TAGUSDELTA - 3D high-resolution seismic stratigraphy of the Tagus Delta - imaging and modeling of tsunami evidence for geological hazards assessment

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The Tagus River delta landslide, off Lisbon, Portugal. Implications for Marine geo-hazards

Publication . Terrinha, Pedro; Duarte, Henrique; Brito, Pedro; Noiva, João; Ribeiro, Carlos; Omira, Rachid; Baptista, Maria Ana Carvalho Viana; Miranda, Miguel; Magalhaes, Vitor; Roque, C.; Rosa, Marcos; Alves, Paulo; Teixeira, Francisco; Tagusdelta Cruise Team

The stratigraphy of the Tagus river ebb-tidal delta off Lisbon (Portugal) is investigated using high resolution multichannel seismic reflection profiles with the purpose of searching for sedimentary or erosive features associated with landslides. The Tagus delta is sub-divided in two prograding seismic units of 17 ky to 13 ky and 13 ky to Present based on the calibration of seismic lines using gravity and box-cores in the Tagus pro-delta. We report the existence of a buried landslide with 11 km of length, 3.5 km of width and a maximum thickness of 20 m that accounted for the collapse of half of the upper unit of the Tagus river delta front in Holocene times. The non-collapsed half of the delta front contains extensive shallow gas of still unknown origin and nature. An estimated age of similar to 8 ky BP for the Tagus delta landslide is proposed based on stratigraphic correlation. The trigger mechanisms of the newly identified Tagus landslide are discussed as well as of the several landslides also found in the lower delta unit. These findings present a first step towards a future assessment of the susceptibility of the nearby coastal areas and the off-shore infrastructures to hazards related to such large collapses.

2019-10Journal article

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The 1531 Lisbon Earthquake: A Tsunami In the Tagus Estuary?

Publication . Baptista, Maria Ana Carvalho Viana; Miranda, Jorge Miguel; Batllo, J.

On 26 January 1531, a strong-magnitude earthquake heavily impacted Lisbon downtown. Immediately after the earthquake, the eyewitnesses reported large waves in the Tagus estuary, mainly north of the city and along the northern bank of the river. Descriptions include large impacts on ships anchored in the estuary and even morphological changes in the riverbed. We present a synthesis of the available information concerning both the earthquake and the water disturbance as a basis for the discussion of the probable tectonic source and the magnitude of the associated river oscillations. We hypothesize that the initial disturbance of the water can be attributed to the coseismic deformation of the estuary riverbed, and we use a nonlinear shallow water model to simulate the tsunami propagation and inundation. We show that the Vila Franca de Xira fault is the most probable source of the 1531 event. The largest inundation effects of the model correlate well with the historical descriptions: the impact is relevant in the inner Tagus estuary, but inundation in downtown Lisbon is small.

2014-10Journal article

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