Update: This story was originally published on September 18 2021. Since then, the Cumbre Vieja volcanic ridge has begun to erupt. Multiple eruption points, some producing lava flows, have been recorded. Local evacuations on the island of La Palma are now underway
A series of small earthquakes in Spain's Canary Islands has put authorities on alert for a possible volcanic eruption, but a much larger danger may lie beneath the seas off the island chain that some scientists say could result in waves hundreds of metres high radiating out into the Atlantic.
Authorities have detected more than 4200 tremors in what scientists are calling an "earthquake swarm" around La Palma island since last Saturday.
La Palma is the fifth-largest island in the volcanic chain, which sits 100km off the coast of Morocco in the Atlantic Ocean.
Earthquake swarms can indicate an approaching eruption and, in this case, the quakes have raised concerns about Cumbre Vieja, a dormant volcanic ridge that last erupted in 1971.
Before that, the volcano erupted in 1949 and it is the changes wrought by that eruption that some believe put the entire structure of the volcano at risk.
The alarm was first raised at the turn of the millennium by Dr Simon Day and his colleagues at University College London.
This was a few years before the devastating Boxing Day tsunami reminded humanity of the terrifying force of the seismically driven waves.
A year later, in 2001, Day teamed up with Dr Steven N Ward, from the University of California, to author a study on the destructive potential of the Cumbre Vieja volcano.
Day's original research in 2000 had found that any future eruption of Cumbre Vieja could dislodge a massive section of the island of La Palma.
Though subsequent studies have tempered some of Day and Ward's catastrophic predictions, there is still a risk.
After the 1949 eruption, ruptures were found on the surface. In Day and Ward's later study, these ruptures were described as "ominous" because they were evidence of a subsurface detachment.
In other words, the crack doesn't stop at the surface.
In the event of a collapse driven by a fresh eruption, Day found that anywhere between 150 to 500 cubic kilometres of rock could slide into the ocean at 100 metres per second.
The immense force caused by such a landslide would generate huge waves, hundreds of metres high, that would spread across the Atlantic and hit the coast of the Americas at heights of up to 25 metres.
The pair found that, although there is no historical record of a mega-tsunami caused by the lateral collapse of an oceanic volcano, the geological record shows clear evidence of their power.
That is to say, modern humans may not have been around to witness it, but it has happened.
And could happen again.
The path of the waves
Day and Ward's 2001 paper reveals their theory on how the waves could radiate out from La Palma, bringing widespread devastation across four continents.
Firstly, as the massive amount of rock and earth smashes into the ocean, a huge water dome would build, reaching heights of up to 900 metres.
Within 5 minutes, this wave would have outrun the landslide that birthed it and, 50km from La Palma would have reduced in height.
To 500 metres.
Within 10 minutes, wave hundreds of metres high would smash into the western islands of the Canary chain.
Between 15 minutes and an hour after the event, the eastern islands of the Canaries would be hit and waves would eventually reach the mainland of Africa, at heights of between 50 and 100 metres, bringing with them immense devastation and loss of life.
But to the west, in the vast expanse of the Atlantic Ocean, the waves would be spreading out across 500km, with wave heights still hitting 60 metres.
Past the three-hour mark, the long arc would curve back around to strike Europe, with Spain and England experiencing wave heights of 5 to 7 metres.
Then, as waves of around 10 metres hit the North American coastline at Newfoundland, larger waves of 15 to 20 metres would strike coastal Brazil and other northern regions of South America.
A full nine hours after the event, the US state of Florida would face the waves.
Day and Ward predict the waves "parading in a dozen cycles or more" and reaching up to 25 metres in height.
For low-lying Florida, the resulting inundation would stretch far inland.
For context, the largest wave recorded during the 2004 tsunami was estimated at between 15 and 30 metres, recorded on the west coast of the Indonesian province of Aceh.
Will it happen?
Day and Ward's research shocked many when it was released in 2001, but subsequent studies of the potential consequences of a collapse at Cumbe Vieja have significantly played down the risk.
In later years, and often with the benefit of experience hard-won from the 2004 Boxing Day tsunami, scientists using different models have found that any waves created could be considerably smaller.
Some have argued that any collapse of the ridge could not occur with the force described in Ward and Day's paper, with some positing that a collapse would happen in stages.
Others have pointed out that undersea topography could reduce the height of the waves in many areas.
But most agree that if a collapse were to occur, large waves would be generated that would prove tragic for the Canary Islands themselves.
Where scientists differ is how far away effects would be felt and to what extent.
Officials in the Canary Islands have said they had no indication an eruption was imminent, and a scientific committee monitoring the activity said the number of tremors and their magnitude had fallen by Thursday of this week.
Even so, the Scientific Committee for the Special Civil Protection Plan and Emergency Response for Volcanic Risks warned there could be a rapid, renewed surge in quakes and kept the public warning level on yellow, according to private Spanish news agency Europa Press.
"The decrease in seismic activity may be transient and does not necessarily imply a halt to the reactivation," the regional emergency services said in a statement after a meeting with politicians, volcano experts and civil defence authorities.
Volcano warnings are announced in accordance with the level of risk, rising through green, yellow, orange and red.
Scientific Committee for the Special Civil Protection Plan and Emergency Response for Volcanic Risks reported that ground depressions up to 10 centimetres deep have formed — an occurrence often attributed to magma movements.
Before a volcano erupts, there is a gradual increase in seismic activity that can build up over a prolonged period.
And even if the volcano does erupt, it's not certain that the landslide would occur.
In 2001, Dr Simon Day said: "The collapse will occur during some future eruption after days or weeks of precursory deformation and earthquakes. An effective earthquake monitoring system could provide advanced warning of a likely collapse and allow early emergency management organisations a valuable window of time in which to plan and respond."
"Eruptions of Cumbre Vieja occur at intervals of decades to a century or so and there may be a number of eruptions before its collapse," Day added.
"Although the year to year probability of a collapse is therefore low, the resulting tsunami would be a major disaster with indirect effects around the world. Cumbre Vieja needs to monitored closely for any signs of impending volcanic activity and for the deformation that would precede collapse."
Gary McMurty, a scientist who specialised in landslides, told the New Scientist in 2001 that we shouldn't panic about Ward and Day's original findings.
"These events are very rare and shouldn't worry anyone who has a lifetime of less than a hundred years," he said.
- with Associated Press