Seismic readings by University of Utah scientists reveal unseen vibrations of the 120 m high sandstone formation
At about the same rate that your heart beats, the Castleton Tower in Utah gently vibrates, keeping time and keeping watch over the sandstone desert. Swaying like a skyscraper, the red rock tower taps into the deep vibrations in the earth-wind, waves and far-off earthquakes.
New research from University of Utah geologists details the natural vibration of the tower. Understanding how this and other natural rock forms vibrate, they say, helps us keep an eye (or ear) on their structural health and helps us understand how human-made vibrations affect seemingly unmovable rocks. The results are published in the Bulletin of the Seismological Society of America.
Castleton Tower is a spire of Wingate Sandstone nearly 400 feet (120 m) tall that stands over Utah’s Castle Valley. It’s one of the largest freestanding rock towers and a major destination for climbers.
„Most people are in awe of its static stability, in its dramatic freestanding nature perched at the end of a ridge overlooking Castle Valley,“ says geologist Jeff Moore, who led the study. „It has a kind of stoic power in its appearance.“
Moore and his colleagues study the vibrations of rock structures, including arches and bridges, to understand what natural forces act on these structures. They also measure the rocks’ resonance, or the way the structures amplify the energy that passes through them.
Sources of this energy can be as local as wind gusts or traffic on a nearby road or as distant as far-off earthquakes and even ocean waves. „Because nothing is truly static, there is always energy propagating throughout the earth, which serves as a constant vibration source for the rock,“ says Riley Finnegan, a graduate student and co-author on the paper.
Moore, Finnegan and graduate student Paul Geimer use seismometers placed all over the rock to measure even the slightest movement in three dimensions.
Analyzing their data, the researchers found that the tower’s 2 primary resonance modes were at frequencies of 0.8 and 1.0 hertz, respectively. One hertz is equal to one cycle per second, so these results mean that the tower naturally sways around once per second.
And that small sway is constant, Geimer says. „The distant sources that excite the unique resonance of Castleton Tower are always active and transferring energy into the rock mass.“
Geimer will use the data to see if repeated measurements can assess damage to the structures, while Finnegan is studying how vibrational energy, both from natural and human sources, may impact the structural integrity of structures like Castleton Tower. „While some forces humans create might appear minor,“ Moore says, „our research is addressing the long-term effects of these forces on the rate of erosion and structural degradation over time.“
So far, the team can tell that Castleton Tower’s vibrational modes sit in a quiet part of the frequency spectrum, relatively unaffected by traffic or even small earthquakes.
A sound profile and a visualization of the movement are available on the University of Utah’s webpage.
Source: University of Utah
(05.09.2019, USA: 09.05.2019)