The November 11 event began off the coast of Mayotte and rippled diagonally across Africa according to National Geographic, hitting sensors in Kenya, Zambia, and Namibia. The waves were also picked up by sensors in Canada, New Zealand, Chile, and Hawaii and repeated for about 20 minutes, but were apparently not strong enough for people to feel and report them. “I don’t think I’ve seen anything like it,” said Columbia University seismologist Göran Ekström after seeing the graph image of the anomaly (first shared on Twitter by user @matarikipax). “It doesn’t mean that, in the end, the cause of them is that exotic.” While maybe not exotic, the waves are certainly weird, primarily because there wasn’t an earthquake event that would have normally caused this particular brand of seismic wave. Or was there?
Ekström suggests that maybe a magnitude-5 “slow earthquake” caused the waves, but that it went unnoticed because of how they occur (over time instead of all at once). “The same deformation happens, but it doesn’t happen as a jolt,” the seismologist said. National Geographic points out that slow earthquakes are often linked to volcanic activity, which tracks with the theory that the mystery event was linked to the Mayotte quakes because Mayotte was formed by a volcano. There hasn’t been an eruption in 4,000 years, but that doesn’t mean that things aren’t happening beneath the surface.
Others have shared educated guesses as to what caused the anomaly, but the truth is that no one really knows. The French Geological Survey is monitoring the area of the swarm and plans to survey the ocean floor for more answers, but it’s possible that they will never know what rocked the world that day.