Of course La Palma has undergone a previous flank collapse event, and there have been similar collapses elsewhere in the Canary Islands.\u00a0 Interestingly, none of these appear to have generated widespread tsunami deposits around the Atlantic basin.\u00a0 The key to the idea such an event developing again is the events of the major eruption in 1949.\u00a0 During this event, a fault structure was observed to develop along a part of the Cumbre Vieja ridge.\u00a0 This has been interpreted as indicating movement of the flank of the volcano towards the west, and thus the development of a potential flank collapse landslide on the southern part of La Palma.\u00a0 I spent a day up on Cumbre Vieja, with the main aim of taking a look at this fault scarp. \u00a0\u00a0 Armed with a map from one of the key papers I roamed up and down the mountains on a most beautiful day.\u00a0 The feature that I found is remarkably unremarkable in landslide terms.\u00a0 Based upon the maps, the most obvious feature that I could find is the scarp shown below:-<\/p>\n
![\"La](\"https:\/\/blogs.agu.org\/landslideblog\/files\/2017\/07\/IMG_9479-e1500013368959.jpg\")
The possible fault scarp on the Cumbre Vieja volcano on La Palma, viewed from the north<\/p><\/div>\n
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This scarp is about 3 m high at the maximum – if you look carefully there is a person walking down the path for scale.\u00a0 It is reasonably clear in the landscape for some hundreds of metres before merging into the flank of one of the volcanic craters:<\/p>\n
![\"La](\"https:\/\/blogs.agu.org\/landslideblog\/files\/2017\/07\/IMG_9401-e1500013609927.jpg\")
The apparent fault scarp on La Palma, viewed from the south. Note the people for scale.<\/p><\/div>\n
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![\"La](\"https:\/\/blogs.agu.org\/landslideblog\/files\/2017\/07\/IMG_9402-e1500013737553.jpg\")
The trace of the fault scarp of Cumbre Vieja, merging into one of the volcanic craters.<\/p><\/div>\n
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I have to admit that I struggle to believe that this feature is a key component of the mega-landslide hypothesis given its small size, but I could not find any more convincing alternative.\u00a0 Perhaps I missed the correct feature. The scarp suggests limited movement on the landslide, even if it is a correct interpretation, suggesting in turn that an actual failure would be unlikely in the foreseeable future.<\/p>\n
This fault scarp did not reactivate in the smaller 1971 eruption.\u00a0 Monitoring of the flanks suggest that there is no sign of rapid current movement on this scale, but that there may be some signs from satellite data of very low velocity creep on this slope, which is not a surprise (this may well be true of all large slopes in weak materials).\u00a0 Thus, the mega-landslide hypothesis is that this volcano would only become unstable in a future eruption, and that in such circumstances the flank could collapse in a single coherent block to generate the tsunami.\u00a0\u00a0 The paper that modeled the tsunami, Ward and Day (2001)<\/a> modeled a landslide of about 450 cubic kilometres – i.e. they took the very largest volume that is imaginable.\u00a0 This seems a little odd to me – the rear scarp of their mega-landslide appears not to mobilise the scarp shown above, but one considerably to the east, creating a much larger block.\u00a0 I am not sure that I understand the reasoning for this.\u00a0 The model then assumes a series of extreme scenarios:<\/p>\n A change (reduction) in any of these parameters would yield a much smaller tsunami.\u00a0 For example, subsequent work (Abadie et al.<\/em> 2012<\/a>) has taken the “\u201ccredible worst case scenario\u201d (derived using slope stability analysis), to have a volume of 80 cubic kilometres (but note the factor of safety of the slope was found to be considerably higher than one, indicating that the slope is not particularly unstable). \u00a0\u00a0 Modelling of the tsunami generated by such a landslide, using a more refined tsunami simulation,\u00a0 does generate a very significant wave close to La Palma.\u00a0 This wave would be significant as it crossed the continental shelf off the east coast of North America, but would lose a great deal of energy due to frictional effects in this region. \u00a0 Thus, for a 80 cubic kilometre\u00a0“credible worst case scenario\u201d flank collapse on La Palma,\u00a0 wave heights on the east coast of the USA were found by Tehranirad et al.<\/em> (2015)<\/a> to be less than 2 metres along the coastline.<\/p>\n This is not the disastrous scenario that the newspapers have so enjoyed featuring.<\/p>\n Abadie<\/span> S., Harris<\/span> J.C., Grilli<\/span> S.T. and R. Fabre<\/span>, 2012. Numerical modeling of tsunami waves generated by the flank collapse of the Cumbre Vieja Volcano (La Palma, Canary Islands) : tsunami source and near field effects<\/a>. Journal of Geophysical Research<\/em>, 117<\/strong>: C05030.<\/p>\n Tehranirad, B., Harris, J.C., Grilli, A.R. et al.<\/em> 2015.\u00a0 Far-Field Tsunami Impact in the North Atlantic Basin from Large Scale Flank Collapses of the Cumbre Vieja Volcano, La Palma<\/a>. Pure and Applied Geophysics<\/em> 172<\/strong>: 3589.<\/p>\n\n
References<\/h5>\n