Il contributo della geologia alla comprensione dei terremoti

SESSIONE 1
Struggling from surface observations to seismogenic structures, and
the contribution of seismic reflection profiles
Key words: Active tectonics, earthquake geology, seismogenic
structures, seismic reflection profiles.
INTRODUCTION
Field geology data can greatly help defining the parameters
that characterize active faults. Features like fault direction, fault
length, and fault segmentation can often be constrained by careful
field studies (eg., GALLI &BOSI, 2003). These studies are of
paramount importance in paleoseismology, particularly as a way
to assess the recurrence time interval of larger earthquakes.
However, they address only the surface expression of tectonic
processes that affect the upper kilometers of the Earth crust. On
the other hand, earthquakes, particularly those of larger
magnitude, tend to originate near the base of the brittle crust
(MARONE &SCHOLZ, 1988). Therefore, the link between the
seismogenic fault, located at some kilometers depth, and its
surface expression is not always straightforward. The finding that
in several instances, particularly in compresissonal regime,
active faults can be blind (e.g., MYERS et alii, 2003) adds further
complexities, sometimes resulting in poorly constrained inference
on the seismogenic source. Data on fault length and fault slip can
be used to define the seismogenic source. However, simplified
fault slip models are usually adopted to fit surface observation,
and the result is greatly affected by the quality and spatial
coverage of available constraints.
The seismicity of the Italian region has been well defined by
20 years of instrumental rcording by the INGV seismic network
(CHIARABBA et alii, 2005). Active tectonics occurs with a variety
of structural styles, but deformation rates are rather low,
(VANNUCCI et alii, 2004; SERPELLONI et alii, 2007), for instance
with respect to the Anatolian and Aegean regions; therefore, the
geomorphological/stratigraphic signal of blind faulting can be
subdue, particularly when competing with processes like
differential compaction, gravitational adjustments and fluid
escape.
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(*) ISMAR-CNR, Bologna, andrea.argnani@ismar.cnr.it
ANDREA ARGNANI (*)
2
SOME EXAMPLES
Examples illustrating some of the open and debated issues
concerning active tectonics in the Italian region will be presented,
with particular emphasis on case studies from marine areas. Three
examples of marine areas characterized by large historical
earthquakes are briefly introduced in the followings.
Peri-Gargano region - Seismic data support the conclusion
that the Pliocene-Quaternary deformation offshore of the
Gargano Promontory is progressively localized along a narrow E-
W belt in the South (ARGNANI et alii, 2009a) and along a NE-SW
belt in the North (ARGNANI et alii, 2002), whereas in the same
time span the whole of the Gargano Promontory has been
uplifted. The Present-Day deformation around the Gargano
Promontory is mainly restricted to the regions located to the N-
NE of the promontory. Deformation is typically rather diffuse and
of limited extent; basement was likely involved, as suggested by
both depth of earthquakes and trends of the structures, that bear
little relationship with those of the adjacent fold and thrust belts.
Compressional reactivation of extensional Mesozoic faults is
considered as the most likely mechanism for Neogene
deformation. Messina Straits – In the Messina Straits it is difficult
to find a single fault that is long enough to account for the Mw
7.1, 1908 Messina earthquake (ARGNANI et alii, 2009b, and
references therein). Moreover, the trend of the observed faults,
though consistent with faults onshore Calabria, is different from
the trend of many of the faults proposed on the basis of inversion
of seismological and geodetic data. Seismic data show that there
is no N-S fault cutting the sedimentary successions south of the
epicenter the 1908 earthquake. In fact, the only long fault that is
cutting the sea floor is located at the SW tip of Calabria, trends
NW-SE and dips to the west with a low angle. Such fault
parameters are not compatible with the 1908 earthquake, as the
hypocentral depth would be located too far to the west. A system
of interconnected faults, partly exploiting pre-existing fault
planes, can better describe the observed geological (i.e., long
term) deformation; an interpretation that leaves the possibility to
have more than a fault active at the same time. At present it is
difficult to say whether the observed active faults are just the
surface expression of a single, deeper and blind seismogenic
fault, or if they truly represent the complex response of an area
where more than one fault can be active at the same time. The