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

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

3. 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

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

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

6. 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

7. Evidence of past Landslides and Tsunamis
In The Bahamas, there are 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

8. 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

9. 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

10. 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

11. 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)

12. 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

13. 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

14. 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

15. 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

16. 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

17. Landslide Dimensions 1
The mass of rock likely to slide is the same size as The Isle of Man
The volume is estimated between 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

19. 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 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

20. Landslide Dynamics
The landslide is likely to move as a coherent block for at least km before breaking up
It will cascade down to a depth of 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

21. 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

22. 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

23. 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

24. 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 further eruptions to fully destabilise it

25. 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

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

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

28. 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 metres in height

29. 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

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

31. 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 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

32. The End
Tsunami Generator!