Geological and geophysical characterization of the Paganica – San Gregorio area after the April 6, 2009 L’Aquila earthquake (Mw 6.3, central Italy): implications for site response.

We present the results of a geological and geophysical investigation, aimed at the seismic microzonation for post-earthquake reconstruction, of the Paganica-San Gregorio area damaged by the April 6, 2009 L'Aquila earthquake. The area is characterised by a thick pile of lacustrine and alluvial deposits (up to 190 m thick or more) accumulated syntectonically within a NW-SE extensional tectonic basin and covering a morphologically and structurally articulated carbonate bedrock. The extensive cover of continental deposits justifies the widespread local stratigraphic amplifications of the ground motion measured by passive seismology (weak motion and noise), with some apparently contradicting results such as the absence of significant amplification peaks within the large Paganica alluvial fan. In general, we found an excellent agreement between the results from the spectral ratio analysis of the ground motion and the structural-stratigraphic setting obtained by independent geological (surface and well data) and geophysical (ERT) constraints. Besides the 1D amplification effects, spectral ratio analyses revealed significant amplifications over a broad frequency range along both the horizontal and vertical components suggesting that 2D (or possibly 3D) site effects are related to the architecture of the buried bedrock.