Seismic site effects at near-fault strong-motion stations along the Aterno river valley during the Mw=6.3 L’Aquila earthquake.

An Mw=6.3 earthquake originating at a normal fault in proximity of the city of L’Aquila produced asignificant amount of near-fault strong-motion data recorded by an array deployed in the upper AternoRiver Valley, NW of L’Aquila. This set of data deserves attention since it is the first well documentedearthquake occurring in a near-fault area which was recorded from a moderate magnitude earthquakeon a normal fault. For most of the stations geotechnical conditions are ascertained by means of drillingand geophysical investigation. To provide insights on the relevance of site conditions on measurednear-source ground motions, peak and spectral values of acceleration and velocity recordings at stiffsoil sites are compared with those at ‘‘rock’’ reference sites. Events of magnitude ranging between3.0 and 6.3 (main event) were used. The results show that peak and response spectra values of groundmotion, for the horizontal component and to a lesser extent for the vertical one, are affected by soilprofile characteristics, especially for the station located on alluvium at the centre of the valley. For thisstation, observed amplification ratios were compared with numerical ones computed from 1D siteresponse analyses. Evidence is presented that 1D numerical modelling is not able to completely explainthe recorded motions.