The Mw 6.3, 2009 L'Aquila earthquake: Linear and nonlinear site effects

Ground motion has been computed for the 6 April 2009 earthquake (Mw=6.3) by using a hybrid method (modal summation and finite difference techniques) along a geological section crossing L'Aquilla. Sources have been simulated both as point and extended, with unilateral and bilateral ruptures. Spectral amplifications of about 10 have been found in correspondence of the cover of the Aterno river alluvial deposits and span a wide range of frequencies, that is various typologies of structures. In addition, the modelling has evidenced that whereas strong heterogeneitiies are present, for the same set of synthetic seismograms, the H/V spectral ratios are not able to evidence the spectral amplifications obtained with the 2D/1D response spectra ratios. Possible nonlinear soil effects have been evaluated for PGA values higher than 0.1-0.2 g, depending on the strain-dependent soil properties, with the linear equivalent approach, by using literature variations of shear moduli and damping with strain.