May 18, 2022 8:33 pm

The mountains move “like trees swayed by the wind”

Kerry Taylor-Smith Meteor United Kingdom 4 min
The Matterhorn moves
The Matterhorn vibrates like a tree swaying in the wind, but on a much smaller scale.

Each object vibrates at a natural frequency depending on the geometry and its material properties. Just like tall buildings and bridges, the mountains also vibrate, stimulated by seismic energy from the Earth’s oceans, earthquakes and anthropogenic activity.

Although these vibrations are very subtle, imperceptible to the human eye, an international team of researchers has measured this resonant oscillation and has made its movement visible through computer simulations.

Two seismic measurement stations were set up at the Matterhorn in the Alps: one at the summit, 4,470 m above sea level, and one in an emergency shelter at Hörnligrat, at 4,004 m above sea level. A third station at the foot of the alpine mountain served as a reference, and the data from all three was automatically transmitted to the Swiss Seismological Service in ETH Zurich.

The seismometers recorded all the movements of the mountain at high resolution, which allowed the researchers to determine the frequency and direction of the resonance. The data revealed that the Matterhorn oscillates in a north-south direction at a frequency of 0.43 Hertz, and in an east-west direction at a similar frequency.

The researchers sped up ambient vibration measurements 80 times, making the Matterhorn’s vibration audible to the human ear.

Greater movement at the summit

The movements of the Matterhorn were small, in the range of nanometers to micrometers, but those at the top of the mountain were up to 14 times stronger than those recorded at the reference station at the bottom. This increase in ground motion with altitude is explained by the fact that the top moves freely, while the foot is fixed, a bit like a tree swaying in the wind.

Such amplification could be measured during earthquakes, which could have important implications for slope stability in the event of strong seismic shocks.

“Areas of mountains experiencing amplified ground motion are likely to be more prone to landslides, rockfalls and damage on rocks when they are shaken by a strong earthquake,” explains Jeff Moore of the University of Utah, who initiated the study on the Matterhorn.

all mountains move

The Matterhorn is not unique. In fact, many mountains are expected to vibrate in a similar way. To confirm this, researchers from the Swiss Seismological Service carried out a complementary experiment on the Grosse Mythen, a similar but smaller peak in central Switzerland.

Grosse Mythen vibrates at a frequency around four times higher than the Matterhorn , which scientists expected as it is significantly smaller in size.

the mountains move
Grosse Mythen vibrates like the Matterhorn, but at a higher frequency.

Scientists at the University of Utah simulated the resonance of the Matterhorn and Grosse Mythen using computer models, thus making the resonant vibrations visible. Previously, scientists had simulated smaller objects, such as rock arches in Arches National Park in the United States.

“It was exciting to see that our simulation approach also works for a big mountain like the Matterhorn and that the measurement data confirmed the results,” concluded Moore.

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