The 2009 MW 6.1 L'Aquila fault system imaged by 64k earthquake locations

On April 6 2009, a MW 6.1 normal-faulting earthquake struck
the axial area of the Abruzzo region in central Italy. We investigate the complex
architecture and mechanics of the activated fault system by using 64k high-resolution
foreshock and aftershock locations. The fault system is composed by two major SW
dipping segments forming an en-echelon NW trending system about 50 km long: the
high-angle L’Aquila fault and the listric Campotosto fault, located in the first 10 km
depth. From the beginning of 2009, foreshocks activated the deepest portion of the
mainshock fault. A week before the MW 6.1 event, the largest (MW 4.0) foreshock
triggered seismicity migration along a minor off-fault segment. Seismicity jumped
back to the main plane a few hours before the mainshock. High-precision locations
allowed us to peer into the fault zone showing complex geological structures from
the metre to the kilometre scale, analogous to those observed by field studies and
seismic profiles. Also, we were able to investigate important aspects of earthquakes
nucleation and propagation through the upper crust in carbonate-bearing rocks such
as: the role of fluids in normal-faulting earthquakes; how crustal faults terminate
at depths; the key role of fault zone structure in the earthquake rupture evolution
processes.