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The tsunami signature on a submerged promontory: the case study of the Atacames Promontory, Ecuador

Abstract : Shelf promontories exhibit very specific bathymetric features with regards to tsunamis. Because of their submerged cape morphology, a potential tsunami generated seawards of the promontory will exhibit a specific mode of propagation and coastal impact. To identify this peculiar tsunami signature, the Atacames Promontory, Ecuador, was chosen as a case study (another example is the shelf of the Nile delta, Egypt). The area is tectonically very active, hosts earthquakes among the most powerful recorded, as well as areas of slope instabilities that have triggered significant submarine landslides in the past (several cubic kilometres of volume). Both types of events are likely to be tsunamigenic. To examine the tsunami behaviour at the coastal area of the promontory and at its vicinity, we have considered two examples of tsunamigenic landslides of which scars were identified near the base of the continental slope. We also took into consideration two earthquake scenarios that are likely to represent most classes of earthquakes possibly occurring in this area depending on their locations and subsequent tsunami directivity, that is, a sensitivity test investigation. We took two distinct earthquake scenarios which are based on the 1942 and 1958 events that stroke the area. Then we computed their derived tsunamis and analysed their coastal impact. We found that significant tsunamis can be generated by either landslides or earthquakes. However, the maxima of wave amplitude occur offshore (but still above the underwater promontory): the concave-type shape of the bathymetric field often yields a refraction/focusing area that is located on the shelf promontory and not at the coast area of the promontory: the wave propagates first through the focusing area before striking the considered coast. This area may be considered as a sheltered zone. Besides, in the vicinity of the promontory (not exactly concerned by the study), the city of Esmeraldas, is relatively sheltered due to the presence of the underwater canyon at its termination and due to diverging waves.
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M. Ioualalen, Gueorgui Ratzov, J.-Y. Collot, E. Sanclemente. The tsunami signature on a submerged promontory: the case study of the Atacames Promontory, Ecuador. Geophysical Journal International, Oxford University Press (OUP), 2011, 184 (2), pp.680-688. ⟨10.1111/j.1365-246X.2010.04878.x⟩. ⟨hal-00585747⟩

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