Hazard Potential of Volcanic Flank Collapses Raised by New Megatsunami Evidence

Tuesday, 15 December 2015: 14:25
309 (Moscone South)
Ricardo S Ramalho1, Gisela Winckler2, José Madeira3, George R Helffrich4, Ana Hipólito5, Rui Quartau6, Katherine Adena1 and Joerg M Schaefer7, (1)University of Bristol, School of Earth Sciences, Bristol, United Kingdom, (2)Lamont -Doherty Earth Observatory, Palisades, NY, United States, (3)Instituto Dom Luiz, Universidade de Lisboa, Lisboa, Portugal, (4)Tokyo Institute of Technology, Earth-Life Science Institute, Tokyo, Japan, (5)Universidade dos Açores, Centro de Vulcanologia e Avaliação de Riscos Geológicos, Ponta Delgada, Portugal, (6)Instituto Português do Mar e Atmosfera, Divisão de Geologia e Georecursos Marinhos, Lisboa, Portugal, (7)Columbia University of New York, Palisades, NY, United States
Large-scale gravitational flank collapses of steep volcanic islands are hypothetically capable of triggering megatsunamis with highly catastrophic effects. Yet evidence for the existence and impact of collapsed-triggered megatsunamis and their run-up heights remains scarce and/or is highly contentious. Therefore a considerable debate still exists over the potential magnitude of collapse-triggered tsunamis and their inherent hazard. In particular, doubts still remain whether or not large-scale flank failures typically generate enough volume flux to result in megatsunamis, or alternatively operate by slow-moving or multiple smaller episodic failures with much lower tsunamigenic potential. Here we show that one of the tallest and most active oceanic volcanoes on Earth – Fogo, in the Cape Verde Islands – collapsed catastrophically and triggered a megatsunami with devastating near-field effects ~73,000 years ago. Our deductions are based on the recent discovery and cosmogenic 3He dating of tsunamigenic deposits - comprising fields of stranded megaclasts, chaotic conglomerates, and sand sheets – found on the adjacent Santiago Island, which attest to the impact of this megatsunami and document wave run-up heights exceeding 270 m. The evidence reported here implies that Fogo’s flank failure involved at least one sudden and voluminous event that resulted in a megatsunami, in contrast to what has been suggested before. Our work thus provides another line of evidence that large-scale flank failures at steep volcanic islands may indeed happen catastrophically and are capable of triggering tsunamis of enormous height and energy. This new line of evidence therefore reinforces the hazard potential of volcanic island collapses and stands as a warning that such hazard should not be underestimated, particularly in areas where volcanic island edifices are close to other islands or to highly populated continental margins.