The local seismic response and the effects on the buildings at L'Aquila downtown of the 2016 Central Italy earthquake

The historical center of L'Aquila was founded on a terrace that raises about 50 meters above the Aterno riverbed and is formed by al-luvial Quaternary breccias consisting of limestone clasts in a marly matrix. The breccias are over imposed to a 200 meters thick lacustrine sediments formed mainly of silty and sandy layers and minor gravel beds, which lie over calcareous bedrock 300 - 400 meters deep. This geological context deeply affects the dynamic behavior of the for-mations and the local seismic response on the surface. The microzon-ing studies, indeed, evidenced the presence of low frequency (about 0.4 - 0.6 Hz) amplification widespread in the historical center. The presence of this particular site effects, produce local amplification in the range of fundamental frequencies of tall buildings and seismic iso-lated structures. In the historical center of L'Aquila, most of the build-ings are two or three storey high and only few building in the southern part of the downtown (XX Settembre street ) rise over six/ seven sto-rey. Conversely, after the earthquake which struck the town April, 6 2009 and the reconstruction phase which followed and still ongoing, seismic isolation system was adopted in many new public (C.A.S.E. project, ANAS headquarter etc...) and private buildings or to retrofit existing structures. In the historic center of L'Aquila, many seismic isolation works have been carried out, also on historical masonry buildings. The paper analyzes the effects of the August, 24 and Octo-ber, 30 2016 recent earthquakes on long fundamental period buildings located in L'Aquila downtown, in particular seismic isolated buildings, focusing on the proper assessment of the seismic action to adopt in low frequency amplification site, also with reference to the NTC 2008 provisions.