Basin-effects observed during the 2012 Emilia earthquake sequence in Northern Italy

Abstract

During the 2012 Emilia earthquake sequence, prolonged shaking associated with long-period motions were observed in the Po Plain basin in Northern Italy. Such anomalous characteristics of ground-motion were unnoticed at the rocky sites outside the Po Plain basin. To explain these phenomena, a series of detailed analyses were carried out using the strong motion records from May 20 to May 29 main events. The observed amplification, calculated using the spectral ratio method indicates a fundamental resonance period at 5s. Well-dispersed surface Rayleigh waves of periods between 3 and 10s were noticed and the contribution of surface waves to the total motion was significant despite the source being located beneath the Plain. The late arriving long-period surface waves significantly increased the duration of ground shaking. The envelope delay spectrum shows that the duration lengthening of ground motion could be well correlated with the dispersion of surface waves. The greatest lengthening of the records was observed around the fundamental period of the basin. Large peak ground-motions were observed in the near-field region, especially in the vertical component which is attributed to source effects (predominantly vertical movement of the causative fault) while the prolonged duration of motions seems caused by surface waves.