Subsurface profiling of buried valleys in central alps (northern Italy) using HVSR single-station passive seismic

Abstract

Active and passive seismic techniques permit to draw images of relevant buried morphologies in mountainous areas. Here we apply the Horizontal-to-Vertical Spectral Ratio (HVSR) passive seismic technique to draw the shape of a buried valley in the core of the Italian Central Alps, at the confluence among the fault-controlled Valtellina, Valchiavenna and Lake Como depressions (“Colico knot”). Twenty-three single-station, ambient-noise measurements were acquired and the site-specific fundamental frequency f0 was obtained by computing the HVSR in the 0.5 Hz to 5 Hz frequency band.Shear wave velocity (Vs)-versus-depth profiles were computed from the inversion of the HVSR curves, under the assumption of a 1D structure. To limit uncertainty, the results were calibrated by comparison with data from two sources: 1) HVSR profiles acquired in a comparable setting in Valtellina, at a site where estimates of Vs for the seismic basement and cover sediments were available; 2) borehole logs which crossed the sediments above the metamorphic bedrock at the Colico knot site. Independent data were used to constrain the best fit of the experimental HVSR peaks at the Colico knot site, in terms of frequency f0 and HVSR amplitude, thus retrieving information about Vs variations related to the bedrock versus sediments unconformity.A SW-NE elongated buried trough, as deep as 150 m below sea-level and up to 2 km wide was drawn from the present-day Adda valley to the Lake Como depression to the South. The morphology of this abandoned and buried course of the Adda river is controlled by active deep-seated gravitational slope deformation along the adjacent mountain slope. The valley was abandoned before the Last Glacial Maximum (latest Pleistocene), plausibly owing to narrowing and uplift induced by deep-seated slope deformation. In fact, the LGM Adda glacier reshaped the valley slopes and the bedrock watershed that separates at present the post-Glacial Adda river course from the paleo-Adda trough.