’On the morning of November 11, just before 9:30 UT, a mysterious rumble rolled around the world.
The seismic waves began roughly 15 miles off the shores of Mayotte, a French island sandwiched between Africa and the northern tip of Madagascar. The waves buzzed across Africa, ringing sensors in Zambia, Kenya, and Ethiopia. They traversed vast oceans, humming across Chile, New Zealand, Canada, and even Hawaii nearly 11,000 miles away. These waves didn’t just zip by; they rang for more than 20 minutes. And yet, it seems, no human felt them. Only one person noticed the odd signal on the U.S. Geological Survey’s real-time seismogram displays. An earthquake enthusiast who uses the handle @matarikipax saw the curious zigzags and posted images of them to Twitter. That small action kicked off another ripple of sorts, as researchers around the world attempted to suss out the source of the waves. Was it a meteor strike? A submarine volcano eruption? An ancient sea monster rising from the deep?
“I don’t think I’ve seen anything like it,” says Göran Ekström, a seismologist at Columbia University who specializes in unusual earthquakes. “It doesn’t mean that, in the end, the cause of them is that exotic,” he notes. Yet many features of the waves are remarkably weird—from their surprisingly monotone, low-frequency “ring” to their global spread. And researchers are still chasing down the geologic conundrum. Why are the low-frequency waves so weird? In a normal earthquake, the built-up tensions in Earth’s crust release with a jolt in mere seconds. This sends out a series of waves known as a “wave train” that radiates from the point of the rupture, explains Stephen Hicks, a seismologist at the University of Southampton. The fastest-traveling signals are Primary waves, or P-waves, which are compression waves that move in bunches, like what happens to an extended slinky that gets suddenly pushedat one end. Next come the secondary waves, or S-waves, which have more of a side-to-side motion. Both of these so-called body waves have relatively high frequencies, Hicks says, “a sort of ping rather than a rumbling.”
Earthquakes 101 Finally, chugging along at the end come slow, long-period surface waves, which are similar to the strange signals that rolled out from Mayotte. For intense earthquakes, these surface waves can zip around the planet multiple times, ringing Earth like a bell, Hicks says. However, there was no big earthquake kicking off the recent slow waves. Adding to the weirdness, Mayotte’s mystery waves are what scientists call monochromatic. Most earthquakes send out waves with a slew of different frequencies, but Mayotte’s signal was a clean zigzag dominated by one type of wave that took a steady 17 seconds to repeat. “It’s like you have colored glasses and [are] just seeing red or something,” says Anthony Lomax, an independent seismology consultant.
Mayotte’s volcanic roots Based on the scientific sleuthing done so far, the tremors seem to be related to a seismic swarm that’s gripped Mayotte since last May. Hundreds of quakes have rattled the small nation during that time, most radiating from around 31 miles offshore, just east of the odd ringing. The majority were minor trembles, but the largest clocked in at magnitude 5.8 on May 15, the mightiest in the island’s recorded history. Yet the frequency of these shakes has declined in recent months—and no traditional quakes rumbled there when the mystery waves began on November 11.
The French Geological Survey (BRGM) is closely monitoring the recent shaking, and it suggests that a new center of volcanic activity may be developing off the coast. Mayotte was formed from volcanism, but its geologic beasts haven’t erupted in over 4,000 years. Instead, BRGM’s analysis suggests that this new activity may point to magmatic movement offshore—miles from the coast under thousands of feet of water. Though this is good news for the island inhabitants, it’s irksome for geologists, since it’s an area that hasn’t been studied in detail. “The location of the swarm is on the edge of the [geological] maps we have,” says Nicolas Taillefer, head of the seismic and volcanic risk unit at BRGM. “There are a lot things we don’t know.” And as for the November 11 mystery wave, he says, “it’s something quite new in the signals on our stations.”
Motion in the ocean Though baffled, scientists aren’t without leads. For one, they know that Mayotte is on the move. Since mid-July, GPS stations on the island have tracked it sliding more than 2.4 inches to the east and 1.2 inches to the south, according to a BRGM report from November 12. Using these measurements, Taillefer notes, the agency estimates that a magma body that measures about a third of a cubic mile is squishing its way through the subsurface near Mayotte.…’