Alternative interpretations of horizontal to vertical spectral ratios of ambient vibrations: new insights from theoretical modeling

Abstract

Different positions exist about the physical interpretation of horizontal to vertical spectral ratios (HVSR) deduced from ambient vibrations. Two of them are considered here: one is based on the hypothesis that HVSR are mainly conditioned by body waves approaching vertically the free surface, the other one assumes that they are determined by surface waves (Rayleigh and Love, with relevant upper modes) only. These interpretations can be seen as useful approximations of the actual physical process, whose reliability should be checked case-by-case. To this purpose, a general model has been here developed where ambient vibrations are assumed to be the complete wave field generated by a random distribution of independent harmonic point sources acting at the surface of a flat stratified visco-elastic Earth. Performances of the approximate interpretations and complete wave field models have been evaluated by considering a simple theoretical subsoil configuration and an experimental setting where measured HVSR values were available. These analyses indicate that, at least as concerns the subsoil configurations here considered, the surface-waves approximation seems to produce reliable results for frequencies larger than the fundamental resonance frequency of the sedimentary layer. On the other hand, the body waves interpretation provides better results around the resonance frequency. It has been also demonstrated that the HVSR curve is sensitive to the presence of a source-free area around the receiver and that most energetic contribution of the body waves component comes from such local sources. This dependence from the sources distribution implies that, due to possible variations in human activities in the area where ambient vibrations are carried on, significant variations are expected to affect the experimental HVSR curve. Such variations, anyway, only weakly affect the location of HVSR maximum that confirms to be a robust indicator (in the range of 10%) of the local fundamental resonance frequency.

Extensive characterization of Italian accelerometric stations from single-station ambient-vibration measurements

“…In these latter cases, the authors have found that, depending on the investigated site, alternatively higher Rayleigh-wave modes or S-wave or Love waves could be dominant. Recently, Albarello and Lunedei (2010) have observed, for the subsoil structures tested in their study, characterized by a single sharp impedance contrast, that vibrations at frequencies above an HVSR peak are dominated by surface waves, while body waves are predominant around and below the fundamental frequency. These considerations imply that around a fundamental frequency, a mixture of seismic phases exists whose relative contributions mainly depend on the subsoil structure.…”

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

“…Indeed, regardless of the nature of the noise wave field, the H /V peaks are observed at the resonance frequency of the S waves even when the noise is dominated by Rayleigh waves (Asten, 2004). However there are difficulties in inferring amplification factors from HVNR, because the amplitude of the H /V spectral ratios, even though correlated to amplification factor, may not be representative of its actual value and can change according to the nature of the noise wave field (Albarello and Lunedei, 2009).…”

“…Albarello and Lunedei (2009), investigating the simple case of flat horizontal layering with a low impedance layer overlying a stiffer substratum, found that peak amplitude of noise H /V ratios in different situations may reflect S waves amplification or Rayleigh wave ellipticity (i.e. the ratio between horizontal and vertical component of the elliptical Rayleigh particle motion): S waves amplification appears to have a major influence on H /V ratios around the site fundamental resonance frequency and for closer noise sources, whereas for higher frequencies and more distant sources the effect of Rayleigh wave ellipticity seems to prevail, causing an increase of H /V ratios beyond the amplification factor observed during earthquakes.…”

Application of Surface-Wave Methods for Seismic Site Characterization

“…As an example, the NHV values around the maximum have been discarded by Parolai et al (2006) and instead taken into account by D'Amico et al (2008). Recent theoretical studies Albarello and Lunedei 2010;Tuan et al 2011) indicated that the NHV curve around the fundamental resonance frequency f 0 (i.e., around the NHV maximum) can be significantly affected by the damping profile in the subsoil and by the distribution of sources around the receiver. In particular, they showed that sources located within a few hundred meters of the receiver can generate seismic phases that strongly affect the shape of the NHV curve around and below f 0 .…”