Seismic site characterisation of Red Soil and soil-building resonance effects in L’Aquila downtown (Central Italy)

We present a fine-scale shallow geological setting and its possible influence on the seismic building behaviour of L’Aquila downtown which suffered many casualties and building damage caused by the 6 April 2009, Mw 6.1 earthquake. This study draws mainly on the integration of 666 borehole logs and 274 microtremor recordings interpolated with GIS-based techniques. The shallow geology of L’Aquila downtown is characterised by Middle Pleistocene calcareous L’Aquila breccias. Their top surface consists of a weathered epikarst zone that is mantled by Red Soil (reddish colluviated Alfisols) formed during the late Pleistocene. The areal distribution and thickness of Red Soil are causative of the medium microtremor frequency (3–13 Hz). The map of cross-correlation between the medium frequencies, estimated through the empirical Eurocode 8 equation, shows buildings with possible seismic coupling due to the shallow geology setting and the fundamental building period. This technique is further supported by the areal distribution of seismic building damages caused by the 2009 and 1703 L’Aquila earthquakes. This approach, carried out with a large geological and geophysical dataset, is useful in defining seismic site effects and may be effective in mitigating the seismic risk of cities with notable historical heritage in Central Italy, such as the L’Aquila downtown area.