Seismicity
The huge collection of P- and S-waves arrival times recorded by permanent seismic networks operating in Italy yields a very complete seismic catalogue (CSI1.1, Castello et al., 2006). The distribution of thousands of earthquakes that have occurred in the past 30 years give a consistent picture of seismicity in northern-central Apennines (Figure 1). Seismicity follows the Apennines range and its lateral geometrical offsets and rotations. The seismic belt consists of a western stripe of shallow seismicity with hypocenters mostly located above 8-12 km depth and an eastern belt with deeper crustal earthquakes (hypocentral depth of about 18-20 km). The northern portion of the Apennines has a more intense and diffuse seismicity (Chiarabba et al., 2005), with respect to the central Apennines that was mostly silent in the past 20 years before the 2009 L'Aquila earthquake, with a few and sparse earthquakes (Bagh et al., 2007).
Figure 2. Focus on the normal fault system.
Details of seismicity in the region struck by the recent normal faulting earthquakes. White dots are epicenters form the CSI catalog. The mainshocks and aftershocks of the 1979 Norcia, 1997 Colfiorito, 2009 L’Aquila seismic sequences are shown with a colour scale relative to hypocentral depths. Fault segments from .. are shown. Not the striking continuity of the NNW-striking normal faulting system.
There are two main regions with intermediate-depth earthquakes (35 > z >70 km): one located in the north-eastern part and one in the central region (Umbria-Marche), separated by an area where this type of seismicity is poor (Figure 1). None of the intermediate-depth earthquakes occur further west, where a high velocity anomaly is found in the shallow mantle by teleseismic tomography (white dashed line, modified after Giacomuzzi et al, 2010). Numerous studies on subduction zones document that earthquakes in the Benioff zone, sometimes delineating a double seismic layer (Brudzinski et al., 2007), occur for metamorphism and dehydration of slabs (Hacker et al., 2003). In our case, deep focus earthquakes are likely generated by the dehydration of the lower crust in the flexed Adria lithosphere, down to the depth after which the eclogitization is probably complete. The positive velocity anomaly at 70 km depth defines the top of the eclogitized Adria lithosphere.
The external belt of deep crustal earthquakes is characterized by reverse faulting mechanisms (Frepoli and Amato, 1997; Piccinini et al., 2006; Chiarabba et al. 2009a) with compression at mid – low crustal depths. Generally, this deep seismicity has moderate magnitude. Historical earthquakes such as the 1741, 1799, 1873 and the 1943, Imax<9, are those characteristic for this region.
Figure 3. Fault geometry by aftershocks.
Vertical sections of aftershocks across the 1997 Colfiorito, 2009 L’Aquila and 2009 Laga Mt faults (locations from Chiarabba et al., 2009c and Chiarabba et al., 2009b). The seismicity delineates gently dipping faults that flatten at depth.
Figure 4. Normal faults vs. thrusts from tomography.
Details of the upper crust structure across two main thrusts of the central-northern Apennines (Fema-Cavallo Mts., and Teramo thrust) from the seismic tomography by Chiarabba et al. (2009c) and Chiarabba et al. (2010). In the first case, the Mw5.4 October 14 1997 earthquakes originated on a deep steep ramp of the Fema thrust (bold. dashed white line), while the upper portion is more propagated eastward on a flat located at 2-3 km depth (white dashed line).
The western belt of seismicity underneath the mountain range accommodates the extension of the Apennines wedge revealed by geodetic data (Hunstad et al., 2003; Serpelloni et al., 2005; D'Agostino et al., 2008; Devoti et al., 2008). The recent large magnitude normal faulting earthquakes occured on southeastward gently dipping (40-50°) normal faults (Chiaraluce et al., 2004; Chiarabba et al., 2009b). Figure 2 shows the past year's seismicity of the normal faulting belt in the central area from Umbria-Marche to Abruzzi. It is strikingly evident the NNW-elongated fault system, along which moderate and large magnitude earthquakes develop, spanning from the 1979 Norcia to the 2009 L'Aquila events (Figure 2). The normal fault system is almost continuous, but fragmented into segments with length in the order of tens of kilometres. The distribution of aftershocks in vertical sections of aftershocks, across the portions of the fault system for which high resolution seismological data are available (Figure 3), reveal that the normal faults have dip less than 40-50° and present flattening both on decollements at depth (between 6 and 9 km) and in some cases upward (see section 2). The strictly linear stripe of normal faulting earthquakes does not mirror the surface trend of pre-existing faults, except in a few cases. Although such an argument has been used to contrast the possibility that normal faults re-activate old thrusts (see Chiarabba et al., 2009c and references therein for a discussion), the striking linearity of the fault system and the gentle dip of the ruptured faults are strong support to this concept. One explanation for this inconsistency could be a de-coupling of deformation between upper, sometimes rootless, nappes and the deep units occurring along shallow decollements, mostly developed within the Mesozoic cover, (2-4 km depth). Recently developed local earthquake tomographic studies of the Umbria-Marche 1997 and Abruzzi regions (Chiarabba et al., 2009; Chiarabba et al., 2010) shows that the upper sedimentary cover is deformed by compressional tectonics differently from the deeper portion of the upper crust and the main thrusts seems to be decolled at shallow levels (figure 4). The shallow portion of the stack units has average displacements of a ten of kilometers, in agreement with geologic data (Mazzoli et al., 2005), while at depth the thrusts have high angle ramps within the basement.