Mountains Sway to the Seismic Song of Earth

Every object "wants" to vibrate at certain frequencies depending on its shape and what it is made of (a property known as resonance). Familiar examples include tuning forks and wineglasses; when energy of a resonant frequency hits the object, it shakes harder. Moore and his colleagues hypothesized that mountains — like tall buildings, bridges and other large structures — also vibrate at predictable resonances on the basis of their topographic shape.

But unlike the world of civil engineering, in which one can test what frequencies are resonant by placing large shakers on the structure or waiting for vehicles to drive over them, it would be impractical to excite something so large as a mountain.

Instead, Moore and his international team of collaborators sought to measure the effects of ambient seismic activity on perhaps one of the most extreme mountains: the Matterhorn.

Located on the border of Italy and Switzerland in the Alps, the pyramid-shaped Matterhorn is the most photographed mountain in the world. It towers nearly 15,000 feet (4,500 meters) in elevation, and its four faces face the cardinal directions.

Researchers helicoptered up the Matterhorn to set up one solar-powered seismometer roughly the size of a "big cup of coffee" at the summit. Another was placed under the floorboards of a hut a few hundred meters below the peak, and a third was placed at the foot of the mountain as a reference, says Samuel Weber, a researcher at the WSL Institute for Snow and Avalanche Research in Switzerland and the lead author of the study.

The seismometers continuously recorded movements and allowed the team to extract the frequency and direction of the resonance.

The movements are small, on the order of nanometers at the baseline to millimeters during an earthquake, Moore says. "But it's very real. It's always happening."

The measurements showed that the Matterhorn consistently oscillates in the north-south direction at a frequency of 0.42 hertz, or slightly less than once every two seconds, and in the east-west direction at a similar frequency.

Comparing the movement on top of the mountain with measurements from the reference seismometer at its base, the researchers found that the summit was moving much more than the base.

"It was quite surprising that we measured movement on the summit, which was up to 14 times stronger than next to the mountain," Weber says.

The researchers also made measurements on Grosser Mythen, a similarly shaped (albeit smaller) Swiss mountain, and found similar resonance.

"I just think it's a clever combination of choices in terms of the location being so iconic and the careful placement of instruments," says David Wald, a seismologist with the U.S. Geological Survey who was not involved in the study. Choosing a smooth mountain like the Matterhorn also removed the problems brought by soil and sediment, which would have added another layer of complexity to measuring movement.

The baseline vibrations of mountains like the Matterhorn are caused by the hum of seismic energy.

"A lot of this comes from earthquakes rattling all over the world, and really distant earthquakes are able to propagate energy and low frequencies," Moore says. "They just ring around the world constantly."

But the data also pointed to another, unexpected source: the oceans.

Ocean waves moving across seafloors create a continuous background of seismic oscillations, known as a microseism, which can be measured around the world, Moore says. Intriguingly, the microseism had a frequency similar to the resonance of the Matterhorn.

"So the interesting thing was that there's ... some connection between the world's oceans and the excitation of this mountain," Moore says.

The research has practical applications in understanding how earthquakes could affect steep mountains where landslides and avalanches are a constant worry.

But it also brings to life a new way of appreciating the Matterhorn and all other mountains swaying in their own way to a music hidden deep beneath Earth.

"You come to one of these landforms with this idea that you're trying to capture something hidden, something new and unknown about it," Moore says. "It's actually a lot of fun because it makes you sit up quietly and think about the mountain in a different way."

Richard Sima is a science writer based in Baltimore, Maryland. He has a Ph.D. in neuroscience from Johns Hopkins University and an undergraduate degree in neurobiology from Harvard College.

This article is republished from Eos under a Creative Commons license. You can find the original article here.