Seismic noise measurements along the slope of the L'Aquila terrace

Abstract

L'Aquila downtown (Central Italy) suffered strong damages during the April 6th 2009 Mw 6.3 earthquake. In particular many collapses and strong damages on buildings have been observed near the southern edge of the L'Aquila downtown. L'Aquila is bounded southward by the Aterno river valley along a steep scarp. The downtown is settled on a calcareous terrace, mainly composed of a Pleistocene stiff calcareous breccia over-imposed to ancient lacustrine sediments. The upper portion of the L'Aquila breccia in the area of Via XX Settembre, which borders the southern edge of the top of the morphological terrace of L'Aquila downtown, is irregularly affected by the presence of residual soils known as "red soils" or filling material. The continental deposits are characterized by a velocity inversion at a depth ranging from few tens up to one hundred meters. We performed extensive ambient vibration measurements along the south-western slope of the L'Aquila terrace, where some RC structures without anti-seismic design were heavily damaged during the Mw 6.3 mainshock. Noise measurements were conducted along several sections starting from the top of the terrace toward the base of the Aterno river valley. The aim was to investigate the relationship between horizontal-to-vertical amplitude spectra ratio (H/V method) with the geological and geomorphological variations existing along the flanks of the L'Aquila terrace. We also performed noise measurements within some open underground caves that are present along the slope of the calcareous terrace. We integrate our geophysical data with the available geological, geotechnical and geophysical data, performed for the seismic microzonation and the reconstruction of the damaged buildings. We find that the resonance frequency is varying from about 0.6 to 1 Hz for sites at the top of terrace and in proximity of the Aterno river valley, respectively. Larger amplitude of the resonance peak is observed at sites on the top of the terrace rather than the measurements performed at the base. Additionally, the H/V curves at the bottom of the L'Aquila terrace show an amplification in a broader frequency band. The variation in the H/V shapes, moving along the flanks of the L'Aquila terrace, is mainly related to a different behavior of the vertical component. Indeed the vertical recordings show a spectral minimum shifted at higher frequency for measurements nearby the Aterno river.