|This illustration shows how magma in a reservoir deep underground ascended to form a submarine volcano in the Indian Ocean. (Image credit: James Tuttle Keane/Nature Geoscience (2020))|
Mysterious seismic hums detected around the world were likely caused by an unusual geologic event — the rumblings of a magma-filled reservoir deep under the Indian Ocean, a new study finds.
These odd hums were an unconventional geologic birth announcement. A few months after the sounds rippled around the Earth, a new underwater volcano was born off the coast of the island of Mayotte, located between Madagascar and Mozambique in the Indian Ocean.
Since May 2018, an unusual sequence of earthquakes has been recorded off the coast of the island of Mayotte in the Comoros archipelago between Africa and Madagascar. Seismic activity began with a swarm of thousands of "seemingly tectonic" earthquakes, culminating in an earthquake of magnitude 5.9 in May 2018. Mostly, since June 2018, however, a completely new form of earthquake signal has emerged that was so strong that it could be recorded up to 1000 kilometers away.
These 20- to 30-minute-long signals are characterized by particularly harmonic, low frequencies, almost monochromatic, similar to a large bell or a double bass, and are called very long period (VLP) signals. Although the centre of the seismic activity was located almost 35 kilometers offshore the east of the island, a continuous lowering and eastward motion of the Earth's surface at Mayotte had begun at the same time as the massive swarms of VLP events started, accumulating to almost 20 cm to date.
The special frequency content of the VLP signals is caused by the resonance oscillation of the buried magma chamber. The deeper the vibrations, the larger the magma reservoir. However, the earthquake swarms under the ocean floor were much deeper than with other volcanoes and the resonance tones of the VLPs were unusually low and strong.
An international team led by GFZ scientist Simone Cesca analyzed seismological and geodetic data from the region to study these observations and their evolution over time. However, the investigations were complicated by the fact that there was no seismic network on the ocean floor, and therefore, measurements were only available at great distances on Mayotte, Madagascar and in Africa. "We tried to improve the unfavorable initial situation by developing special new analytical methods such as cluster and directional beam methods," says Cesca.
The team identified different activity phases within the sequence of events from May 2018 to today. The initial swarm phase indicated a rapid upward movement of magma from a deep mantle reservoir more than 30 kilometers below the Earth's surface. Once an open channel had formed from the Earth's mantle to the seabed, the magma began to flow unhindered and form a new underwater volcano. A French oceanographic campaign recently confirmed the birth of the submarine volcano, whose location coincides with the reconstructed magma rise.
In this phase, the apparent tectonic earthquake activity decreased again, while the lowering of the ground on the island of Mayotte began. Likewise, long-lasting monofreququent VLP signals started. "We interpret this as a sign of the collapse of the deep magma chamber off the coast of Mayotte," explains Eleonora Rivalta, co-author of the scientific team. "It is the deepest (~30 km) and largest magma reservoir in the upper mantle (more than 3.4 cubic kilometers) to date, which is beginning to empty abruptly.
"Since the seabed lies 3 kilometers below the water surface, almost nobody noticed the enormous eruption. However, there are still possible hazards for the island of Mayotte today, as the Earth's crust above the deep reservoir could continue to collapse, triggering stronger earthquakes," says Torsten Dahm, head of the section Physics of Earthquakes and Volcanoes at the GFZ.
In all, about 0.4 cubic miles (1.5 cubic km) of magma drained out of the reservoir, the researchers calculated. However, given the vast size of the volcano, it's likely that even more magma was involved, Cesca noted.
Although the volcano is now formed, earthquakes may still rattle the area.
"There are still possible hazards for the island of Mayotte today," study senior researcher and head of the section Physics of Earthquakes and Volcanoes at the GFZ Torsten Dahm, said in a statement "The Earth's crust above the deep reservoir could continue to collapse, triggering stronger earthquakes."
The above story is based on materials provided by Helmholtz Association of German Research Centres.