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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Mar 2018

Research article | 19 Mar 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Natural Hazards and Earth System Sciences (NHESS).

Reanalysis of the 1761 transatlantic tsunami

Martin Wronna1,3, Maria Ana Baptista1,2, and Jorge Miguel Miranda1,3 Martin Wronna et al.
  • 1Instituto Dom Luiz, Faculdade de Ciências da Universidade de Lisboa, Portugal
  • 2Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, Portugal
  • 3Instituto Português do Mar e da Atmosfera, IP, Lisboa, Portugal

Abstract. The tsunami catalogues of the Atlantic include two transatlantic tsunamis in the 18th century the extensively studied 1st November 1755, and 31st March 1761. The latest event struck Portugal, Spain, and Morocco around noontime. Several sources report a tsunami following the earthquake as far as Cornwall (United Kingdom), Cork (Ireland) and Barbados (Caribbean). An earlier analysis of macroseismic information and its compatibility with tsunami travel time information located the epicentre circa 34.5°N 13°W close to the Ampere Seamount at the eastern end of the Gloria Fault (North East Atlantic). The estimated magnitude of the earthquake is 8.5. In this study, we propose a tectonic source for the 31st March 1761 earthquake compatible with the tsunami observations in the Atlantic. We revisit the tsunami observations, reevaluate tsunami travel time data, and include a report from Cadiz not used before. The global plate kinematic model NUVEL 1A computes a convergence rate of 3.8mm/y in the area of the presumed epicentre. We propose a source mechanism for the parent earthquake compatible with the geodynamic constraints in the region capable of reproducing most of the tsunami observations. The results of our study support the hypothesis that the 1761 event took place in the area of Coral Patch and Ampere seamounts, SW of the 1st November 1755, mega-earthquake source. Finally, this study shows the need to include the 1761 event in all seismic and tsunami hazard assessments in the Atlantic Ocean.

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Short summary
We investigate the source of the 1761 earthquake and tsunami. The reanalysis of the tsunami travel times agrees with an earlier suggested source area. We check the geodynamical setting of the area and place a fault as an extension to an identified fault. We use numerical tsunami modelling to test several hypotheses at this fault and check the results against historical observation. One of our theories is compatible with the geodynamic setting and reproduces well the observed tsunami parameters.
We investigate the source of the 1761 earthquake and tsunami. The reanalysis of the tsunami...