The island of Mayotte sits between Mozambique and Madagascar. It has been getting a lot of attention recently because it has been the location of some unique geological ongoings in the past year. Since May of last year, the French Geological Survey had been monitoring a series of earthquakes that were occurring off the island’s eastern shores. The biggest of the quakes reached a magnitude of 5.8. But then last Veterans Day things turned up a level when seismographs around the globe registered a very low-frequency tremor that lasted roughly 30 minutes. A part of the tremor even sounded like multiple high-frequency blips, with approximately one minute in between.
When this occurred, scientists took to Twitter to debate out the event. Stephen Hicks, a seismologist at the University of Southampton, commented in a November 12 tweet “Something biggggg, yet strangely slow, sent seismic rumblings around the surface of much of the planet yesterday.”
Yet at the time of the tremor, scientists weren’t really sure what it was. And they’re still not entirely sure, though researchers at the French Geological Survey and France’s Ecole Normale Supérieure have taken a stab at hypothesizing what could have caused it. Their research has been uploaded in pre-print form to the public server EarthArXiv.
The conclusion that the scientists have come to – though they make no presumption of having all the answers – is that the tremor originated from a volcanic event which moved a huge (no, really, HUGE) volume of magma underneath the seafloor that resulted in the ground deflating. The event is thought to be one of the largest offshore volcanic events to be registered by scientific instruments.
According to the data from the onshore GPS stations and seismic signals, the volcanic activity centered around Mayotte shows that the island’s eastern seafloor is sinking at a rate of about 0.4 inches per month all while the island itself is shifting eastward at a rate of 0.63 inches per month. According to Gizmodo, these movements indicate serious underground movement.
“The nature of these tremors suggest that the magmatic source is centered at a depth of 16 miles beneath the seafloor. In the first six months of the sequence alone, at least 0.24 cubic miles of magma has shifted around. That’s roughly equivalent to 385 Great Pyramids of Giza,” Gizmodo writes.
Samuel Mitchell, an expert in underwater eruptions at the University of Hawaii at Mānoa, commented on the potential hugeness of the event if it were to culminate in an eruption. “The 2018 event at Mayotte does appear to show a substantial volume of magma leaving a deep storage region which, if erupted, would make this indeed one of the largest recent submarine eruptions documented,” he said. That plus the long, low-frequency tremor is enough to keep seismologists curious for at least a while.