The Canary Islands and the Tsunami Threat I.G.Kenyon

The Canary Islands comprise 7 volcanic islands that rise 6 to 8 km from the seafloor Eruptions occur on average every 30 years

Landslide History of the Canary Islands 1 At least 14 large landslides have been mapped offshore from the Canary Islands Most of these landslides have been dated within the last 1 million years

Landslide History of the Canary Islands 2 Recurrence interval is 100,000 years for all islands and around 300,000 years for individual islands Landslides comprise 50 to 500 km³ of debris avalanches spread over 130 km from their source Around 25% of the mass of the islands have been removed by landslides

Submarine avalanche deposits around the Western Canary Islands Most likely to collapse next Most Recent

Evidence of past Landslides and Tsunamis Canary Island volcanoes have experienced at least fourteen major collapses over several million years Submarine landslides occur as fans of debris on the sea floor and occur once every 10,000 to 100,000years All the islands except La Gomera have submarine avalanche deposits around them The most recent collapse was the El Golfo avalanche on El Hierro 15–20,000 years ago

Evidence of past Landslides and Tsunamis In The Bahamas, there are 1000+ tonne blocks of coral limestone found high above current sea level. The only explanation for these is that they were transported there by a large tsunami in the recent geological past. The most recent volcanic island landslide is thought to be around 4,000 years old and occurred in the Pacific Ocean from the flanks of Reunion Island

Landslide deposits around La Palma Cumbre Vieja volcano is likely to be the next collapse Last collapse on La Palma occurred between 175,000 and 536,000 years ago

Basic Geographical Facts La Palma lies at the North West edge of the Canarian chain Triangular in shape, it covers an area of 706km² Maximum height 2426m (Roque de la Muchachos) Rests on ocean floor 4000 metres in depth Considered the steepest sided island in the world with slopes averaging 15º to 20º

Major normal fault that has slipped 4 metres towards the west Geology map of La Palma showing the locations and dates of historical eruptions from 1470 Major normal fault that has slipped 4 metres towards the west

Geological Structure 1 La Palma is made up of two distinct volcanic structures A circular 25km diameter shield volcano in the north (Northern Shield) Extinct for last 400,000 years, the Northern Shield has a deeply eroded radial network of barrancos Has a 6km diameter erosional depression on its SW flank (Caldera de Taburiente)

Geological Structure 2 A north-south elongated 20km long Cumbre Vieja rift in the south Cumbre Vieja volcano highly active for last 120,000 years Six eruptions in last 500 years, two of them in the 20th century (1949 and 1971) Most active volcano in the Canary Islands over the last 125,000 years

Recent Eruptions Cumbre Vieja is the active volcanic centre and comprises the southern third of the island Has erupted 6 times in the last 500 years Dormancy periods have vary from 22 to 237 years

Cross section showing the internal structure of Cumbre Vieja Deformation zone of many small faults Present day profile of Cumbre Vieja Steep 1949 fault Sea level Lower part of block saturated by seawater will lower frictional cohesion here Cumbre Vieja sequence lies unconformably on older avalanche deposits from the earlier Cumbre Nueva collapse

Possible Trigger Factors 1 Dyke emplacement may initiate collapse Rising magma may increase the pore-water pressure within the volcano causing a reduction in friction along the fault plane Dyke emplacement may initiate collapse Rising magma may generate small earthquakes which may also help further destabilise the faulted block

Possible Trigger Factors 2 Climate change-rising sea levels and wetter conditions may reduce friction within the faulted block as more water penetrates into the structure Warming of the oceans destabilises gas hydrates stored in marine sediments

Landslide Dimensions 1 The mass of rock likely to slide is the same size as The Isle of Man The volume is estimated between 150 and 500 km³ of rock Maximum dimensions are 25km long x 20km wide x 1-2km thick Landslide is like a half submerged wedge of cheddar cheese lying on its side with the thin half under water

Landslide Dimensions 2 Landslide estimated dimensions are based on the average volume of observed avalanche deposits around the Canary Islands If the block that detaches is smaller, say only 250 km³ and moves at only 50 metres per second it will generate a tsunami only 25-40% the size of the worst case scenario Although this landslide is half the magnitude and intensity, the waves will still match the size and destructive capacity of the 2004 Asian Tsunami

Landslide Dynamics The landslide is likely to move as a coherent block for at least 15km before breaking up It will cascade down to a depth of 4000 metres to the ocean floor at an average speed of 360 kilometres per hour As the avalanche spreads out on the ocean floor it will cover an estimated area of 3,500km²

3D Block diagram of La Palma showing the major structural features La Palma is very steep sided with slopes of >15-20º Next likely collapse Atlantic Ocean floor 4,000 metres deep Older avalanche deposits

Why is La Palma so unstable? Instability initiated during the 1949 eruption Fractures formed along the flanks of Cumbre Vieja An entire flank separated from the rest of the volcano and dropped 4 metres towards the sea A north-south trending normal fault that dips west extends for a distance of 4 kilometres The 1971 eruption further to the south caused no further movement of the detached block Monitoring during the 1990’s suggest that the landslide is moving seawards at a rate of between 0.5 – 1.0 cm per year

The fault scarp exposed near the summit of Cumbre Vieja Fault scarp here is approximately 2 metres in height Upthrown side Dip of fault plane Downthrown side A normal fault

Another eruption is likely in the next decade or so Surprisingly the 1971 eruption did not cause the faulted block to reactivate movement westwards 1971 eruption by day 1971 eruption at night Another eruption is likely in the next decade or so The next eruption may cause the faulted block to collapse or it might take another 5, 10 or even 20 further eruptions to fully destabilise it

Likely position of head scar following slope failure La Palma – birds eye view showing likely position of the next landslide 1949 eruptions that produced major faulting on the flank of Cumbre Vieja 1971 eruption Teneguia Likely position of head scar following slope failure

Tsunami Generation 1 (A) Within 2 minutes of the initial failure, a dome of water 900 metres high may be generated (B) Within 5 minutes the leading wave height will drop to 500 metres after 50km of travel (C) Waves of over 200 metres high hit the westernmost islands of the Canary chain

Tsunami Generation 2 From 15 to 60 minutes waves sweep eastwards through the rest of the Canary Islands and 50-100 metre waves make first landfall on the African mainland D,E and F

Tsunami Generation 3 From 3-6 hours the tsunami expands across the Atlantic with waves of 10 metres hitting Newfoundland, whilst Florida and South America can expect waves of 15-25 metres in height

Tsunamis will reach the eastern seaboard of the USA within 8-9 hours Tsunami Generation 4 Tsunamis will reach the eastern seaboard of the USA within 8-9 hours Even with maximum warning time, it is unlikely that all areas at risk could be sufficiently evacuated In some cases the best escape may be vertical in high rise buildings in Miami, New York, Boston and other port cities

Towards the northeast, Spain and England are likely to experience waves of 5 to 7 metres in height

Could the faulted block be quarried out to remove the hazard? The block is 25km long x 20 km wide and 1-2 km thick Assuming a truck could remove 10 cubic metres of rock in a single journey Would need 15 – 50 billion journeys to remove it If a truck left every minute of every day it would take 10 – 35 million years This does not include excavation time nor that the lower part of the block is under water

The End Tsunami Generator!