Evidence of Topographic Effects through the Analysis of Ambient Noise Measurements

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Amplification of the ground motion in a topographic irregularity is generally linked to the focalization of seismic waves at its topmost part due to the existence of diffraction, reflection, and conversion of the incident waves (Bard 1982). The amplification effects at the topmost part of a hill are frequency-dependent so that resonance phenomena occur when the wavelength of the incident wave is comparable to the horizontal dimension of the hill. In addition, the influence of the topography on ground motion is linked to the sharpness of the ridge crest (Geli et al. 1988; Bard and Riepl-Thomas 1999). The upper part of a hill shows increasing resonant motion with respect to the whole of the structure. This ground-motion amplification mechanism at ridge crests is in principle similar to the well-known effects in the seismic design of buildings, which appears to apply at a larger scale to mountains as well (Buech et al. 2010). Moreover, significant directional effects, transverse to the major axis of the ridge, are often observed (Spudich et al. 1996). Several analytical and numerical methods have been developed to study incoming seismic waves when crossing a hill-shaped morphology ( e.g. , LeBrun et al. 1999; Paolucci 2002). On the other hand, experimental studies using earthquake instrumental records are relatively few, and only in recent times have data from small arrays started to be commonly used ( e.g. , Buech et al. 2010; Pischiutta et al. 2010). In a review article, Geli et al. (1988) point out that an important disagreement between observed and predicted amplification on a topographic feature often occurs. Several studies (Tucker et al. 1984; Catchings and Lee 1996; Steidl et al. 1996; Bouchon and Barker 1996) have shown that this difference derives from the difficulty of finding a reference station not …