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

The number of seismic isolated buildings has increased in recent years in Italy. In particular, in the city of L'Aquila, mostly affected by the 2009 earthquake, many new seismic isolated buildings were built and seismic isolation systems were also incorporated into many existing buildings. The frequencies range in which these buildings operate is substantially different from that of classical buildings. In particular, the seismic isolation, allowing the periods of the first modes of vibration to be generally moved beyond 2 s, in the absence of local amplifications, drastically reduces the stresses on the structure due to seismic events. On the other hand, special conditions relating to the morphology and stratigraphy of soils can produce amplifications, even considerable, in the operating frequencies typical of such buildings. This appears to be the case for L'Aquila. The historical centre of the town, indeed, was founded on a terrace rising about 50 m above the Aterno riverbed and is formed by alluvial Quaternary breccias consisting of limestone clasts in a marly matrix over imposed to 200 m thick lacustrine sediments which lie over a 300-400 m deep calcareous bedrock. This geological context deeply affects the dynamic behaviour of the formations and the local seismic response on the surface. The microzoning studies outlined a large presence of low frequency amplification (about 0.4-0.6 Hz) in the historical centre. The paper focus on the proper assessment of the seismic action to adopt in low frequency amplification sites with reference to the current Italian seismic regulations, also in perspective to the design of seismic isolated buildings, and evaluate the effectiveness of 1D and 2D local seismic response methods to predict surface amplification effects.