Anthropogenic sources stimulate resonance of a natural rock bridge

Abstract

The natural modes of vibration of bedrock landforms, as well as the sources and effects of stimulated resonance remain poorly understood. Here we show that seismic energy created by an induced earthquake and an artificial reservoir has spectral content coincident with the natural modes of vibration of a prominent rock bridge. We measured the resonant frequencies of Rainbow Bridge, Utah using data from two broadband seismometers placed on the span, and identified eight distinct vibrational modes between 1 and 6 Hz. A distant, induced earthquake produced local ground motion rich in 1 Hz energy, stimulating a 20 dB increase in measured power at the bridge's fundamental mode. Moreover, we establish that wave action on Lake Powell, an artificial reservoir, generates microseismic energy with peak power ~1 Hz, also exciting resonance of Rainbow Bridge. These anthropogenic sources represent relatively new energy input for the bridge with unknown consequences for structural fatigue.

Fracture Network Imaging on Rock Slope Instabilities Using Resonance Mode Analysis

“…Ambient seismic vibration measurements have been increasingly used to characterize unstable slopes (Del Gaudio et al, 2014;Del Gaudio & Wasowski, 2011;Galea et al, 2014;Iannucci et al, 2018;Jongmans et al, 2002;Kleinbrod et al, 2017), prone-to-fall rock columns Colombero et al, 2018;Lévy et al, 2010;Valentin et al, 2017), and rock arches (Moore et al, 2016;Moore et al, 2018;Starr et al, 2015). Of particular interest is such seismic mapping in case of brittle rock failure, where only little prefailure displacements can be registered.…”

Lakes as a Source of Short‐Period (0.5–2 s) Microseisms

“…A recent survey of ambient seismic noise recorded in North America by the Earthscope Transportable Array (Koper & Burlacu, ) found that several stations in the western U.S. had a peak, instead of a trough, in spectral power near a period of 1 s. These stations were located close to lakes in the Utah region, and Moore et al () suggested that wave action in the lakes was responsible for the 1 s noise peaks. Although less commonly studied than ocean‐generated microseisms, lake‐generated microseisms have previously been reported for Lake Ontario (Kerman & Mereu, ; Kerman et al, ), the Great Lakes (Lynch, ), the Great Salt Lake (Goddard et al, ), the Great Slave Lake (Kerman et al, ; Koper et al, ; Weichert & Henger, ), and possibly the Lesser Slave Lake (Gu & Shen, ), usually with dominant periods near 1 s. The shorter period of lake‐generated microseisms relative to ocean‐generated microseisms may be related to the smaller length of open water that can be acted on by wind (the fetch), which is an important factor in determining the dominant period of ocean wave spectra (Bromirski & Duennebier, ; Webb, ).…”

Characterization of the 3‐D fracture setting of an unstable rock mass: From surface and seismic investigations to numerical modeling

“…New effort is thus needed toward the comprehension of these phenomena in more complex and full 3‐D site conditions. With this aim, Moore et al [] measured the resonance frequencies of a natural sandstone arch (Rainbow Bridge, Utah), identifying seven distinct modes of vibration between 1 and 6 Hz, with distinct polarization directions. The 3‐D numerical analysis, using a photogrammetric model of the bridge, succeeded in reproducing the measured frequency values and orientations and provided valuable detail on the complete displacement field for each vibration mode.…”