First evidences of fast creeping on a long-lasting quiescent earthquake normal-fault in the Mediterranean

A key issue in our understanding of the earthquake cycle and seismic hazard is the behaviour
of an active fault during the interseismic phase. Locked and creeping faults represent two
end-members of mechanical behaviours that are given two extreme rupturing hazard levels,
that is, high and low, respectively. Geophysical and space geodetic analyses are carried out over
the Pollino Range, an extensional environment within the Africa–Eurasia plate boundary, to
disclose the behaviour of the long-lasting quiescent Castrovillari normal fault. Fault trenching
evidenced at least four large earthquakes (6.5–7.0 M w ) in the past and an elapsed time of
1200 yr since the last event. Inversion of Differential Interferometric Synthetic Aperture
Radar and Global Positioning System over a decade shows fast creeping at all depths of the
fault plane. The velocity-strengthening creeping zone reaches maximum rates 20 mm yr −1
against an average rate of about 3–9 mm yr −1 . It limits the southern-weakening locked part of
the fault. An essential condition for the generation of a large earthquake on the Castrovillari
fault, as has occurred in the past, is a rupture through the velocity-strengthening zone. The
Castrovillari fault yields the best evidence for being both a strong and weak fault during
its earthquake cycle. Creeping at rates faster than its tectonically driven ones, it must thus
consist of a mix of unstable and conditionally stable patches ready to sustain a sizeable
earthquake. Quantifying and mapping the slip rate over the fault plane is important because they
influence fault moment budget estimate and helps to constrain constitutive laws of fault zones.
Aseismic slip also redistributes stress in the crust, thereby affecting the locations of future
earthquakes.