A reversed hierarchy of active normal faults: the 6 April 2009, Mw 6.3, L’Aquila earthquake (Italy)

Understanding the relationship between seismogenic slip at depth and surface deformation is fundamental in any seismic hazard analysis because the assessment of the earthquake potential of large continental faults relies largely on field investigations. The well-documented 5 6 April 2009, Mw 6.3, L’Aquila earthquake affords a unique opportunity to explore the relationships between the activity of the deep source and its surface evidence. We used available high-resolution geologic, geodetic and seismological data aided by analogue modeling to reconstruct the geometry of the seismogenic rupture in relation with surface and sub-surface faults. We contend that the earthquake was caused by a blind fault, controlled at depth by pre-existing discontinuities and expressed at the surface by pseudo-primary breaks resulting from coseismic crustal bending. Finally, we propose a scheme for hierarchizing normal faults that explains all surface occurrences related to blind faulting in the frame of a single, mechanically coherent, interpretative model. Failure to appreciate such complexity may result in severe over–or under-estimation of the local seismogenic potential.