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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS017 Oral Presentation 2366
Brittle-ductile transition and cutoff depth of seismicity in the Apennines
belt
Prof. Vincenzo Pasquale
DIPTERIS Universit di Genova Universit di Genova IASPEI
Paolo Chiozzi, Gianluca Gola
The thickness of the seismogenic layer in the continents is closely related to the mechanical strength of
the lithosphere and accordingly to the occurrence of large earthquakes. The seismicaseismic boundary
is thought to be related to the thermally-controlled brittle-ductile transition and the largest earthquakes
in the crust occur near the cutoff depth of seismicity. Nevertheless, in some young orogenic belts, the
connection between the thermal field and mechanical properties is still unclear. The paper focuses on
this topic and analyses lateral variation of lithosphere rheology beneath the central-northern sector of
the Apennines belt (Italy). The seismic activity recorded in the time period 1981-2002 is scattered along
the entire belt. The number of quakes and the energy released are larger in the Internal Zone of the
belt, where the maximum focal depth is 97 km. In this zone, the quakes define a slab dipping westward
at an angle of 45 and the event with maximum local magnitude (5.9) occurred at 21 km depth. In the
eastern part of the belt (External zone), the distribution of earthquakes versus depth presents two
peaks; one at 5 km and another, more prominent, at 25 km narly at the depth of the maximum
magnitude (5.1) earthquake. The b-value of the frequency-magnitude relation decreases with depth and
eastwards; the largest value (0.955) is inferred in the western part of the belt (Peri-Tyrrhenian Zone),
which shows extensional tectonics, few earthquakes (maximum magnitude 4.3) and higher terrestrial
heat-flow density (80-120 mW/m2). Since the lithosphere thermal state across the belt is far from
equilibrium and at different stages of relaxation, rheological modelling was based on a transient thermal
approach. Consequently, the expected mechanical behaviour is a function of time and position. The
bottom depth of the brittle layer is 10 km in the Peri-Tyrrhenian Zone and 28 km in the External Zone,
and the corresponding temperature range is of 310-380 C. In the External Zone, characterized by a
compressional tectonic regime and large average mechanical strength, a 6 km thick brittle layer in the
upper mantle with limiting temperature of 600 C below a ductile crustal layer is expected. In the
Internal Zone, the deeper earthquakes are related to thermal conditions controlled by the vertical
descent rate and time since the beginning of the lithosphere subduction. The empirical relation between
the maximum depth of seismicity and the age of subduction allowed us to infer a convergence velocity
of 2 cm/yr and a minimum temperature at the seismicity cutoff depth of 750 C.
Keywords: heat flux, rheology, seismicity