The nature of noise wavefield and its applications for site effects studies A literature review

The aim of this paper is to discuss the existing scientific literature in order to gather all the available information dealing with the origin and the nature of the ambient seismic noise wavefield. This issue is essential as the use of seismic noise is more and more popular for seismic hazard purposes with a growing number of processing techniques based on the assumption that the noise wavefield is predominantly consisting of fundamental mode Rayleigh waves. This survey reveals an overall agreement about the origin of seismic noise and its frequency dependence. At frequencies higher than 1 Hz, seismic noise systematically exhibits daily and weekly variations linked to human activities, whereas at lower frequencies (between 0.005 and 0.3 Hz) the variation of seismic noise is correlated to natural activities (oceanic, meteorological...). Such a surface origin clearly supports the interpretation of seismic noise wavefield consisting primarily of surface waves. However, the further, very common (though hidden) assumption according which almost all the noise energy would be carried by fundamental mode Rayleigh waves is not supported by the few available data: no “average” number can though be given concerning the actual proportion between surface and body waves, Love and Rayleigh waves (horizontal components), fundamental and higher modes (vertical components), since the few available investigations report a significant variability, which might be related with site conditions and noise source properties.