The seismic ground motion of a test area in the eastern district of Naples was computed with a hybrid technique based on the rnode surnrnation and the finite difference methods. This technique allowed the realistic modelling of source and propagation effects, including local soil conditions. In the modelling, as seismic source we considered the 1980 Irpinia earthquake, a good example of strong shaking for the area of Naples, located about 90 km from the source. Along a profile through Naples, trencling N86°W, the subsoil is mainly formed by alluvial (ash, stratified sand and peat) and pyroclastic materials overlying a pyroclastic rock (yellow Neapolitan tuff) representing the Neapolitan bedrock. The detailed information available on the subsoil mechanical properties and its geometry warrants the application of the sophisticated hybrid technique. For SH waves, a comparison was made between a realistic 2-D seismic response and a standard I-D response, based on the vertical propagation of waves in a plane layered structure. As expected the sedimentary cover caused an increase in the signal's amplitudes and duration. If a thin uniform peat layer is present, the amplification effects are reduced, and the peak ground accelerations are similar to those observed for the bedrock model. This can be explained by the backscattering of wave energy at such a laqer. The discrepancies evidenced between the l -D and the 2-D seismic response suggest that serious caution must be taken in the formulation of seismic regulations. This is particularly true in the presence of the thin peat laqer where the misinatch between the l -D and the 2-D amplification functions is particularly evident in correspondence of the dominant peak and of the second significant peak.
<p>Measuring ambient seismic vibration provides a promising tool to monitor unstable rock slopes due to its independence from actual surface deformations. It is generally observed that the seismic wavefield, arising from ambient vibrations, polarizes perpendicular to open fractures and that unstable slopes exhibit strong wavefield amplifications compared to stable reference sites. Rock slope instabilities dominated by deep persistent fracture sets exhibit normal mode behaviour due to standing wave phenomena within individual compartments of the unstable volume. Techniques to assess such behavior are well established in mechanical and civil engineering to assess the dynamic response and possibly the structural integrity of the structure studied.</p><p>&#160;</p><p>We performed enhanced frequency domain decomposition modal analysis on ambient vibration data acquired in real-time on an unstable rock site with a volume larger than 150,000&#160;m<sup>3</sup> near Preonzo, Switzerland. We tracked the resonance frequency and normal mode polarization of the first two modes over a period of four years. In addition, we show the development of the modal damping ratio of the fundental mode over time, which is a measure of energy dissipation within and out of the system. We found that the dynamic properties of the rock structure experienced annual variations and that they are primarily controlled by temperature and only secondarily by the exension and closure of large-scale fractures. Even though no large slope failure was observed during the monitoring period, the dataset provides a reference model for ongoing slope monitoring, as the resonance frequency and damping ratio is expected to change significantly prior to failure.</p>
<p>The Preonzo rock slope instability in southern Switzerland partly collapsed in 2012, releasing a volume of ~210&#8217;000 m3 and leaving behind an unstable rock mass of about 140&#8217;000 m3. Shortly after the collapse, a small-aperture seismic array measurement was performed on the remaining unstable volume. The analysis of these data showed a fundamental resonance frequency of about 3.5 Hz and strong wavefield amplifications with factors of more than 30 in direction perpendicular to open tension cracks. Normal mode analysis by frequency domain decomposition using the fundamental and several higher modes allowed for mapping the fracture network of the instability.<br>However, the observed amplification factors and mode shapes could not be explained solely by the open tension cracks visible at the surface. Strong amplifications, especially at frequencies of higher modes, were observed on the uphill part of the rear fracture, which was supposed to be outside the presumed unstable area. The zone where amplifications rapidly decreased in the uphill direction coincides roughly with a geomorphological lineament in the field, interpreted as an additional, but hidden, rear fracture.&#160;<br>We performed active seismic refraction tomography across this lineament and discovered distinct low velocity anomalies in the transition zone from high to low amplifications, supporting the interpretation of an additional fracture. Considering this new finding, the volume of the unstable rock mass increases by about 40 %.&#160;</p>
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