L. Lenti

Self-excitation process due to local seismic amplification responsible for the reactivation of the Salcito landslide (Italy) on 31 October 2002

[1]xa0An integrated numerical and geophysical approach was used to investigate the role of local seismic amplification in the reactivation of the Salcito landslide (Italy) after the Molise earthquake of 31 October 2002. Numerical stress-strain analysis with FLAC 5.0 FDM software, performed in dynamic configuration, showed that the 1 Hz frequency was consistent with landslide instability conditions. The Fourier spectrum of the triggering earthquake showed two main peaks occurring at 1 and 2 Hz, respectively, which could be related to local effects. The analysis of H/V spectral ratios of ambient noise records obtained in the landslide area, evidenced amplification effects in the 1–3 Hz range. On the basis of an engineering geology model, numerical modeling of both 1-D and 2-D seismic wave propagation was conducted using linear and nonlinear solutions. The simulation outputs showed (1) a 1 Hz amplification ascribable to 2-D effects due to a landslide mass lying within a basin-like geological structure; (2) a double-peak amplification at about 2 and 3 Hz, respectively, ascribable to 1-D resonance of the landslide mass; and (3) 1-D plus lateral wave effects within the landslide mass in the 2.5–3.5 Hz frequency range. These findings suggest that local amplification of ground motion by the Salcito landslide mass may generate a self-excitation process responsible for its reactivation.

Evidence of two-dimensional amplification effects in an alluvial valley (Valnerina, Italy) from velocimetric records and numerical models

Abstract The results of field surveys, velocimetric records, and numerical models are reported with the goal of analyzing the local seismic response in a section of the Nera River alluvial valley in central Italy. Alluvial deposit geometry and dynamic properties were defined by geological and geophysical investigations. 2D amplification effects were pointed out by horizontal-to-vertical spectral ratios (HVSRs) as well as by horizontal-to-horizontal spectral ratios (HHSRs) to a reference station; HVSRs were derived from both noise and weak-motion records, while HHSRs were only computed from weak motions. Where travertine deposits are interlayered within the alluvia, 2D amplification effects are shown only by HHSRs. The observed amplifications: (1)xa0consist of peaks of the amplification functions A ( f ) in the range 1–10xa0Hz, (2)xa0generally correspond to two frequency ranges whose values mainly depend on the valley shaped ratio and on the local heterogenities of the alluvial deposits, and (3)xa0can be regarded as due to a 1D resonance combined with lateral wave propagation. Numerical models via the finite difference method were performed by the Italian National Institute for Geophysics and Volcanology Web Interface for Seismological Application and by the Fast Lagrangian Analysis of Continua codes. The obtained results: (1)xa0show a good agreement with HHSRs, (2)xa0prove that some 2D amplification effects are constrained by both the shape ratio of the valley and the heterogeneity of the alluvial deposits, (3)xa0reveal focalization effects within the alluvial deposits strictly related to the shape of the valley, and (4)xa0allow the comparison of the A ( f ) functions in both linear and nonlinear dynamic behavior and the computation of the shear strains due to ground motion within the alluvial deposits.

Parametric numerical study of observed amplification effects on the colle di roio limestone ridge (Central italy)

Following the seismic microzonation studies of the L’Aquila area, Central Italy, hit by the Mw 6.3 L’Aquila earthquake of 6th April 2009, we observed ground motion amplification at the hilltop of the Colle di Roio limestone ridge. In particular, horizontal-to-vertical spectral ratio amplitude of weak motion waveforms showed average peak level above 5 in the frequency range 4–5 Hz. In order to evaluate the contribution of topography and subsoil heterogeneity on the observed response, we performed 2D numerical simulations of wave propagation through the ridge using a finite difference approach. Different hypothesis of model configuration, consistent with the available geological data, were tested. A homogeneous model was considered to evaluate the effect of surface topography on ground motion amplification, while several heterogeneous models were considered in order to examine the interaction between topography and heterogeneity on the seismic response. Results were analyzed in terms of synthetic horizontal-to-vertical (H/V) spectral ratios and the latter were compared to the observed ones. The results showed that the observed ground motion amplification level cannot be explained only by surface topography effect and that the assumption of an heterogeneous model with complex inner geometry may fairly reproduce the observations.

Seismometric Monitoring of Hypogeous Failures Due to Slope Deformations

Results from a seismometric monitoring of rock mass failures affecting a karstified slope are here presented. The slope, located in Central Apennines (Italy), hosts a drainage plant and is involved in gravity-induced deformations.