Seismic site effects at near-fault strong-motion stations along the Aterno River Valley during the Mw=6.3 2009 L'Aquila earthquake

Abstract

An Mw=6.3 earthquake originating at a normal fault in proximity of the city of L'Aquila produced a significant amount of near-fault strong-motion data recorded by an array deployed in the upper Aterno River Valley, NW of L'Aquila. This set of data deserves attention since it is the first well documented earthquake occurring in a near-fault area which was recorded from a moderate magnitude earthquake on a normal fault. For most of the stations geotechnical conditions are ascertained by means of drilling and geophysical investigation. To provide insights on the relevance of site conditions on measured near-source ground motions, peak and spectral values of acceleration and velocity recordings at stiff soil sites are compared with those at “rock” reference sites. Events of magnitude ranging between 3.0 and 6.3 (main event) were used. The results show that peak and response spectra values of ground motion, for the horizontal component and to a lesser extent for the vertical one, are affected by soil profile characteristics, especially for the station located on alluvium at the centre of the valley. For this station, observed amplification ratios were compared with numerical ones computed from 1D site response analyses. Evidence is presented that 1D numerical modelling is not able to completely explain the recorded motions.