Abstract
Amplification of seismic energy in steep topography plays an important role controlling the location of earthquake-induced landslides. Alpine mountains represent extreme topography, therefore large amplification may be anticipated, however suitable data needed to probe the limits of topographic effects in these demanding locations are rare. Here we present new ambient vibration data from seismic stations on the summit and ridge of one of the tallest freestanding mountains in the Swiss Alps – the Matterhorn – comparing these to a nearby local reference. Results show elevated spectral power at mountain stations between 0.4 and 1 Hz, and directional site-to-reference spectral amplitude ratios up to 14, which we attribute in part to topographic resonance. We used ambient vibration modal analysis and numerical eigenfrequency modeling to identify the fundamental mode of the Matterhorn at 0.42 Hz, as well as evidence for a second, mutually-perpendicular mode at a similar frequency. We identified high modal damping ratios of ∼20% for these modes, which we ascribe to radiative energy loss. A short campaign measurement at another mountain of comparable shape but smaller scale showed similar modal properties with a higher fundamental frequency of 1.8 Hz and peak spectral ratios of 6. Tracking of resonant frequencies over one year at the Matterhorn revealed no measurable seasonal variations related to near-surface environmental changes (e.g. temperature, ice). Our results demonstrate large spectral amplifications linked to resonance of high-relief mountain landforms, which is likely to be a widespread effect making such areas more prone to co-seismic rock damage and landslides.
Highlights
Topographic amplification of seismic energy arises due to resonance and wave focusing in steep topography, resulting in amplified ground motions at slope crests and convex breaks as compared to adjacent valley-bottom reference sites (Boore, 1972; Davis and West, 1973; Celebi, 1987; Meunier et al, 2008)
We propose that the spectral amplification evident in our ambient vibration data results to the first order from resonance of the large-scale mountain landforms
Our results further provide several key outcomes useful for predicting aspects of topographic amplification of ground motion on mountain slopes and summits, relevant for assessment of earthquake induced landslides: a) likely lower-limit range for resonant frequencies and a possible upper-limit value for spectral amplification related to topographic resonance, b) experimental data showing high modal damping related to efficient energy radiation, and c) confirmation of the utility of simple numerical models to estimate the resonant frequencies of mountain landforms
Summary
Topographic amplification of seismic energy arises due to resonance and wave focusing in steep topography, resulting in amplified ground motions at slope crests and convex breaks as compared to adjacent valley-bottom reference sites (Boore, 1972; Davis and West, 1973; Celebi, 1987; Meunier et al, 2008) This phenomenon is distinguished from localized site effects resulting from near-surface soil stratigraphy or weathered rock shear-wave velocity contrasts, as well as the presence of rock fractures (Borcherdt, 1970; Havenith et al, 2002; Moore et al, 2011; Burjánek et al., 2012; Häusler et al, 2019). Suitable broadband seismic data from these locations are rare, in part due to difficult and often dangerous site access as well as limited measurement locations, and data needed to probe the range of topographic amplification remain limited
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