Seismic response and wavefield characterization using a very dense 2D seismic array on an active landslide (Cavola, Italy)

Abstract

A dense 2D array (95 broad-band stations) was installed in a grid-like configuration over a 130x56 m area on the active landslide of Cavola in northern Apennines. Outcropping rocks consist of Monte Piano (fractured flysch and clayey melanges) and Ranzano (muddy and sandstone turbidite) formations of Eocene-Oligocene age. These units are covered by a 4-km long landslide with maximum thickness more than 60 m. In historical times there are three instances of activity triggered by meteorological events, in 1938, 1940, and 1960. The latter event was the most damaging, involving an area of 1.3 km2 and partial destruction of the Cavola village. Recent SAR interferometric analyses coupled to inclinometer data for the supposedly dormant period 1995-2001 show movements between 10-15 mm/yr affecting mainly areas close to the landslide side-scarps and to a new industrial district with active sliding surface at a depth of 10-15 m. The landslide structure beneath the array was reconstructed by means of active and passive seismic surveys, in-situ velocity measurements and geoelectrical tomography obtaining clay thickness ranging from 0 to 45 m and with average shear-velocity of 350 m/s. The inferred model is used for numerical 2D simulation of earthquake ground motion for different incidence angle and type of waves. The data from 11 of more than 100 local earthquakes recorded at the array have been used for the comparison with model results. The long data records (3 months of continuous recording) allow investigation of properties of ambient noise in terms of temporal stability and wavefield composition. We find that local industrial sources affect the H/V spectral ratios and their interpretation in terms of resonant frequencies. Analysis of noise polarization is also performed, revealing different patterns depending on the nature of the noise source.