The anatomy of the 2009 L’Aquila normal fault system (central Italy) imaged by high resolution foreshock and aftershock locations

摘要

On 6 April (01:32 UTC) 2009 aM_W 6.1 normal faulting earthquake struck the axial area of the Abruzzo region in central Italy. We study the geometry of fault segments using high resolution foreshock and aftershock locations. Two main SW dipping segments, the L’Aquila and Campotosto faults, forming an en echelon system 40 km long (NW trending). The 16 km long L’Aquila fault shows a planar geometry with constant dip (～48°) through the entire upper crust down to 10 km depth. The Campotosto fault activated by three events with 5.0 ≤ M_W ≤ 5.2 shows a striking listric geometry, composed by planar segments with different dips along depth rather than a smoothly curving single fault surface. The investigation of the spatiotemporal evolution of foreshock activity within the crustal volume where the subsequent L’Aquila main shock nucleated allows us to image the progressive activation of the main fault plane. From the beginning of 2009 the foreshocks activated the deepest portion of the fault until a week before the main shock, when the largest foreshock (M_W 4.0) triggered a minor antithetic segment. Seismicity jumped back to the main plane a few hours before the main shock. Secondary synthetic and antithetic fault segments are present both on the hanging and footwall of the system. The stress tensor obtained by inverting focal mechanisms of the largest events reveals a NE trending extension and the majority of the aftershocks are kinematically consistent. Deviations from the dominant extensional strain pattern are observed for those earthquakes activating minor structures.