Abstract

An experimental study aimed at investigating potential topographic amplification of seismic waves was conducted on a 50‐m‐tall and 185‐m‐wide soft‐rock ridge located at Los Alamos National Laboratory near Los Alamos, New Mexico. Ten portable broadband seismograph stations were placed in arrays across the ridge and left to record ambient vibration data for 9 hours. Clear evidence of topographic amplification was observed by comparing spectral ratios calculated from ambient noise recordings at the toe, slope, and crest of the instrumented ridge. The inferred resonance frequency of the ridge obtained from the experimental recordings was found to agree well with several simple estimates of the theoretical resonance frequency based on its geometry and stiffness. Results support the feasibility of quantifying the frequency range of topographic amplification solely using ambient vibrations, rather than strong or weak ground motions. Additionally, comparisons have been made between a number of widely used experimental methods for quantifying topographic effects, such as the standard spectral ratio, median reference method, and horizontal‐to‐vertical spectral ratio. Differences in the amplification and frequency range of topographic effects indicated by these methods highlight the importance of choosing a reference condition that is appropriate for the site‐specific conditions and goals associated with an experimental topographic amplification study.

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