Site effect investigation in the Aterno valley using earthquake data after the Mw 6.3 April 6 L'Aquila earthquake

On April 6, 2009, a Mw 6.3 normal faulting event struck the Abruzzo region close to the city of L'Aquila, central Italy, causing many casualties and serious damage. Soon after the mainshock, a dense network of about 30 velocimetric and accelerometric stations was installed in the epicentral area along the Aterno valley (south-east of the city of L'Aquila). Several Institutions cooperated in the deployment of the network with the aim of studying possible site effects and the response of the buildings. Here we present preliminary results based on spectral analyses of selected earthquakes that occurred between April 8 and April 27 with local magnitudes larger than 3.0, including the 9 April (Mw 5.4) strong aftershock. Both horizontal-to-vertical (H/V) spectral ratio and standard spectral ratio (SSR) techniques were used. In general, significant site effects are found for some settlements in the middle portion of the Aterno valley, located both on recent soft alluvial soil and on mid-Pleistocene silts. The high ground-motion amplifications partially explain the observed macroseismic intensity values (≥ IX on MCS scale). Nearby villages, only a few kilometers apart, show lower amplification effects in agreement with the observed level of damage (VI to VII MCS).