Recorded ground motion and site effects evaluation for the April 6, 2009 L'Aquila earthquake

On April 6, 2009 a ML = 5.8 earthquake hit the city of L’Aquila on the Apennine chain in central Italy. Notwithstanding the moderate-size event the L’Aquila city and several small villages along the Aterno river valley suffered severe damage, because of the unusual strong motions, mainly due to proximity to the fault (estimated hypocentral depth of about 10 km). In this paper the main features of the recorded motion are discussed. Four accelerometric stations were located within the surface projection of the fault and recorded peak values ranging from 0.4 to 0.6 g. The recorded motions were characterised by short durations and high peak accelerations both in the horizontal and vertical directions. The strong portions of vertical and horizontal motions occurred almost simultaneously due to the short travel paths of P and S waves from the fault to the ground surface near the fault area. Hence site response analyses were performed for the sites where recording stations were located. The geotechnical subsoil model was derived by boreholes, in situ dynamic tests (D-H and SDMT) and by laboratory tests (RCT). One-dimensional numerical analyses were carried out employing the well known computer code EERA. The numerical model was calibrated, in the linear equivalent range, by comparing numerical results with the horizontal acceleration recorded components.