Abstract
We present a revision and a seismotectonic interpretation of deep
crust strike-slip earthquake sequences that occurred in 1990-1991
in the Southern Apennines (Potenza area). The revision is motivated
by: i) the striking similarity to a seismic sequence that occurred
in 2002 \~140 km NNW, in an analogous tectonic context (Molise
area), suggesting a common seismotectonic environment of regional
importance; ii) the close proximity of such deep strike-slip seismicity
with shallow extensional seismicity (Apennine area); and iii) the
lack of knowledge about the mechanical properties of the crust that
might justify the observed crustal seismicity. A comparison between
the revised 1990-1991 earthquakes and the 2002 earthquakes, as well
as the integration of seismological data with a rheological analysis
offer new constraints on the regional seismotectonic context of crustal
seismicity in the Southern Apennines. The seismological revision
consists of a relocation of the aftershock sequences based on newly
constrained velocity models. New focal mechanisms of the aftershocks
are computed and the active state of stress is constrained via the
use of a stress inversion technique. The relationships among the
observed seismicity, the crustal structure of the Southern Apennines,
and the rheological layering are analysed along a crustal section
crossing southern Italy, by computing geotherms and two-mechanism
(brittle frictional vs. ductile plastic strength) rheological profiles.
The 1990-1991 seismicity is concentrated in a well-defined depth
range (mostly between 15 and 23 km depths). This depth range corresponds
to the upper pat of the middle crust underlying the Apulian sedimentary
cover, in the footwall of the easternmost Apennine thrust system.
The 3D distribution of the aftershocks, the fault kinematics, and
the stress inversion indicate the activation of a right-lateral strike-slip
fault striking N100E under a stress field characterized by a sub-horizontal
N142-trending sigma1 and a sub-horizontal N232-trending sigma3, very
similar to the known stress field of the Gargano seismic zone in
the Apulian foreland. The apparent anomalous depths of the earthquakes
(> 15 km) and the confinement within a relatively narrow depth range
are explained by the crustal rheology, which consists of a strong
brittle layer at mid crustal depths sandwiched between two plastic
horizons. This articulated rheological stratification is typical
of the central part of the Southern Apennine crust, where the Apulian
crust is overthrusted by Apennine units. Both the Potenza 1990-1991
and the Molise 2002 seismic sequences can be interpreted to be due
to crustal E-W fault zones within the Apulian crust inherited from
previous tectonic phases and overthrusted by Apennine units during
the Late Pliocene-Middle Pleistocene. The present strike-slip tectonic
regime reactivated these fault zones and caused them to move with
an uneven mechanical behaviour; brittle seismogenic faulting is confined
to the strong brittle part of the middle crust. This strong brittle
layer might also act as a stress guide able to laterally transmit
the deviatoric stresses responsible for the strike-slip regime in
the Apulian crust and may explain the close proximity (nearly overlapping)
of the strike-slip and normal faulting regimes in the Southern Apennines.
From a methodological point of view, it seems that rather simple
two-mechanism rheological profiles, though affected by uncertainties,
are still a useful tool for estimating the rheological properties
and likely seismogenic behaviour of the crust.
Users
Please
log in to take part in the discussion (add own reviews or comments).