Page 2 - Seismogenic_evidence_2007

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210 G. LAVECCHIA ET ALII

1. INTRODUCTION Sicilian crust and emerging at the outer front of the Sicil-
ian thrust system, along the Sciacca-Catania belt.
The Apennine-Maghrebian fold-and-thrust belt sys- In this paper, we present a contribution to this discus-
tem extends with continuity from the Po Plain-Adriatic sion focusing attention on the kinematics and the distribu-
region to southern Sicily (fig. 1a). It developed since late tion of the historical and instrumental seismicity located in
Oligocene times through the progressive nucleation of the areas to the rear of the Pescara-Pesaro segment of the
inward-dipping crustal slices thrust over the Adriatic- Po Plain-Adriatic front and of the Sciacca-Catania segment
Pelagian foreland (LAVECCHIA, 1988). According to sev- of the Sicilian front. In particular, we will investigate if the
eral authors, the entire belt ended its activity in early major earthquakes may be linked to reverse shearing along
Pleistocene times (PATACCA et alii, 1990; BUTLER et alii, two crust-scale basal thrusts, here respectively called Adria-
1992; LICKORISH et alii, 1999; DI BUCCI & MAZZOLI, 2002; tic Basal Thrust (ABT) and Sicilian Basal Thrust (SBT).
PATACCA & SCANDONE, 2004), but according to some oth- With this aim we will reconstruct the ABT and SBT three-
ers it is still undergoing compression, at least in eastern- dimensional shape; we will compile an updated and inte-
central Italy and in Sicily. In particular, on the basis of grated dataset of all the major events that occurred within
the combined analysis of historical, seismological and the two study areas during the last two thousands years
geomorphologic data, VANNOLI et alii (2004) outlined a and, through the analysis of the associated macroseismic
rather regularly segmented active fault system running fields, we will define likely depth ranges of the seismogenic
parallel to the shoreline from Ancona to Rimini (NW-SE), sources. By comparing the 3-D shape of the basal thrusts
made up of blind thrusts accompanied by anticlinal with the kinematics and with the surface and depth distrib-
growth. MONTONE et alii (2004), through the analysis of ution of the overlying earthquakes, we will define the
borehole breakouts, showed that the maximum horizon- boundaries of homogeneous seismogenic provinces. After
tal stress is generally perpendicular to the strike of the verifying the completeness of the seismological dataset, we
shallow (0-6 km depth) middle Pliocene to Quaternary will also try to compute and quantitatively compare the
thrust structures of the Po Plain and Coastal-Adriatic energy release in the two study areas.
domain. On the basis of the analysis of instrumental seis-
mological data, several authors considered that this
domain is undergoing active compression (FREPOLI & 2. THE ABT AND SBT BASAL THRUSTS
AMATO, 1997 among many others). In fact, earthquake
focal mechanisms show a predominance of thrust and The ABT overthrusts the Umbria-Marche meso-ceno-
strike-slip solutions with P-axes generally perpendicular zoic sedimentary cover and the overlying Plio-Pleistocene
to the trend of the outer thrust system (PONDRELLI et alii, foredeep terrains above the Adriatic foreland. It emerges
2004). LAVECCHIA et alii (2003) identified an individual, with an eastward convex shape along the Pesaro-Adriatic
regional scale structure, responsible for such seismicity in Sea-Pescara front and deepens westward, cutting through
the outermost westward-dipping thrust plane of the Apen- the entire crust and reaching its base beneath the Umbria
nine fold-and-thrust belt, which emerges along the Po region (BARCHI et alii, 1998; FINETTI et alii, 2001; LAVEC-
Plain-Adriatic front. CHIA et alii, 2003). The SBT overthrusts terrains pertaining
In Sicily, BOUSQUET & LANZAFAME (1986) were the to the Sciacca-Saccense tectonic unit and to the Gela-
first to document Quaternary E-W contractional deforma- Catania early-middle Pleistocene foredeep deposits above
tion, at the external limit of the thrust belt in eastern the Pelagian-Hyblean foreland (GHISETTI & VEZZANI,
Sicily. MONACO et alii (1996), based on seismological, 1984; CATALANO et alii, 1989; BUTLER et alii, 1992;
structural and morphotectonic observations, suggested TORELLI et alii, 1998). It emerges with a southward convex
Late Quaternary activity of the Sciacca thrust, an inner shape along the Sciacca-Gela-Catania front and deepens
splay of the outer basal thrust. LANZAFAME et alii (1997) northward, not only cutting upper crust Plio-Pleistocene
documented field evidence of Late Pleistocene-Holocene sediments, but also mid-crust and lower crust levels as
N-S compression in the Etna area, with a shortening of a imaged by recent seismic profiles (FINETTI et alii, 2005b).
few kilometres. TORELLI et alii (1998) recognised, in a The ABT and SBT are very similar in size and 3-D shape.
seismic line across the Catania foredeep, a clear incipient This is well highlighted in fig. 2 by the geometry of their
northward-dipping reverse shearing, which cuts obliquely depth-contour lines, which we have schematically recon-
through the Pleistocene sediments. CARBONE et alii (2000) structed through the use of a large number of regional
pointed out geomorphologic evidence of reactivation of geological and seismic cross-sections sections available in
out of sequence thrusts beneath Etna and in the Nebrodi the literature (fig. 1b-1c and references therein). In central
area. Furthermore, CATALANO et alii (2004 and 2006) Italy, the isobaths have an arched shape that substantially
showed evidence of Late Quaternary tectonic inversion of follows the surface ABT trajectory. They are more closely
the northern border faults of the Scordia-Lentini Plei- spaced near the northern and southern ends and, on aver-
stocene graben, at the front of the Gela-Catania thrust age, dip about 20° westward reaching a depth of about
plane, with formation of post 40 ka E-W trending folds. 30 km beneath the Apennine mountain chain. In Sicily,
COCINA et alii (1997) and NERI et alii (2005) documented also, the isobaths have an arched shape that substantially
N-S compressional seismogenic activity at depths follows the surface SBT trajectory; they are more closely
between 10 and 30 km in eastern Sicily, beneath the Etna spaced near the western and eastern concavity and more
volcanic area, whereas CACCAMO et alii (1997) and JENNY distant in the central sector. This feature reflects the SBT
et alii (2006) recognise N-S seismogenic compression in dip-attitude, which is steeper in proximity to the Mazara-
western and central Sicily. LAVECCHIA et alii (2006) iden- Sciacca and Gela-Catania areas, and flatter in the Agri-
tified an individual, regional scale structure, possibly gento-Canicattì-Licata area. On average, the SBT dips
responsible for such a compressional seismicity, in a Plio- about 25° northward reaching a depth of about 30 km
Quaternary north-dipping thrust zone cutting the entire beneath the northern coast of Sicily.