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The Cumbre Vieja tsunami hazard refers to the risk that a volcanic eruption on the island of La Palma, Canary Islands, Spain, could cause a large landslide triggering a megatsunami in the Atlantic Ocean. Volcanic islands and volcanoes on land frequently undergo large landslides/collapses, which have been documented in Hawaii for example. A recent example is Anak Krakatau, which collapsed to cause the 2018 Sunda Strait tsunami.
Steven N. Ward and Simon Day in a 2001 research article proposed that a Holocene change in the eruptive activity of Cumbre Vieja volcano and a fracture on the volcano that formed during an eruption in 1949 may be the prelude to a giant collapse. They estimated that such a collapse could cause tsunamis across the entire North Atlantic and severely impact areas as far away as North America. Later research has debated whether the tsunami would still have a significant size far away from La Palma, as the tsunami wave may quickly decay in height away from the source and interactions with the continental shelves could further reduce its size.
Evidence indicates that most collapses in the Canary Islands took place as multistage events that are not as effective at creating tsunamis, and a multi-stage collapse at La Palma likewise would result in smaller tsunamis. The recurrence rate of similar collapses is extremely low, about one every 100,000 years or less in the case of the Canary Islands. Recent eruptions, including the 2021 event, did not result in a collapse. Other volcanoes across the world are at risk of causing such tsunamis.
Sector collapses and tsunamis caused by them
Giant landslides and collapses of ocean island volcanoes were first described in 1964 in Hawaii and are now known to happen in almost every ocean basin. As volcanoes grow in size they eventually become unstable and collapse, generating landslides and collapses such as the failure of Mount St. Helens in 1980 and many others. In the Hawaiian Islands, collapses with volumes of over have been identified.
A number of such landslides have been identified in the Canary Islands, especially in the more active volcanoes El Hierro, La Palma and Tenerife where about 14 such events are recorded through their deposits. They mostly take the form of debris flows with volumes of that emanate from an amphitheatre-shaped depression on the volcanic island and come to rest on the seafloor at depth. They do not appear to form through individual collapses; multi-stage failures lasting hours or days appear to be more common as has been inferred from the patterns of landslide-generated turbidite deposits in the Agadir Basin north of the Canary Islands. The most recent such event took place at El Hierro originally dated at 15,000 years ago but later re-dated to have occurred between 87,000±8,000 (margin of uncertainty) and 39,000±13,000 years ago.
Many processes are involved in the onset of volcano instability and the eventual failure of the edifice. Mechanisms that destabilize volcanic edifices to the point of collapse include inflation and deflation of magma chambers during the entry of new magma, intrusion of cryptodomes and dykes, and instability of slopes under loading from lava flows and oversteepened lava domes. Periodic collapses have been found at some volcanoes, such as at Augustine and Volcan de Colima. Shield volcanoes have different mechanical properties than stratovolcanoes as well as flatter slopes and undergo larger collapses than the latter. Finally, the mechanical stability both of the volcanic edifice and the underlying basement and the influence of climate and sea level changes play a role in volcano stability.
Tsunami hazards
Large collapses on volcanoes have generated tsunamis, of which about 1% relates to volcanic collapse; both small collapses and earthquake-linked landslides that took place in historical times generated tsunamis. The 1998 Papua New Guinea earthquake in particular drew attention to this hazard. The most recent such tsunami is the 2018 Sunda Strait tsunami, which was caused by a flank collapse of Anak Krakatau and caused at least 437 fatalities. The possibility of a large collapse of this volcano causing a tsunami was known already before the 2018 event, and the disaster drew attention to the dangers associated with non-earthquake tsunamis.
Other historically recorded examples include the 1929 Grand Banks earthquake, the 1958 Lituya Bay tsunami, numerous tsunamis at Stromboli including a 2002 tsunami that caused severe damage to coastal settlements, the 1888 tsunami caused by the Ritter Island collapse which killed about 3,000 people and is the largest historical tsunami-forming collapse with a volume of , and the 1792 Shimabara collapse of Unzen volcano in Japan which claimed approximately 15,000 victims. In total, volcanically generated tsunamis are responsible for about 20% of all fatalities related to volcanic eruptions.
Prehistoric landslides that caused tsunamis include the Storegga Slide 8,200 years ago, a submarine landslide off Norway that generated a tsunami recorded from geological evidence in the Faroe Islands, Norway and Scotland. The landslide has been modelled as a retrogressive failure that moved at a rate of . Another landslide-induced tsunami inundated Santiago, Cape Verde, 73,000 years ago after a collapse of the neighbouring Fogo volcano. More controversial is the evidence of past landslide-induced tsunamis at Kohala and Lanai in the Hawaiian Islands and at Gran Canaria in the Canary Islands, and other candidate deposits of landslide-induced tsunamis have been reported from Bermuda, Eleuthera, Mauritius, Rangiroa and Stromboli.
The size of such tsunamis depends both on the geological details of the landslide (such as its Froude number) and also on assumptions about the hydrodynamics of the model used to simulate tsunami generation, thus they have a large margin of uncertainty. Generally, landslide-induced tsunamis decay more quickly with distance than earthquake-induced tsunamis, as the former, often having a dipole structure at the source, tend to spread out radially and has a shorter wavelength (the rate at which a wave loses energy is inversely proportional to its wavelength, in other words the longer the wavelength of a wave, the slower it loses energy) while the latter disperses little as it propagates away perpendicularly to the source fault. Testing whether a given tsunami model is correct is complicated by the rarity of giant collapses. The term "megatsunami" has been defined by media and has no precise definition, although it is commonly taken to refer to tsunamis over high.
Regional context: Cumbre Vieja and the Atlantic Ocean
The Cumbre Vieja volcano lies on the southern third of La Palma (Canary Islands) and rises about above sea level and above the seafloor. It is the fastest growing volcano in the archipelago and thus dangerous in terms of collapses and landslides. Several collapses took place since the Pliocene, followed by the growth of Cumbre Vieja during the last 125,000 years. The latest eruption began on 19 September 2021 following a week of seismic activity.
During the Holocene volcanic activity on Cumbre Vieja has become concentrated along a north–south axis, which may reflect an incipient detachment fault under the volcano. During the 1949 eruption a long normal fault developed along the crest of Cumbre Vieja; it has been inactive since then and prior eruptions did not form such faults, which do not have the appearance of graben faults. Geodesy has not identified ongoing movement of the flank. Unlike on Hawaii, flank movements at Canary Islands appear to occur mainly during volcanic episodes.
Tsunamis are less common in the Atlantic Ocean than in the Pacific or the Indian oceans, but they have been observed e.g. after the 1755 Lisbon earthquake. Apart from fault lines, submarine volcanoes such as Kick'em Jenny and landslides are sources of tsunamis in the Atlantic. Tsunamis are not unique to the sea; a landslide in the Vajont Dam in 1963 caused a megatsunami resulting in around 2000 fatalities, and evidence of past tsunamis is recorded from Lake Tahoe.
Models
Model by Ward and Day 2001
Ward and Day 2001 estimated that the unstable part of Cumbre Vieja would be at least wide in north–south direction. In light of the behaviour of other documented sector collapses such as at Mount St. Helens, the headscarp of the unstable part of Cumbre Vieja is likely east from the 1949 fault and the toe of the sector lies at depth below sea level. Bathymetric observations west of La Palma support this interpretation. They had not enough information with which to estimate the thickness of the block but assumed that it would have a volume of about and the shape of a wedge, comparable to the Cumbre Nueva giant landslide 566,000 years ago also on La Palma.
The authors used linear wave theory to estimate the tsunami induced by the simulated Cumbre Vieja. They used a scenario of a collapse of that moves at a rate of about on top of a layer of mud or landslide breccia, which lubricate its movement, and eventually spreads to cover a jug-shaped area of . Ignoring that the landslide excavates part of the flank of Cumbre Vieja, thus assuming that it does not contribute to any tsunami generation, they estimated the following timing of the tsunami: 2 minutes: A thick dome of water rises above the landslide. 5 minutes: The dome collapses to a height of as it advances by ; additionally, wave valleys form. 10 minutes: The landslide is now over. Waves reaching heights of hit the three western Canary Islands. 15–60 minutes: high waves hit Africa. A wide train of waves advances across the Atlantic. 3–6 hours: The waves hit South America and Newfoundland, reaching heights of and , respectively. Spain and England are partially protected by La Palma, thus tsunami waves there only reach . 9 hours: Waves approach Florida; they are not expected to grow farther as they hit the coast.
France and the Iberian Peninsula would be affected as well. Further, the authors concluded that the size of the tsunami roughly scales with the product of the landslide speed and its volume. They suggested that traces of past such tsunamis may be found in the southeastern United States, on the continental shelf, in northeast Brazil, in the Bahamas, western Africa.
Later models
Mader 2001 employed a shallow water code that includes friction and the Coriolis force. Assuming shallow-water behaviour of the wave even with runup the eventual tsunami heights in the US and the Caribbean would not exceed and in Africa and Europe it would not be higher than . Mader 2001 also estimated that dispersion along the US coast could reduce tsunami amplitude to less than .
Gisler, Weaver and Gittings 2006 used public domain bathymetric information and the so-called "SAGE hydrocode" to simulate the tsunami stemming from variously shaped landslides. The landslides generate a single wave that eventually detaches from the landslide as the latter slows down. The waves have shorter wavelengths and periods than teletsunamis and thus do not spread as effectively as the latter away from the source and decay away roughly with the inverse of the distance. Such tsunamis would be a greater danger to the Canary Islands, the eastern Lesser Antilles, Iberia, Morocco and northeastern South America than to North America where they would be only a few centimetres high.