'Unprecedented' mini quake swarm in La Palma island

by 11/10/2017 20:06:00 0 comments 1 Views
  • Dozens of mini-earthquakes recorded on the seabed under La Palma island
  • The strongest was hit 2.7 on the Richter scale, at 17.4 miles under the sea
  • Some experts say a volcanic eruption in the area could trigger a large chunk of land to fall into the ocean creating a 'megatsunami'
  • However, leading geologists describe the theory as 'rubbish' 
  •  La Palma, a popular holiday destination, last saw a volcanic eruption in 1971

By Mark Prigg and Sara Malm In London For Mailonline

Published: 19:59 EDT, 11 October 2017 | Updated: 20:06 EDT, 11 October 2017

The Canary holiday island of La Palma has recorded dozens of mini-earthquakes over the weekend, scientists report. 

More than 40 tremors were recorded in just 48 hours, all between 1.5 and 2.7 on the Richter scale, although they took place at such depth under the sea that residents on the island did not feel them.

It has raised fears the islands' huge Cumbre Vieja volcano could erupt, which some scientists have warned could then collapse into the sea, causing a a mile-high tidal wave that would hit Spain, Britain and the east coast of the US - although leading experts describe this theory as 'rubbish'.

Shaken: More than 40 tremours were recorded in just 48 hours, all between 1.5 and 2.7 on the Richter scale, by seismologists on La Palma in the Spanish Canary Islands
Shaken: More than 40 tremours were recorded in just 48 hours, all between 1.5 and 2.7 on the Richter scale, by seismologists on La Palma in the Spanish Canary Islands

Shaken: More than 40 tremours were recorded in just 48 hours, all between 1.5 and 2.7 on the Richter scale, by seismologists on La Palma in the Spanish Canary Islands

THE QUAKE SWARM 

The largest of the tremors, which took place at 1pm on Saturday hit 2.7 on the Richter scale and was located at a depth of 17.4miles.

In the following hours, another ten tremors were recorded, taking the total of mini-earthquakes until Tuesday to 50, according to the National Geographic Institute (IGN). 

La Palma is the most north-westerly island of the Canary Islands, and is home to some 86,000 people - a population which increases significantly during tourist season.

Like the other Canary Islands, La Palma is volcanic and is considered the most 'active' in the archipelago.

The most recent eruption on the island, which saw the Cumbre Vieja - 'Old Summit' - volcano erupt, took place in 1971.

 

The controversial 2013 study by Dr Steven Ward, of the University of California, and Dr Simon Day, of the Benfield Greig Hazard Research Centre at University College London, claimed that if the Cumbre Vieja – an active but dormant volcano – were to erupt, the western flank of the mountain could tumble into the sea.

Writing in Geophysical Research Letters, they said a build-up of groundwater could destabilise a block of rock up to 500 cubic km in size, which could break off, smashing into the sea at up to 350 km an hour (220 mph).

This eruption could cause the landslide, which will unleash a deadly wall of water, initially almost 3,000ft high and several miles wide, to hurtle at speeds of up to 800 km an hour (500 mph).

However,  experts have slammed the study as a 'scare'.

'This scare should be consigned to the garbage can once and for all,' wrote Dave Petley in a post for the American Geophysical Union.

He said the phenomenon the researchers studied, known as a flank collapse, would only have generated small waves.

 'Previous flank collapses have occurred as a series of distinct events rather than as a single coherent block,' he wrote.

'Each of these could have been able to generate a very large wave, and even a local tsunami. However, they would not have generated a megatsunami. 

'There is no reason to believe that a future event will behave differently.' 

Professor Iain Stewart the director of the Sustainable Earth Institute at Plymouth University, told Devon Live: 'I'm not going to say it's rubbish - but it hasn't happened since the beginning of civilisation.

Rare: Such seismic activity is not usual on Palma but, experts say, not abnormal 
Rare: Such seismic activity is not usual on Palma but, experts say, not abnormal 

Rare: Such seismic activity is not usual on Palma but, experts say, not abnormal 

'The short answer is that it's not happened in the last 10,000 years.

'At the moment there is definitely nothing to be disturbed about.'

The largest of the tremors, which took place at 1pm on Saturday hit 2.7 on the Richter scale and was located at a depth of 17.4miles.

In the following hours, another ten tremors were recorded, taking the total of mini-earthquakes until Tuesday to 50, according to the National Geographic Institute (IGN). 

La Palma is the most north-westerly island of the Canary Islands, and is home to some 86,000 people - a population which increases significantly during tourist season.

Like the other Canary Islands, La Palma is volcanic and is considered the most 'active' in the archipelago.

The most recent eruption on the island, which saw the Cumbre Vieja - 'Old Summit' - volcano erupt, took place in 1971.

The current event has been dubbed a 'seismic swarm', and while unusual, large numbers of these smaller tremours are not abnormal, the director of the IGN in the Canary Islands, María José Blanco, told Canarias7.

However, she added that they had 'never recorded a similar swarm' since monitoring began on La Palma.

The IGN and the Volcanological Institute of the Canary Islands (Involcan) have increased surveillance on the island to monitor the increase in seismic activity.

HOW COMMON ARE MEGATSUNAMIS? 

In the early 2000s, other researchers started publishing evidence that the Cape Verdes could generate large tsunamis. 

Others have argued that Spain's Canary Islands have already done so. 

Simon Day, a senior researcher at University College London has sparked repeated controversy bywarning that any future eruption of the Canary Islands' active Cumbre Vieja volcano could set off a flank collapse that might form an initial wave 3,000 feet high. 

This, he says, could erase more than nearby islands. Such a wave might still be 300 feet high when it reached west Africa an hour or so later he says, and would still be 150 feet high along the coasts of North and South America. 

So far, such studies have raised mainly tsunamis of publicity, and vigorous objections from other scientists that such events are improbable. 

A 2013 study of deep-sea sediments by the United Kingdom's National Oceanography Centre suggests that the Canaries have probably mostly seen gradual collapses.

Part of the controversy hangs not only on the physics of the collapses themselves, but on how efficiently resulting waves could travel. 

In 1792, part of Japan's Mount Unzen collapsed, hitting a series of nearby bays with waves as high as 300 feet, and killing some 15,000 people. 

On July 9, 1958, an earthquake shook 90 million tons of rock into Alaska's isolated Lituya Bay; this created an astounding 1,724-foot-high wave, the largest ever recorded. 

Two fishermen who happened to be in their boat that day were carried clear over a nearby forest; miraculously, they survived.

These events, however, occurred in confined spaces. 

In the open ocean, waves created by landslides are generally thought to lose energy quickly, and thus to pose mainly a regional hazard.

However, this is based largely on modeling, not real-world experience, so no one really knows how fast a killer wave might decay into a harmless ripple. 

 

A spokesperson for Involcan told Canarias7 that 'seismic swarms' are 'absolutely normal' for an active volcano such as Cumbre Vieja. 

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