Page 5 - Visini_de Nardis_Lavecchia_2010

Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S243–S264 S247

By integrating constraints on tectonic style from seismic compression. According to others (Neri et al. 2005;
lines and geologic data with the traditional constraints from Lavecchia et al. 2007a, b), a nearly E–W striking exten-
historical and instrumental seismicity catalogues, Lavec- sional seismogenic domain is interposed between the two.
chia et al. (2007a, b) have interpreted both the mainland- This last point of view is supported by the presence in the
southern Sicily and the Marche–Adriatic seismotectonic Peloritani and Nebrodi–Madonie areas of northern Sicily of
provinces as corresponding to active deformation volumes a neat belt of extensional focal mechanisms, which are in
at the hanging-wall of inward-dipping regional-scale basal continuity with the Apennine and Calabria extensional
thrusts named, respectively Sicilian Basal Thrust (SBT) seismic belt (Pondrelli et al. 2006).
and Adriatic Basal Thrust (ABT) (Fig. 2 a4, b5). Both the In the current paper, we will adopt this last point of view
SBT and the ABT could penetrate the entire crust to a and will compute the corresponding pattern and rate of
depth of at least 25 km with an average dip of nearly 25– deformation, separately.
30°. In both regions, the thickness of the seismogenic layer
deduced from seismic data coherently deepens inward from
shallow depths to mid and lower crustal depths well sup-
Procedure
porting such a configuration. Therefore, in a 3D view the
Adriatic and Sicilian provinces may be schematically
In order to estimate the velocity tensor representative of
represented as crust-scale wedge-shaped seismogenic
the active deformation within the Marche–Adriatic,
volumes. In map view, the two provinces correspond to
mainland-southern Sicily and Southern Tyrrhenian seis-
arc-shaped, outward convex polygons, whose boundaries
motectonic provinces, we have used the procedure pro-
coincide with the map-view thrust front trajectory and with
posed by Papazachos and Kiratzi (1992), integrating
the surface projection of the basal thrust 25 km depth
analyses of seismological data and geological information
contour line.
referring to the geometry of the deformed volume. This
More complicated and highly questionable are the
procedure requires not only knowledge of complete his-
geometry and nature of the seismogenic structures
torical and instrumental records over a certain magnitude
responsible for the E–W seismic belt of the Southern
threshold and of the associated Gutenberg–Richter (G–R)
Tyrrhenian, where a relevant component of the Europa–
parameters, but also knowledge of the shape and size of
Nubia NW–SE convergence is considered to be accom-
the seismogenic volume, as well as that of the kinematics
modated (D’Agostino and Selvaggi 2004; Goes et al. 2004;
of the active deformation deduced from reliable fault
Pondrelli et al. 2004; Serpelloni et al. 2005). Such a
plane solutions.
compressional strip began its activity in Late Pleistocene
The procedure is articulated in the following steps:
times (Goes et al. 2004), inverting the pre-existing exten-
_
sional structures of the thin-crust Tyrrhenian domain. Some a. computation of the scalar seismic moment rate M o
authors (Jenny et al. 2006) associate the seismic activity from historical and instrumental earthquakes,

with a north-verging back thrust developed at the hanging- b. computation of the moment tensor F ij from focal
wall of the subduction plane which dips northward beneath mechanism data,
Sicily and the Tyrrhenian Sea and emerges along the south c. computation of the average seismic strain rate tensor

Sicily thrust front. Other authors (Chiarabba et al. 2005; _ e ij ;
Serpelloni et al. 2005) hypothesise a south-verging thrust d. computation of the components of the velocity tensor
which would represent the active front of the compres- (U ij ),
sional deformation between the Nubia and European plates e. evaluation of the errors which may be involved in the
now shifted nearly 150 km northward with respect to the above computations.

former, now abandoned, southern Sicily front of the _
The scalar seismic moment rate M o within each
Apennine–Maghrebian compressional belt. Others (Billi
seismogenic volume in the years of the completeness
et al. 2007) assume a general southward vergence of the
interval may be calculated by means of Molnar’s (1979)
belt and associate it with an incipient flip of the subduction
relation:
plane, hypothesising a south-dipping subduction of the
A
oceanic Tyrrhenian crust beneath Sicily. M o ¼ M omax 1 B ð1Þ
_
An open fundamental question in the seismotectonic 1 B
b
interpretation of Sicily involves the spatial and geometric where A = 10 [a?(bd/c)] , B ¼ ; a and b are values of the
c
relations between the Southern Tyrrhenian and the main- Gutenberg–Richter relation, M o max is the scalar moment of
land-southern Sicily compressional provinces. According the largest observed earthquake in the region, M o ¼
to some authors (Kiratzi 1994; Jenny et al. 2006), the two 10 c M s þd ; c and d are constants of the moment–magnitude
are in physical continuity and undergo a common coaxial relation (Kanamori and Anderson 1975). The advantage of
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