Drawing Knowledge from Ambient Noise: Challenges of Ground Dynamic Response Investigations

The improvement in the Civil Protection system capability of mitigating the impact of future large magnitude earthquakes requires an upgrading of methods aimed at providing realistic scenarios of seismic effects. One of the factors that contributes the most to the uncertainty of the previsional assessment of earthquake consequences is the dynamic response of ground to seismic shaking. This is especially relevant in complex geological-geomorphological settings, which can determine dramatic variations of shaking energy and ground failure effects, even within very short distances. The most straightforward approach to the investigation of ground dynamic response consists of the direct recording of seismic ground motion at sites suspected to be liable to amplification phenomena. However, while it is not difficult to collect weak motion observations with relatively short term monitoring, their scaling to strong shaking conditions can provide unreliable results. On the other hand, much time is usually necessary to acquire strong motion data. This and the cost of instruments needed to provide suitable resolution level for site response investigations in geologically heterogeneous areas makes long-term widespread monitoring impractical. These problems promoted the experimentation of alternative approaches based on the analysis of persistent ground vibrations generated by non-seismic sources (wind, sea waves, traffic, etc.), commonly defined as “ambient noise”. The noise signals can be recorded through short time (less than 1 hr) acquisition sessions conducted with lightweight portable instruments, even at sites of difficult accessibility. The outcomes of ambient noise analysis suffer from uncertainties comparable to those of weak motion data interpretation. Nevertheless, noise analysis can benefit from the possibility of an extensive application, also to sites recently hit by strong earthquakes. Such events offer comparative data (e.g. seismic recordings) that can be used to refine the information derivable from noise signals. The most popular technique of ambient noise analysis for site response investigation is the Nakamura (1989) method, also named HVNR (horizontal-to-vertical noise ratios). It relies on the analysis of ratios H/V between spectral amplitudes of horizontal (H) and vertical (V) components of noise recordings, subdivided into series of 20-30 second long time windows. Averaging spectral ratios over several tens of such time windows, the occurrence of amplification conditions can be recognized from the presence of pronounced peaks at site specific frequencies. These were found to correspond to the resonance frequencies deriving from conditions of seismic wave constructive interference, controlled by geometrical and physical properties of surficial geological bodies. Although originally devised to investigate site response of sites with flat horizontal layering, where amplification is caused by wave trapping inside a soft surface layer overlying a stiffer substratum, the Nakamura’s method proved able to reveal resonance conditions also under more complex site conditions (e.g., slopes/topographic relieves with lateral variations of subsoil materials and their mechanical properties). In such conditions, an analysis of azimuthal variation of spectral ratios H/V can show directional variations consistent with those observed in ground motion amplification. The improvement in the Civil Protection system capability of mitigating the impact of future large magnitude earthquakes requires an upgrading of methods aimed at providing realistic scenarios of seismic effects. One of the factors that contributes the most to the uncertainty of the previsional assessment of earthquake consequences is the dynamic response of ground to seismic shaking. This is especially relevant in complex geological-geomorphological settings, which can determine dramatic variations of shaking energy and ground failure effects, even within very short distances. The most straightforward approach to the investigation of ground dynamic response consists of the direct recording of seismic ground motion at sites suspected to be liable to amplification phenomena. However, while it is not difficult to collect weak motion observations with relatively short term monitoring, their scaling to strong shaking conditions can provide unreliable results. On the other hand, much time is usually necessary to acquire strong motion data. This and the cost of instruments needed to provide suitable resolution level for site response investigations in geologically heterogeneous areas makes long-term widespread monitoring impractical. These problems promoted the experimentation of alternative approaches based on the analysis of persistent ground vibrations generated by non-seismic sources (wind, sea waves, traffic, etc.), commonly defined as “ambient noise”. The noise signals can be recorded through short time (less than 1 hr) acquisition sessions conducted with lightweight portable instruments, even at sites of difficult accessibility. The outcomes of ambient noise analysis suffer from uncertainties comparable to those of weak motion data interpretation. Nevertheless, noise analysis can benefit from the possibility of an extensive application, also to sites recently hit by strong earthquakes. Such events offer comparative data (e.g. seismic recordings) that can be used to refine the information derivable from noise signals. The most popular technique of ambient noise analysis for site response investigation is the Nakamura (1989) method, also named HVNR (horizontal-to-vertical noise ratios). It relies on the analysis of ratios H/V between spectral amplitudes of horizontal (H) and vertical (V) components of noise recordings, subdivided into series of 20-30 second long time windows. Averaging spectral ratios over several tens of such time windows, the occurrence of amplification conditions can be recognized from the presence of pronounced peaks at site specific frequencies. These were found to correspond to the resonance frequencies deriving from conditions of seismic wave constructive interference, controlled by geometrical and physical properties of surficial geological bodies. Although originally devised to investigate site response of sites with flat horizontal layering, where amplification is caused by wave trapping inside a soft surface layer overlying a stiffer substratum, the Nakamura’s method proved able to reveal resonance conditions also under more complex site conditions (e.g., slopes/topographic relieves with lateral variations of subsoil materials and their mechanical properties). In such conditions, an analysis of azimuthal variation of spectral ratios H/V can show directional variations consistent with those observed in ground motion amplification. First application tests, however, have also shown some limits in the possibility of drawing information about site response properties. Major problems arise when a strong incoherent background noise is present at frequencies close to that of site resonance, or when local sources of ambient noise do not cover completely the frequency band including the resonance frequencies. These problems imply the need of repeating measurements under different environmental conditions, possibly in different seasons, since a single measurement campaign could encounter unfavourable conditions for the detection of resonance. In addition, unfavourable application conditions are present in sites with amplifications affecting the vertical component of ground motion, which can reduce the values both of H/V ratio derived from the Nakamura’s method and the Rayleigh wave ellipticity estimated by the polarization analysis.