The BIG’95 Submarine Landslide–Generated Tsunami: A Numerical Simulation

Abstract: This article presents a reasonable present-day, sea-level highstand numerical simulation and scenario for a potential tsunami generated by a landslide with the characteristics of the BIG’95 debris flow, which occurred on the Ebro margin in the western Mediterranean Sea in prehistoric times (11,500 cal yr BP). The submarine landslide deposit covers an area of 2200 km2 of the slope and base of slope (200–1800-m water depth), involving a volume of 26 km3. A leapfrog finite difference model, COMCOT (Cornell multigrid coupled tsunami model), is used to simulate the propagation of the debris-flow-generated tsunami and its associated impact on the nearby Balearic Islands and Iberian coastlines. As a requisite of the model, reconstruction of the bathymetry before the landslide occurrence and seafloor variation during landsliding have been developed based on the conceptual and numerical model of Lastras et al. (2005). We have also taken into account all available multibeam bathymetry of the area and high-resolution seismic profiles of the debris flow deposit. The results of the numerical simulation are displayed using plots of snapshots at consecutive times, marigrams of synthetic stations, maximum amplitude plots, and spectral analyses. The obtained outputs show that the nearest shoreline, the Iberian coast, would not be the first one hit by the tsunami. The eastward, outgoing wave would arrive at Eivissa Island 18 min after the triggering of the slide and at Mallorca Island 9 min later, whereas the westward-spreading wave would hit the Iberian Peninsula 54 min after the slide was triggered. This noticeable delay in the arrival times at the peninsula is produced by the asymmetric bathymetry of the Catalano-Balearic Sea and the shoaling effect due to the presence of the wide Ebro continental shelf, which in addition significantly amplifies the tsunami wave (19 m). The wave amplitudes attain 8 m in Eivissa, and waves up to 3 m high would arrive to Palma Bay. Resonance effects produced in the narrow Santa Ponc¸a Bay in Mallorca Island could produce waves up to 9 m high. A similar event occurring today would have catastrophic consequences, especially in summer when human use of these tourist coasts increases significantly

Generación y propagación de tsunamis en el mar Catalano-Balear

[spa] En esta Tesis Doctoral se analiza el peligro de tsunamis en el mar Catalano-Balear inducidos por deslizamientos submarinos, así como los efectos del relieve submarino en la propagación y el impacto en la costa de este tipo de ondas, con especial atención a los cañones submarinos. Con el fin de inferir el potencial tsunamigénico de los deslizamientos submarinos recientes identificados en el mar Catalano-Balear se ha investigado el más extenso y presumiblemente más tsunamigénico de ellos, denominado BIG’95, y cuatro deslizamientos en el canal de Ibiza, probablemente de los menos tsunamigénicos del conjunto dados su escaso volumen, extensión, pendiente y elevada profundidad. Dado que estos últimos pudieron haber ocurrido de manera sincrónica, su potencial tsunamigénico se ha analizado considerando también el escenario de ocurrencia simultánea de los cuatro deslizamientos. Para ello se han aplicado modelos numéricos (COMCOT y landslide-HySEA) utilizado datos de batimetría de multihaz y perfiles de sísmica de muy alta resolución. Los resultados muestran que tanto el deslizamiento BIG’95 como los deslizamientos del canal de Ibiza fueron capaces de generar tsunamis, aunque con alturas de ola de diversa magnitud. Considerando el nivel del mar actual, las costas próximas al deslizamiento BIG’95 sufrirían el impacto de olas con alturas de hasta 9 m, mientras que los tsunamis generados por los deslizamientos del canal de Ibiza apenas superarían los 0,5 m, a excepción de una franja de isla de Formentera, donde se alcanzarían 2 m de altura. El escenario sincrónico de los deslizamientos del Canal de Ibiza incrementaría la altura en la costa hasta un 22%. En vista de estos resultados, otros deslizamientos del mar Catalano-Balear también podrían haber generado tsunamis. Del análisis de estos escenarios se deduce que el margen continental del Ebro, con una plataforma continental especialmente extensa, ejerce un notable efecto de asomeramiento en las olas de tsunami, incrementando la altura de ola y dilatando el tiempo de llegada a la costa. Un efecto similar ocurre al suroeste de la isla de Formentera. Además, las estrechas bahías características de algunos sectores mallorquines podrían sufrir fenómenos de resonancia inducidos por olas de longitud de onda similar a las generadas por el BIG’95. Por otro lado, se han creado una serie de batimetrías teóricas mediante funciones matemáticas reflejando la geometría de un margen continental con una costa rectilínea paralela al borde de plataforma e incidido por cañones submarinos de diversas morfologías, variando su grado de incisión, oblicuidad respecto a la costa y su anchura. Sobre estas batimetrías se han propagado olas de tsunami de diferentes características incidiendo perpendicularmente al margen. A partir de estos escenarios se pudo establecer que la franja de costa situada frente a la cabecera se ve afectada por olas con altura máxima inferior a la de las olas que alcanzan las costas del margen de referencia sin cañón submarino, mientras que, al contrario, las alturas máximas son mayores en las franjas litorales situadas frente a los dos flancos del cañón. Esta variación en las alturas máximas en la franja litoral se acentúa cuanto más próxima está la cabecera del cañón a la línea de costa, lo cual favorece además la formación de ondas de borde. Además, la longitud de la franja litoral afectada por las variaciones en las alturas máximas aumenta en función de la anchura del cañón submarino, y su distribución depende de la oblicuidad del cañón; cuyo incremento produce una mayor altura de ola en la franja de costa situada frente al flanco externo del cañón. Este mismo patrón se observó en la propagación de una onda teórica sobre la batimetría real del sector del margen catalán correspondiente al cañón de Blanes.[eng] In the present PhD Thesis, tsunami hazard induced by submarine landslides in the Catalano-Balearic Sea is analysed, as well as the effects exerted by seafloor morphology, with special attention to submarine canyons, on the propagation and coastal impact of such waves. In order to analyse the tsunamigenic potential of the submarine landslide deposits identified in the Catalano-Balearic Sea, the largest and presumably the most tsunamigenic one, the BIG’95 debris flow, and four slides in the Eivissa Channel, likely among the less tsunamigenic because of their small dimensions, smooth slope and large water depths where they are emplaced, have been chosen. Moreover, the tsunamigenic potential of the four Eivissa Channel Slides has also been analysed considering the simultaneous failure of the four slides. Their tsunamigenic potential has been evaluated by means of numerical modelling and using multibeam bathymetry and very high resolution seismic profiles. The resulting scenarios show that both the BIG'95 and the Eivissa Channel landslides were able to generate tsunamis, though with very different wave heights, according to their characteristics. In the case in which the Eivissa Channel Slides fail synchronously, wave height increases by 22%. The effect on tsunami propagation produced by the presence of a submarine canyon incised in the continental margin has been analysed using both theoretical waves and bathymetries and numerical modelling. The results indicate that the stretch of the coast facing the canyon head is affected by waves with lower maximum wave heights than those reaching the shores of a reference non-canyoned margin, whereas maximum wave heights are higher along the coastal strips placed in front of both canyon flanks. This variation in maximum wave heights along the coast is accentuated the closer the canyon head to the coastline. Additionally, the length of the coastline affected by variations in the maximum wave heights proportionally increases with the width of the submarine canyon, and their distribution, depends on the canyon axis orientation. Similar results have been obtained by propagating the same wave over the section of the Blanes Canyon in the Catalan margin in order to cross-check the theoretical results with a real case

Landslides cause tsunami waves: insights from Aysén Fjord, Chile

On 21 April 2007, an Mw 6.2 earthquake produced an unforeseen chain of events in the Aysén fjord (Chilean Patagonia, 45.5°S). The earthquake triggered hundreds of subaerial landslides along the fjord flanks. Some of the landslides eventually involved a subaqueous component that, in turn, generated a series of displacement waves tsunami- like waves produced by the fast entry of a ubaerial landmass into a water body within the fjord [Naranjo et al., 2009; Sepúlveda and Serey, 2009; Hermanns et al., 2013]. These waves, with run-ups several meters high along the shoreline, caused 10 fatalities. In addition, they severely damaged salmon farms, which constitute the main economic activity in the region, setting free millions of cultivated salmon with still unknown ecological consequences

Landslides cause tsunami waves: insights from Aysén Fjord, Chile

On 21 April 2007, an Mw 6.2 earthquake produced an unforeseen chain of events in the Aysén fjord (Chilean Patagonia, 45.5°S). The earthquake triggered hundreds of subaerial landslides along the fjord flanks. Some of the landslides eventually involved a subaqueous component that, in turn, generated a series of displacement waves tsunami- like waves produced by the fast entry of a ubaerial landmass into a water body within the fjord [Naranjo et al., 2009; Sepúlveda and Serey, 2009; Hermanns et al., 2013]. These waves, with run-ups several meters high along the shoreline, caused 10 fatalities. In addition, they severely damaged salmon farms, which constitute the main economic activity in the region, setting free millions of cultivated salmon with still unknown ecological consequences