Unusual Seismic Waves Rumbling Around the Planet Likely Caused by Magma Shift, Experts Say
The rumbling originated just offshore of Mayotte, an island between the southeast coast of Africa and Madagascar, before shaking through Africa. Locations in Zambia, Kenya, and Ethiopia recorded the rumbling. Even further away, places in Chile, New Zealand, Canada and Hawaii picked up the rumblings that sped around the globe at 9,000 miles per hour.
What the French Geological Survey (BRGM) called the “atypical very low-frequency signal” was a repeating wave that would register about every 17 seconds and lasted some 20 minutes total. Strangely, nobody felt it.
“What’s unusual is you see this very long signal traveling most of the way around the world which hasn’t been detected by operational earthquake detection systems,” University of Southampton seismologist Stephen Hicks told the Guardian.
Even though some seismic stations missed the rumblings, measurements from the sensors that did pick up the waves pointed back to an area off the coast of Mayotte, where the event originated.
Experts were already aware of a several-month-long earthquake swarm that produced several hundred smaller quakes taking place in the same area, but the rumbling was of a low-frequency, not like a typical earthquake which releases high-frequency waves like P- or S-waves.
In a typical earthquake, you have three types of waves. P-waves, otherwise known as compressional waves, shake the ground back and forth in the motion the wave moves. They typically are felt in small jolts or light shaking, though sometimes, they’re not felt at all. S-waves, or shear waves, shake the ground in the direction perpendicular to the movement of the wave. These are felt in larger jolts or stronger shaking, according to the U.S. Geological Survey.
“This source was completely deficient in those waves,” Hicks said. “It wasn’t picked up because the signal had a very low frequency. It was a low, gentle rumbling.”
The third type of wave is surface waves, which are trapped near the Earth’s surface. A rolling motion can be felt with surface waves.
Surface waves are the most comparable type of wave to that of the one recorded Nov. 11. In a large earthquake, surface waves can buzz around the globe multiple times.
But there was no earthquake large enough to fuel a wave like the one recorded across such a widespread area, and no P- or S-waves were recorded. Even more bizarre, the waves that stemmed from Mayotte were too clean-cut and uniform compared to a normal earthquake, which has waves of differing frequencies.
These low-frequency waves have been associated with events like glacial calving, landslides and magma shifts that occur underneath volcanoes.
Eliminate the glacial calving theory since there are no glaciers in the area. The possibility of a landslide is out because nearby hydrophones would’ve picked that up.
A quick drain of magma from a volcanic chamber some 10 miles below the seafloor could’ve ignited the deep shaking that rang around the globe, Hicks thinks. The waves would’ve been strong enough to have been felt by sensitive seismometers, but they waves would cause minimal vibrations.
“It’s something that you wouldn’t perceive,” said Hicks.