CM – Signs of unrest were detectable 4 years before the Cumbre Vieja eruption, a seismic analysis suggests



Kate Baggaley

Published on Dec. 4, 2021 15:00

On September 19, the Cumbre Vieja volcano began an explosive eruption in the Canary Islands that will not abate any time soon. A string of earthquakes only warned about a week before the disaster, which evacuated around 6,400 residents and destroyed more than $ 450 million in infrastructure led to the eruption, were actually detectable four years before the volcano’s explosion. Understanding how this 50-year dormant volcano was reactivated could help improve future eruption predictions and volcanic hazard assessments, says Marc-Antoine Longpré, a volcanologist at Queens College in New York, who published the results in Science on Dec. 2 .

« That is the reason for this consideration – in the hope that in the future, when the volcano awakens again, we will be better prepared to say what could happen and over what period of time, » he says.

Scientists have tracked seismic signals on regularly active volcanoes like Hawaii’s Kīlauea and Sicily’s Mount Etna to investigate how the time between eruptions is related to the reactivation of the volcano. However, this was more difficult with volcanoes that remain inactive for a long time, such as Cumbre Vieja, which is located on the island of La Palma in the Atlantic Ocean. Although it is the most active volcano in the Canary Islands, it has only erupted six times in the last 500 years.

Since the volcano’s last eruption in 1971, the Spanish Instituto Geográfico Nacional and the Instituto Volcanológico de Canarias have set up seismic monitoring networks to Record data from Cumbre Vieja and other parts of the archipelago. « It is the first time that we have seen them reactivate with modern instrumentation, » says Longpré.

He scoured this data to find out how many earthquakes had occurred per month since 2000 and observed very little seismic activity Activity until an earthquake swarm occurred in October 2017. Another group of earthquakes occurred in February 2018. Then, after a break from several years, the swarms of earthquakes increased again in 2020 and 2021. These earthquakes were mostly too small to be felt by people above, but they are the earliest signs that the volcano was about to erupt, says Longpré.

This slow awakening is in contrast to the much shorter ones Run-up times typically seen in this type of basalt volcano, he says. It is possible that other volcanoes with long rest periods may have subtle but lengthy warning periods.

« Volcanoes have their own personalities, so they might do something different next time, » says Longpré. « But at least we know there is a precedent now. »

The tiny earthquakes were caused when magma began to invade and break rocks from miles below the volcano. Just eight days before the eruption began, seismic activity accelerated as the magma neared the surface and swelled the bottom. « This is an additional indication that the magma was moving and fairly shallow under the volcano, » says Longpré.

During this past week, the earthquakes recorded by seismic instruments increased to several hundred a day, grew large enough to be noticed by residents and migrated to the northwest. Eventually, two 200 meter long crevices on the northwest flank of the volcano near the village of El Paraèso opened and began to spew lava and ash.

Over the next six weeks, lava flows destroyed around 2,600 buildings, more than 70 kilometers ( 43.5 miles) of road and 2.3 square kilometers (0.9 square miles) of crops. Meanwhile, the ash rose up to 6 kilometers (3.7 miles) above sea level – higher than researchers expected based on records of past eruptions in the Canary Islands – and within a month it had fallen, and in some places up to 60 centimeters (23.6 in).

In the future, analyzing the chemistry of the rocks ejected from Cumbre Vieja and examining the seismic records could reveal more details about how the volcano is preparing for the violent eruption Longpré says, which could help researchers and officials prepare for future events.

« Predicting eruptions is an imprecise science, » he admits. « What we’ve observed is really useful, but that doesn’t mean that volcanoes like Cumbre Vieja [maybe] will behave differently in the future when they reactivate. »

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