**3. VS models**

que that takes into account source, propagation and local site effects. In order to estimate realistic ground motion we need physical parameters of rocks from surface to depths greater than the earthquake hypocenter. At engineering scale, microzoning activities promoted by the Italian Civil Defense Department [10] have performed VS measurements at depths around 25 m, in gravelly soils with different degree of cementation, alternating to thin layers of finer deposits (sands and/or silts) that often include carbonate boulders (www.cerfis.it). The investigated depths are too shallow to define the vertical and lateral passage from soft sediments to rock basement (VS at least of 800 m/s) which was sporadically found. At regional

Aim of this paper is to retrieve VS models of the shallow crust in the Aterno river valley from the non-linear inversion of the group velocity dispersion curves of the fundamental mode extracted with the FTAN method (e.g. [12,13,14]) from recordings of earthquakes with ML ≥ 2.9 (Table 1) between April 5 and November 10, 2009, in the selected coordinate window of 42.4 ± 0.2 N and 13.4 ± 0.2 E. In addition, VS of the superficial 30 m of Aterno alluvial soils are defined by an active seismic experiment in the Coppito area, and compared with nearby crosshole measurements. The VS profiles vs. depth are then attributed to lithotypes along a geolog‐ ical cross section from the epicenter to a seismic station at L'Aquila. Simulation of the main shock is performed with the NDSHA approach and the computed response spectra and the

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

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].

scale, a physical model is available extending to depths of about 300 km [11].

H/V spectral ratios are compared with those recorded.

80 Engineering Seismology, Geotechnical and Structural Earthquake Engineering

**2. Geological and geophysical setting**

valley to more than 200 m in the middle valley.
