La Palma Tsunami

See Debunked: At risk from megatsunami from the Canary Isles - La Palma on the Debunking doomsday blog - see also Megatsunami on this wiki.

In a BBC television documentary broadcast in 2000, experts said that they thought that a massive landslide on a volcanic ocean island is the most likely future cause of a megatsunami. The size and power of a wave generated by such means could produce devastating effects, travelling across oceans and inundating up to 25 km inland from the coast. This research, however, was later found to be flawed.

The documentary was produced before the experts' scientific paper was published and before responses were given by other geologists. There have been megatsunamis in the past, and future megatsunamis are possible but current geological consensus is that these are only local. A megatsunami in the Canary Islands would diminish to a normal tsunami by the time it reached the continents. Also, the current consensus for La Palma is that the region conjectured to collapse is too small and too geologically stable to do so in the next 10,000 years, although there is evidence for past megatsunamis local to the Canary Isles thousands of years ago. Similar remarks apply to the suggestion of a megatsunami in Hawaii.

Location and geography
La Palma, also San Miguel de La Palma, is the most north-westerly island of the Canary Islands, Spain. La Palma has an area of 706 km2 making it the fifth largest of the seven main Canary Islands. The total population is about 86,000, of which 18,000 (2003 data) live in the capital, Santa Cruz de la Palma and about 20,000 (2004 data) in Los Llanos de Aridane. La Palma has "sister city" status with El Dorado Hills, California. Its highest mountain is the Roque de los Muchachos, at 2,426 metres, being second among the peaks of the Canaries only to the peaks of the Teide massif on Tenerife.

In 1815, the German geologist Leopold von Buch visited the Canary Islands. It was as a result of his visit to Tenerife, where he visited the Las Cañadas caldera, and then later to La Palma, where he visited the Taburiente caldera, that the Spanish word for cauldron or large cooking pot – "caldera" – was introduced into the geological vocabulary. In the center of the island is the Caldera de Taburiente National Park; one of four national parks in the Canary Islands.

Origins and geology
La Palma, like the other islands of the Canary Island archipelago, is a volcanic ocean island. The volcano rises almost 7 km above the floor of the Atlantic Ocean. There is road access from sea level to the summit at 2426 m, which is marked by an outcrop of rocks called Los Muchachos ("The Lads"). This is the site of the Roque de los Muchachos Observatory, one of the world's premier astronomical observatories.

La Palma's geography is a result of the volcanic formation of the island. The highest peaks reach over 2400 m above sea level, and the base of the island is located almost 4000 m below sea level. The northern part of La Palma is dominated by the Caldera de Taburiente, with a width of 9 km and a depth of 1500 m. It is surrounded by a ring of mountains ranging from 1600 m to 2400 m in height. On its northern side is the exposed remains of the original seamount. Only the deep Barranco de las Angustias ("Ravine of Anxiety") ravine leads into the inner area of the caldera, which is a national park. It can be reached only by hiking. The outer slopes are cut by numerous gorges which run from 2000 m down to the sea. Today, only a few of these carry water due to the many water tunnels that have been cut into the island's structure.

From the Caldera de Taburiente to the south runs the ridge Cumbre Nueva – the New Ridge, which despite its name is older than the Cumbre Vieja – Old Ridge. The southern part of La Palma consists of the Cumbre Vieja, a volcanic ridge formed by numerous volcanic cones built of lava and scoria. The Cumbre Vieja is active – but dormant, with the last eruption occurring in 1971 at the Teneguía vent which is located at the southern end of the Cumbre Vieja – Punta de Fuencaliente, (The Point of the Hot Fountain). Beyond Punta de Fuencaliente, the Cumbre Vieja continues in a southerly direction as a submarine volcano.

Volcano
Like all of the Canary Islands, La Palma originally formed as a seamount through submarine volcanic activity. La Palma is currently, along with Tenerife, the most volcanically active of the Canary Islands and was formed three to four million years ago. Its base lies almost 4000 m below sea level and reaches a height of 2426 m above sea level. About a half a million years ago, the Taburiente volcano collapsed with a giant landslide, forming the Caldera de Taburiente. Erosion has since exposed part of the seamount in the northern sector of the Caldera. Since the Spanish occupation, there have been seven eruptions – all of which have occurred on the Cumbre Vieja:


 * 1470–1492 Montaña Quemada
 * 1585 Tajuya near El Paso
 * 1646 Volcán San Martin
 * 1677 Volcán San Antonio
 * 1712 El Charco
 * 1949 Volcán Nambroque at the Duraznero, Hoyo Negro and Llano del Banco vents
 * 1971 Volcán Teneguía

During the 1949 eruption – which commenced on the fiesta of San Juan (St John) 24 June 1949 at the Duraznero, and 8 July 1949 Llano del Banco vents on the Cumbre Vieja – an earthquake, with an epicentre near Jedy, occurred. This is considered to have caused a 2.5 km crack which Bonelli Rubio (1950) named "La Grieta" – (the crack), to form, with a width of about 1 m and a depth of about 2 m. It attains a maximum displacement of ~4 m in the vicinity of the Hoyo Negro to Duraznero vents. It is not traceable southward from the Duraznero vent. North of the Hoyo Negro it traverses downslope and is traceable for ~1500 m. It should be noted that the total distance from the southern rim of the Duraznero vent to the Llano del Banco is ~4 km. In 1951 Ortiz and Bonelli-Rubio published further information in respect of the eruption and associated phenomena that occurred before and during the eruption. There is no indication that the crack has penetrated the edifice of the volcano, and, due to the absence of Minas Galerias (water tunnels) within the Cumbre Vieja, there is no possibility of examining the internal structure of the flank. Carracedo et al.;. This means that claims that the flank is in danger of failing are unfounded. However the lack of supporting evidence has not stopped claims that the flank is in danger of failing.

Short summary
Geologists and volcanologists are in general agreement that the initial study was flawed. The current geology does not suggest that a collapse is imminent. Indeed, it seems to be geologically impossible right now, the region conjectured as prone to collapse is too small and too stable to collapse within the next 10,000 years. They also concluded that a landslide is likely to happen as a series of smaller collapses rather than a single landslide from closer study of deposits left in the ocean by previous landslides. A megatsunami does seem possible locally in the distant future as there is geological evidence from past deposits suggesting that a megatsunami occurred with marine material deposited 41 to 188 meters above sea level between 32,000 and 1.75 million years ago. This seems to have been local to Gran Canaria.

Day and Ward have admitted that their original analysis of the danger was based on several worst case assumptions. A 2008 paper looked into this very worst-case scenario, the most massive slide that could happen (though unlikely and probably impossible right now with the present day geology). Although it would be a megatsunami locally in the Canary Isles, it would diminish in height to a regular tsunami when it reaches the continents as the waves interfere and spread across the oceans.

Details
In a programme transmitted by the British Broadcasting Corporation BBC Horizon broadcast on 12 October 2000, two geologists (Day and McGuire) cited this crack as proof that half of the Cumbre Vieja had moved towards the Atlantic Ocean (Day et al.; 1999, and Ward and Day, 2001 ). They postulate that this process was driven by the pressure caused by the rising magma heating water trapped within the structure of the island. They hypothesised that during a future eruption, the western flank of the Cumbre Vieja, with a mass of approximately 1.5 x1015 kg, could slide into the ocean. This could then potentially generate a giant wave which they termed a "megatsunami" around 650 m-900 m high in the region of the islands. The wave would radiate out across the Atlantic and inundate much of the eastern seaboard of North America about 7 hours later, many of the islands in the Caribbean and northern coasts of South America between six and eight hours later. They estimate that the tsunami will have waves possibly 50 m or higher causing massive devastation along the coastlines. Modelling suggests that the tsunami could inundate up to 25 km inland – depending upon topography. The basis for Ward and Day (2001) modelling the collapse of a much larger portion of the western flank than that that the currently visible surface crack indicates as being potentially unstable, was based on geological mapping by Day et al.; (1999). In this paper they argue that a large part of the western flank has been constructed in the scar of a previous collapse and therefore sits upon unstable debris.

However, nowhere in their paper do Ward and Day, (2001), make any claim about the imminent collapse of the flank. They state that they have modelled the worst-case scenario, and as a result they state "...A future collapse has the potential to cause a tsunami which may devastate the east coast of the USA and other locations...".

The claim was also explored in a BBC docu-drama called End Day which went through several hypothetical scenarios of disastrous proportions.

In 2002 the Tsunami Society (Pararas-Carayannis, 2002 ), published a statement stating "... We would like to halt the scaremongering from these unfounded reports..." The major points raised in this report include: A survey carried out by Moss et al.; (1999) reported that the western flank is stable with no indication of aseismic creep being recorded.
 * The claim that half of Cumbre Vieja dropped 4 m during the 1949 eruption is erroneous, and contradicted by physical evidence.
 * No evidence was sought or shown that there is a fault line separating a "block" of La Palma from the other half.
 * Physical evidence shows a 4 km long line in the rock, but the models assumed a 25 km line, for which no physical evidence was given. Further, there is no evidence shown that the 4 km long line extends beyond the surface.
 * There has never been an Atlantic megatsunami in recorded history.

In 2001 Carracedo et al.; stated that they consider the 1949 crack to be a shallow and inactive surface expression. They do suggest that the crack should be monitored, but consider the possibility that the edifice is unstable as being almost non-existent.

Murty et al.; (2005) claim that the morphology of the Atlantic Ocean prevents the generation and propagation of trans-oceanic tsunamis. However the Tohuko tsunami of 2011 propagated to Hawaii and the coast of California, a distance much greater than a trans-Atlantic crossing.

In 2006 professor Jan Nieuwenhuis of Delft University of Technology simulated several volcanic eruptions and calculated it would take another 10,000 years for the flanks to become sufficiently high and unstable to cause a massive collapse.,

A 2008 paper looked into this very worst case scenario, the most massive slide that could happen (though unlikely and probably impossible right now with the present day geology). They find wave heights in the range 10 to 188 meters in the Canary Isles themselves. But the waves interfere and dissipate as they head out into the Atlantic. They predict 40 meters height for some nearby island systems. For continents, the worst effects are in Northern Brazil (13.6 m), French Guyana (12.7 m), mid-US (9.6 m), Western Sahara (largest prediction at 37 meters) and Mauritania (9.7 m). This is not large enough to count as a megatsunami, with the highest prediction for Western Sahara comparable to the Japanese tsunami, so it would only be a megatsunami locally in the mid Atlantic.

An underwater eruption that began in September 2011 south of the island of El Hierro, gave rise to more speculation about the possibility of a megatsunami. As each island in the archipelago is an independent edifice often several tens of kilometres away from the adjacent island it is geologically impossible that volcanic activity on one island will influence the other islands. El Hierro lies about 100 km south of La Palma, over 100 km south-west of Tenerife. La Gomera – extinct since about 4 million year BP, Gran Canaria and Fuerteventura no eruptive history since about 10000 years BY, Lanzarote last erupted in the 19th century.

On Saturday and Sunday 7–8 October 2017, a series of minor tremors sparked panic on the Canary Islands and bringing up the topic again asking how long the island will last until the next eruption/quake and how they will prepare for such an event (even though the entire island chain would be obliterated by the waves). North of the Spanish archipelago was struck by the 3.5 magnitude quake on 24 October 2017 at 8.25pm.