [Bigot-Cormier et al., 2004; Domzig et al., 2006].
[35] In the south Tyrrhenian area, the inversion of the
passive margin and the subsequent subduction could be
favored by the young and thin Tyrrhenian oceanic lithosphere [Cloething et al., 1982, 1989; Muller and Phillips,
1991; Erickson and Arkani-Hamed, 1993]. On the other
hand, the young and poorly dense Tyrrhenian lithosphere
will be not enough negatively buoyant to initiate a selfsustained subduction [Cloos, 1993]. This suggests that, in
the Tyrrhenian region, the Eurasia-Nubia convergence will
likely lead to an Alpine-type collisional scenario [e.g.,
Beaumont et al., 1996], where continental collision will
follow a brief shortening and partial subduction of a small
and young oceanic basin. A Pacific-type subduction, where
an old and negatively buoyant ocean is able to long subduct
beneath a continent, is far a less appropriate scenario for the
future evolution of the Eurasia-Nubia margin in the Tyrrhenian region.
6. Conclusions
[36] 1. The use of a new technique of earthquake location
(Bayloc) in the south Tyrrhenian region has significantly
contributed to define the tectonic architecture and kinematics
of the seismogenic apparatus of the Nubia plate compressive margin in this region. Main results show that the
studied compressive belt is characterized by spatially discontinuous epicentral clusters with major NW-SE and SWNE trends and by hypocentral clusters dipping toward the
north by angles greater than 60°. These seismic clusters are
here interpreted as high-angle, S verging, reverse fault zones
forming an early configuration of a young compressive belt
(i.e., younger than about 700– 500 ka). This belt occurs
along and inverts a late Neogene – Quaternary passive
margin, which developed at the southern border of the
Tyrrhenian back-arc basin.
[37] 2. On the basis of instrumental and historical seismic
data combined with tectonic and morphological information, earthquakes with a maximum size close to magnitude 7
are proposed for the south Tyrrhenian compressive seismic
belt. At present, the segmented geometry of this belt should
prevent from the occurrence of stronger earthquakes.
[38] 3. An integrated monitoring system including measurements of deformation and seismicity also in offshore
areas [e.g., Beranzoli et al., 1998, 2000] will significantly
contribute in knowing the evolution of the seismogenic
apparatus and therefore in mitigating the earthquake and
tsunami hazards in this densely populated region.
[39] 4. Results from this work combined with previously
published geodetic data provide clues to predict possible
future scenarios, one of which involves the development of
a new subduction zone in the southern Tyrrhenian area
where the Tyrrhenian oceanic lithosphere would subduct
toward the south beneath the Maghrebian fold-thrust belt in
Sicily.
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[40] Acknowledgments. R. Arculus, S. Goes, F. Pepe, and an anonymous Associate Editor are thanked for patient editorial handling and for
insightful comments. L.A. is thanked for a text proofreading.
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A. Billi and C. Faccenna, Dipartimento di Scienze Geologiche,
Università Roma Tre, Largo S. L. Murialdo, 1, I-00146 Rome, Italy.
([email protected])
G. Neri, B. Orecchio, and D. Presti, Dipartimento di Scienze della Terra,
Università di Messina, I-98166 Messina, Italy.
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