No abstract

Abruzzi region (central Italy) producing vast damage in the L'Aquila town and surroundings. In this paper we present the location and geometry of the fault system as obtained by the analysis of main shock and aftershocks recorded by permanent and temporary networks. The distribution of aftershocks, 712 selected events with M L ! 2.3 and 20 with M L ! 4.0, defines a complex, 40 km long, NW trending extensional structure. The main shock fault segment extends for 15-18 km and dips at 45°to the SW, between 10 and 2 km depth. The extent of aftershocks coincides with the surface trace of the Paganica fault, a poorly known normal fault that, after the event, has been quoted to accommodate the extension of the area. We observe a migration of seismicity to the north on an echelon fault that can rupture in future large earthquakes.

.[1] We studied the anatomy of the fault system where the 2009 L'Aquila earthquake (M W 6.1) nucleated by means of~64 k high-precision earthquake locations spanning 1 year. Data were analyzed by combining an automatic picking procedure for P and S waves, together with cross-correlation and double-difference location methods reaching a completeness magnitude for the catalogue equal to 0.7 including 425 clusters of similar earthquakes. The fault system is composed by two major faults: the high-angle L'Aquila fault and the listric Campotosto fault, both located in the first 10 km of the upper crust. We detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops. We observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays. The width of the L'Aquila fault zone varies along strike from 0.3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution, up to 1.5 km at the fault tips with an increase in the geometrical complexity. These characteristics, similar to damage zone properties of natural faults, underline the key role of aftershocks in fault growth and co-seismic rupture propagation processes. Additionally, we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological (i.e., creeping) discontinuity explaining how normal faults detach at depth.

[1] On 6 April (01:32 UTC) 2009 a M 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.

S U M M A R YThe NW-SE trending Val d'Agri extensional basin is one of the regions in Italy with the highest seismogenic potential. Field data do not univocally define which of the fault systems bordering the basin on the two opposite sides is accommodating the active deformation. In this study, we detect and locate, by using an automatic picking procedure, almost 2000 low-magnitude earthquakes (−0.2 < M L < 2.7) recorded by a dense network during a 13-months-long seismic experiment. Events are mostly located along the southwestern flank of the basin. To the south, intense swarm-type microseismicity defines a major cluster ∼5 km wide from 1 to 5 km depth. To the west, a clear alignment of events, characterized by normal faulting kinematics, defines a NE-dipping normal fault between 1 and 6 km depth. The upward continuation of this structure, ∼5 km long, matches a mapped active normal fault recognized by field and palaeoseismological surveys.A temporal correlation found between the intense swarm-type microseismicity and the water level changes in the nearby artificial Pertusillo lake suggests that this seismicity is reservoir-induced.