Site effects at L’Aquila for the April 6, 2009 (MW=6.3) earthquake.

Ground motion for the 6 April 2009 (Mw 6.3) earthquake has been computed along 2-D cross-sections at L’Aquila by a hybrid method (modal summation plus finite differences) and validated with recordings. A general agreement has been obtained between synthetic and observed response spectra, through parametric studies of S-wave velocities of the shallowest lithotypes. It results that the megabreccia covering on lacustrine soils, characterizing the historical center of L’Aquila, is responsible of spectral amplifications along the vertical (2-7) and horizontal components (2-3) at a wide frequency range (0.6-7 Hz). The covering of alluvial soils in the middle Aterno river valley is responsible of amplifications at 2-7 Hz both in the horizontal and vertical planes of the motion. Such amplifications evidence that site effects might have been responsible of structural damages.