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06 133-20 06(209-222) 25-06-2007 14:24 Pagina 213

SEISMOGENIC EVIDENCE OF ONGOING COMPRESSION IN EASTERN-CENTRAL ITALY AND MAINLAND SICILY: A COMPARISON 213

Good agreement may be also observed between the 12°E 1672 1875 1786 1916 May 14°E
active deformation pattern from focal mechanisms and 44°N 1916 Aug Pesaro,
2000 44°N
that deduced from the borehole breakouts sited in the Montegrimano Ancona
2000 PS 1924
areas above the shallower portions of the ABT and SBT
(MONTONE et alii, 2004). U U Urbino 1930
From two regional seismic catalogues available on Montefeltro 1897 AN 1269 sequence, 1972
1987
line (Catalogo della Sismicità Italiana 1981-2002 in Northern U 1690
Marche, 1991
CASTELLO et alii, 2005 and BOLLETTINO SISMICO ITALIA- U 56 B.C.
0 km 40 U MC
NO, 2003-2005), we extracted the events with Ml≥3.0 1741 Camerino
which occurred within the ABT and SBT surface areas Source depth range and
seismotectonic assignation 1799
(grey areas in fig. 2), at depths <40 km, in the period of the major earthquakes 1873 43°N
1981-2005. The earthquakes sited beneath the Etna vol- “Shallow” source (< 10 km), 100 B.C. P.S.Giorgio sequence, 1987
canic area at depth <10 km, were excluded from the Compressional Province 1943
dataset, given that they would be most likely linked with “Deep” source (10-30 km), TE
the volcanic dynamics rather than with the regional tec- Compressional Province Atri PE Southern
tonic process (CASTELLANO et alii, 1997; AZZARO, 1999; Shallow source (< 10-15 km), 1950 Marche, 1995
Extensional Province 1461 CH
PATANE & PRIVITERA, 2001; MONACO et alii, 2005) that we Uncertain depth
U U
are investigating in this paper. The ABT dataset consists U and/or seismotectonic 1762 15 km 10 km 5km a
assignation 13°E 30 km 25 km
of about 60 events with 3.0≤Ml≤5.0 within an area of
20 km
2
Historical seismicityseismicity
~14500 km ; the SBT dataset consists of about 130 events Instrumental Historical
Instrumental seismicityseismicity
1981-2005
1981-2005
360 B.C.-1980360 B.C.-1980
2
with 3.0≤Ml≤4.6, within an area of ~16200 km . Above the Local Magnitude (M l) Magnitude moment (Maw or Mw) Ancona 1972
and
ABT, 40% of the selected events is sited at depths less Belice 1968
than 10 km and 50% between 10 and 30 km; above the 3.01-3.5 3.51-4.0 4.01-4.49 4.5-4.9 4.5-4.9 5.0-5.4 5.5-5.9 6.0-6.6
SBT, 20% of events occur at depth less than of 10 km and 13°E 14°E 15°E
80% between 10 and 30 km. Belice sequence,1968
In eastern-central Italy and in north-eastern Sicily Etna,1995
Caltanissetta, Nebrodi Maletto,
good quality microseimic data (M l <4.5) registered by 1995 1992 1987 ME
local networks are also available (MONACHESI et alii, PA
2004; NERI et alii, 2005). The event depth distribution TP
30 km
Calatafimi 30 km
shows an inward deepening of the seismogenic source, U 25 km
25 km
down to a depth of about 30 km, which follows well the Marsala 1968 Troina 20 km
20 km
15 km
15 km
westward deepening of the ABT and the northward deep- U EN 10 km
10 km
ening of the SBT. This geometry is highlighted in fig. 3, Raddusa 5km
5 km
Sciacca 361 CL CT
where selected hypocenters (see source of data in the U 1818
Mazara del Vallo, Canicattì
figure caption) have been projected along the traces of 1981 Mineo
AG Caltagirone 1624
two interpretive crust-scale sections, one drawn along the Mt. Etna detail
trace of the DSS profile 1978 (after LAVECCHIA et alii, Gela 37°N
2003) and the other across the Etna volcanic edifice. This
latter section has been reconstructed integrating surface
data with speculative interpretation of deep crust sections
available in the literature such as the land side extension
of the CROP M26 seismological profile (FINETTI et alii, , 14°E 0 km 40 15°E b
2005b), the Castel di Tusa-Caltanissetta-Hyblean plateau 10 km
Fig. 2 - Epicentre distribution of historical and instrumental earthquakes (depth <40 km) within the areas above the 0-to-30 km depth-con-
tour lines (dotted lines) of the Adriatic Basal Thrust (a) and of the Sicilian Basal Thrust (b). The maps are in Italy Lambert Conformal Conic
projection. The shape of the depth-contour lines is based on the critical review and re-interpretation of the regional sections whose traces are
given in fig. 1b and c. The instrumental data are extracted from the CSI 1981-2002 seismic catalogue in (CASTELLO et alii, 2005) and from the
BOLLETTINO SISMICO ITALIANO, 2003-05, assuming Ml>3.0, GAP<200°, ErrH and ErrZ<4 km, RMS<0.8 s, phase readings >8 and depth <40
km. The historical earthquakes are mainly extracted from the CPTI04 parametric catalogue (WORKING GROUP CPTI, 2004) with integration
from other catalogues and detailed papers (see tabs. 2a and 2b and reference therein). The different epicentral colours of the major (Maw
≥4.5) earthquakes represent the inferred depth range of the associated seismogenic source and the proposed seismotectonic assignation.
Focal mechanisms refer to major (magnitude moment ≥4.0) earthquakes since 1968 (parametric data and references are given in tab. 1). The
inset in fig. 2b shows a selection of focal mechanisms computed by NERI et alii (2005) for microseismic events (2.2≤Ml≤3.7) recorded in the
period 1988-2001 at depth >10 km beneath the Etna volcano; the arrows represent the average compressional stress axis computed by NERI
et alii (2005) from the focal mechanism stress inversion.
– Distribuzione epicentrale della sismicità storica e strumentale localizzata a profondità <40 km nell’area compresa tra le isobate 0-30 km (linee
tratteggiate) dei sovrascorrimenti basali adriatico (a) e siciliano (b). La proiezione delle mappe è nella rappresentazione conica conforme di Lam-
bert. La ricostruzione dell’andamento delle isobate è basata sulla revisione critica delle sezioni regionali le cui tracce sono riportate in fig. 1 b e c. I
dati strumentali sono stati estratti dal catalogo sismico CSI (CASTELLO et alii, 2005) per l’intervallo 1981-2002 e dal Bollettino Sismico Italiano
per l’intervallo 2003-05, assumendo Ml>3.0, GAP<200°, ErrH e ErrZ<4 km, RMS<0.8 s, numero di fasi >8 e profondità ipocentrale <40 km. I ter-
remoti storici derivano principalmente dal catalogo parametrico CPTI04 (WORKING GROUP CPTI, 2004) con integrazioni (vedi il testo e riferimen-
ti bibliografici associati) nelle tabb. 2a e 2b). Gli epicentri dei terremoti maggiori (M aw ≥4.5) sono stati colorati diversamente a seconda della ipo-
tizzata profondità della sorgente sismogenetica e della proposta provincia sismotettonica di appartenza. I meccanismi focali si riferiscono ai
maggiori terremoti (magnitudo momento ≥4.0) a partire dal 1968 (i parametri focali e la bibliografia sono dati in tab. 1). Il riquadro in basso a
sinistra mostra una selezione di meccanismi focali elaborati da NERI et alii (2005) relativamente alla microsismicità (Ml tra 2.2 e 3.7) registrata
nel periodo 1988-2001 nell’area del Mt. Etna, a profondità >10 km; le frecce rappresentano l’orientazione media dell’asse dello sforzo di massima
compressione associato calcolato dagli stessi autori sulla base dell’inversione dei meccanismi focali.