Evidence for the multi-phase nature of the Monte Amarelo flank collapse based on reflection seismic data offshore Fogo, Cape Verde

The relatively fast and continuous build-up of a volcanic island can lead to an unstable edifice which might be prone to collapse under its own weight. Evidence for major flank-collapse events are found offshore a wide range of volcanic islands, suggesting this phenomenon to be a common process with potentially disastrous consequences as they can generate devastating tsunamis. Fogo Island is situated in the southeastern part of the Cape Verdean Archipelago, about 700 km west of Dakar, NW Africa. During the Monte Amarelo event, approx. 73,000 years ago, its subaerial eastern flank collapsed into the ocean, triggering a mega tsunami with run-up heights of up to 270 m as witnessed by dated tsunami deposits found on the neighboring island of Santiago, ~60 km eastwards from Fogo. Whether the collapse occurred in a single event or multiple phases remains unknown and a volume estimation of the collapse difficult and uncertain as the base of the deposit could not have been captured offshore before. Such information, however, is essential for modeling a more reliable tsunami generation scenario. During the research cruise M155 in June 2019, a set of high-resolution multichannel (MSC) reflection seismic data along with hydroacoustic data was gathered. In most of the MSC profiles, a common reflector is identifiable and regarded as the base of the Monte Amarelo deposit, allowing the reassessment of the volume of failed and remobilized material. In addition, prominent internal reflectors within the chaotic facies defining the Monte Amarelo deposit in the MSC data, strongly suggest the collapse might have happened in multiple phases. Such observation is critical to better assess tsunami modeling associated with volcanic flank collapses