**5. Conclusions**

A realistic estimation of the ground motion at L'Aquila for the MW 6.3 earthquake is obtained by the NDSHA approach, an innovative modeling technique that takes into account source, propagation and local site effects [8,9]. A key point is the definition of VS models representative of the seismic path, like those obtained from the non-linear inversion of Rayleigh group velocities of the fundamental mode extracted with the FTAN method from earthquake recordings and active seismic surveys. Very fractured carbonatic rocks with VS of ~1.4 km/s, covered by alluvial soils with a maximum thickness of ~0.4 km in the center of the Aterno valley are retrieved. The top of the carbonates with the average velocities of 5 and 2.8 km/s, compression and shear respectively, lays at 2-2.5 km of depth and rises to 1 km in the NW part of the valley. This result contradicts available seismic [17] and gravity [10] modeling of the carbonate horizon with VS=2.5 km/s and density ρ =2.6 g/cm3 at some hundred meters of depth. The carbonate horizon with VS of ~3 km/s is found at 4 km of depth. Moreover, the shallowest 30 m of alluvial soils have average VS of ~0.2 km/s against ~0.5 km/s as obtained from crosshole measurements at the AQV station, about 500 m distant. Such velocity difference evidences that strong lateral and vertical geological heterogeneities are present and that the cross-hole (and down-hole) point-like measurements, even though quite precise, may not be represen‐ tative of the average seismic path.

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The soundness of the synthetics is in the good fitting of the recorded H/V spectral ratio and response spectra, despite the point-source approximation. The lateral and vertical geological variability mainly due to the covering of megabreccias on soft soils are responsible of spectral amplifications, mostly for the vertical component, for a wide frequency range (0.5-7 Hz). Taking into account that the majority of the buildings, generally 2-5 floor, at the historical center of L'Aquila suffered serious damage, we can argue that spectral amplifications might have been responsible for damage, beside the near-field conditions. This study shows that realistic ground motion can be computed in advance for the several active faults of the L'Aquila district and that a sounded building code can be formulated for the restoration of the existing damaged buidings and for new building design.
