Roberto De Franco

Polarization filter with singular value decomposition

We present a singular value decomposition (SVD) based algorithm for polarization filtering of triaxial seismic recordings based on the assumption that the particle motion trajectory is essentially 2-D (elliptical polarization). The filter is the sum of the first two eigenimages of the SVD on the signal matrix. Weighing functions, which are strictly dependent on the intensity (linearity and planarity) of the polarization, are applied. The efficiency of the filter is tested on synthetic traces and on real data, and found to be superior to solely covariance‐based filter algorithms. Although SVD and covariance‐based methods have similar theoretical approach to the solution of the eigenvalue problem, SVD does not require any further rotation along the polarization ellipsoid principal axes. The algorithm presented here is a robust and fast filter that properly reproduces polarization attributes, amplitude, and phase of the original signal. A major novelty is the enhancement of both elliptical and linear polariz...

Earth pyramids: Precarious structures surviving recurrent perturbations

Earth pyramids are tall tapered spires, slender or stocky, made of rock or soil material. This type of features have been marginally studied. Even if they are often included in parks and geosites. We start from this lack of studies to analyse the geometrical and physical-mechanical characteristics considering some case studies in northern Italy. Because of their geometrical characteristics these elements are intrinsically weak and can be used as indicators of past external perturbations. We performed laboratory characterization and numerical modelling to analyse the involved actions, the stability and the reaction to dynamic perturbations.

Alluvial fan shifts and stream captures driven by extensional tectonics in central Italy

Subsidence over the last 0.78 myr of a basin in central Italy bounded by a normal fault caused the deactivation and uplift of an Early–Middle Pleistocene alluvial fan at the fault footwall. Uplift of the fan occurred with a basin-bounding fault slip-rate of the order of 0.2 mm a−1. Subsidence resulted in the reorganization of the river network due to a fall in base level, which triggered headward erosion, stream piracy effects and drainage inversion. The mapped river inversions and catchment piracy were related to the distribution of a quantile regression of 134 alluvial fans v. basin areas. Despite the fact that the two parameters were well fitted by a power law relationship, all the fans corresponding to the captured rivers lay above the regression line (in the fan area field), whereas those corresponding to the capturing rivers were below the regression line (in the basin area field). We propose a general model of alluvial fan growth in active extensional settings that helps to interpret this scatter of fan v. basin area distribution and to identify the most active fault segments. Such an approach can better constrain fault activity in a time window that bridges long-term deformation and the present day deformation inferred from geodesy and/or seismology, increasing our understanding of the steadiness/unsteadiness behaviour of faults.