Spectral amplification of ground motion linked to resonance of large-scale mountain landforms

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.

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