**2. Geological and geophysical setting**

The epicentral area of the L'Aquila seismic sequence mainly corresponds to the upper and middle Aterno river valley which is bounded by predominantly NW-SE-striking and SWdipping active normal faults (e.g. [15] and references therein) and characterised by the high variability of the geologic and geomorphologic patterns (Fig. 1). The valley is superimposed on a Quaternary lacustrine basin of tectonic origin and surrounded by carbonates. The thickness of the Quaternary deposits is variable, from about 60 m in the upper Aterno river valley to more than 200 m in the middle valley.

The upper and middle crustal structure of the Abruzzo region has been reconstructed from VP and VP/VS images on a 3D grid with a node vertical spacing of 4 km, by using local earth‐ quake tomography and receiver function modeling [16]. Low Vp velocities (smaller than 4.8 km/s) were found in correspondence with the main Plio-Quaternary basins (Fucino, l'Aquila and Sulmona basins) and high VP velocities (Vp larger than 5.0 km/s) were mostly correspond‐ ent with the outcropping Mesozoic carbonates. Low VP and high VP/VS anomalies were found beneath the L'Aquila and Fucino Quaternary basins between 4 and 12 km depth, suggesting the existence of fluid-saturated rock volumes. Very high P-wave (6.7–7.0 km/s) and S-wave (3.6–3.8 km/s) velocity bodies were observed below 8–12 km depth, and interpreted as deep crustal or mantle rocks exhumed before the sedimentation of the Mesozoic cover. This layer has a regional character as it is found at ~15 km of depth below the 1°x1° cell containing L'Aquila epicentral area, and lying on the mantle detected at ~35 km [11].

The analysis of strong and weak motion recordings in 1996-98 put in evidence amplification ef‐ fect at low frequencies (0.6 Hz) in the town of L'Aquila and 2D numerical modeling allowed to fit it along a SW-NE section [17]. A sedimentary basin was inferred, filled by lacustrine sediments, with a maximum thickness of about 250 m, below the breccias formation about 50 m thick.
