New developments in ambient noise analysis to characterise the seismic response of landslide prone slopes

Abstract

Abstract. We report on new developments in the application of ambient noise analysis applied to investigate the dynamic response of landslide prone slopes to seismic shaking with special attention to the directional resonance phenomena recognised in previous studies. Investigations relying on the calculation of horizontal-to-vertical noise spectral ratio (HVNR) were carried out in the area of Caramanico Terme (central Italy) where an ongoing accelerometer monitoring on slopes with different characteristics offers the possibility of validation of HVNR analysis. The noise measurements, carried out in different times to test the result repeatability, revealed that sites affected by response directivity persistently show major peaks with a common orientation consistent with the resonance direction inferred from accelerometer data. At sites where directivity is absent, the HVNR peaks do not generally show a preferential orientation, with rare exceptions that could be linked to the presence of temporarily active sources of polarised noise. The observed spectral ratio amplitude variations can be related to temporal changes in site conditions, which can hinder the recognition of main resonance frequencies. Therefore, it is recommended to conduct simultaneous measurements at nearby sites within the same study area and to repeat measurements at different times in order to distinguish significant systematic polarisation caused by site specific response directivity from polarisation controlled by properties of noise sources. Furthermore, an analysis of persistence in noise recordings of signals with systematic directivity showed that only a~portion of recordings contains wave trains having a clear polarisation representative of site directional resonance. Thus a careful selection of signals for HVNR analysis is needed for a correct characterisation of site directional properties.

Characterization of the 3‐D fracture setting of an unstable rock mass: From surface and seismic investigations to numerical modeling

“…The interpretation of these data is not straightforward as for the cited nearly 2‐D case studies, in which the vibration motion at the first resonance frequency ( f 1) was systematically found to have the highest spectral amplitude and to be oriented perpendicularly to the main fracture set separating the unstable and stable sectors [e.g., Lévy et al, ; Burjánek et al, ; Del Gaudio et al, ; Bottelin et al, ]. The vibration directions are here complicated by the presence of two distinct unstable blocks, which are separated by a complex 3‐D fracture setting and constrained to a sloping basal discontinuity.…”

“…Furthermore, ambient vibration surveys have been recently applied to potentially unstable rock slopes for either investigation or monitoring purposes, showing the capability to detect both reversible and irreversible modifications within the rock mass that are driven by external factors and controlled by the fracture settings [ Larose et al, ]. Several previous studies demonstrated that the presence of spectral peaks in the seismic noise measured on prone‐to‐fall compartments could be associated with their resonance frequencies [e.g., Lévy et al, ; Burjánek et al, ; Del Gaudio et al, ; Bottelin et al, ]. This hypothesis is supported by the absence of similar peaks in the noise spectra recorded on the stable rock mass.…”

Application of the microtremor measurements to a site effect study

“…During the past decades, the effect of local soil condition is known to have caused serious damage during several earthquakes. Some well-known examples include the earthquake in Michoacan, Mexico, in 1985, Loma Prieta, California, USA, 1989, Roodbar, Iran, 1990, Chi-Chi, Taiwan, 1999, Bam, Iran, 2003 In general, site effect has introduced the effects of soils layers and underground topography which alter the specification of strong ground motion (Choobbasti et al 2013;Del Gaudio et al 2013). It is well accepted that, besides the earthquake magnitude and fault distance, local geological conditions, known as site effects, can impose significant influences on the earthquake ground motion at a given place (Rezaei et al 2015;Panzera et al 2013).…”