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

Abstract

[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.

On the complexity of surface ruptures during normal faulting earthquakes: excerpts from the 6 April 2009 L'Aquila (central Italy) earthquake (<i>M</i><sub>w</sub> 6.3)

“…4c), although in nearby areas the observed Vp and Vs values correspond to the characteristic velocities of the Meso-Cenozoic limestones and terrigenous rocks Di Stefano et al, 2011). The sequence is well described by a detailed doubledifference catalog of relocated events (3000 events with M ≥ 1.9 from Chiaraluce et al, 2011; ∼ 64 000 events of all magnitudes from Valoroso et al, 2013). When coupled with moment tensor solutions for the largest shocks (M w ≥2.7; Herrmann et al, 2011), these data allow imaging of individual faults activated during this complex sequence in great detail (Fig.…”

“…It came as the culmination of a long foreshock/aftershock sequence recorded by permanent and temporary INGV seismometers (Chiarabba et al, 2009;Chiaraluce et al, 2011). The earthquake caused intensity up to IX-X MCS effects in a small number of villages located southeast of L'Aquila, but the largest number of collapsed buildings and casualties was reported in L'Aquila itself.…”

“…In this respect we wish to recall that, although very shallow aftershocks were imaged between 1 and 3 km depth near the northern end of the seismogenic source, no aftershocks where observed between the surface and 2-3 km depth in its central portion, i.e., near Paganica (Chiarabba et al, 2009;Chiaraluce et al, 2011;Chiaraluce, 2012;Valoroso et al, 2013). This could be well explained by the velocity strengthening behavior of faults at shallow crustal depth, that is to say, in the upper stability transition (UST) zone defined by Scholz (1988), but two additional explanations are equally likely: (1) the fault normal stress, which controls the effective coefficient of friction of the rupture, might be especially low in the shallowest portion of the fault plane, thus generating stable sliding (e.g., Brace and Byerlee, 1970); or, more simply, (2) there is no fault plane continuity in the shallowest 2-3 km of the crust in the Paganica area.…”

Geodetic model of the 2016 Central Italy earthquake sequence inferred from InSAR and GPS data

“…Geodetic data show that the sequence evolved along a main SW dipping normal fault system, relative to the MGVB alignment. The 24 August M w 6.1 main shock ruptured two distinct segments of this fault system (Figure 4a), corresponding to the northern part of the~50°SW dipping Mount Gorzano Fault, which was partially activated in its southern portion during the 2009 L'Aquila earthquake [Chiaraluce et al, 2011;Bigi et al, 2013;Cheloni et al, 2014], and to the southern part of the~40°SW-dipping Mount Vettore Fault, respectively. The main shock occurred, with a bilateral rupture, between these two fault segments, possibly merging into a single SW dipping structure at the hypocentral depth [Lavecchia et al, 2016;Tinti et al, 2016;Chiaraluce et al, 2017].…”

Spectral models for ground motion prediction in the L'Aquila region (central Italy): evidence for stress-drop dependence on magnitude and depth

“…The most recent seismic sequence started on 2009 April 6 with the extensively studied L'Aquila earthquake (M w 6.1, M l 5.8) and lasted for about one year (Chiaraluce et al 2011;Valoroso et al 2013).…”