March 31, 1761 Tsunami: SW Portugal

1761, March 31, 12:01 am. 34.5 N, 13 W, tsunami magnitude 8.5. Earthquake at Lisbon,(Reference #3952)

1761, March 31, 12:05 am. 37 N, 10 W, intensity IX. (Reference #537)

At Lisbon, the agitation of the sea was very great, the water rising and falling many feet vertically. Ships at anchor in the river, though riding in some fathoms of water, were left dry at intervals. According to another account, the sea began to fall and rise about 13 hr. after the earthquake, about 8 ft. vertically; the movements continued until night.

At Penzance, about 5 p.m., the sea rose suddenly about 6 ft. five times within one hour. At the pier of St. Michael's Mount, the sea rose and fell about 4 ft. at the same time. At Newlyn, the sea rose nearly 6 ft. In the Scilly Isles, it rose 4 ft. at this time, the movements continuing for two hours. Along the Irish coasts, the same phenomena mere observed. At Kinsale, at about 6 p.m., the sea rose suddenly 2 ft. and retreated rapidly in 4 min., this being repeated, though to a less estent, several times. At Carrick, at 4 p.m., the surface of the river Suir rose 1 ft. in five minutes. At Dungarvan, the sea ebbed and flowed five times between 4 and 9 p.m. At Waterford, the sea advanced 30 ft. along the shore, while at Ross (Co. Wexford), a violent agitation of the river occurred at 7 p.m.

To the south and west, similar movements were observed. At Terceira, in the Azores, the sea rose to a great height and then fell so low that the quays were left dry. At Barbadoes, at about 8.5 hours after the earthquake, the sea, then near low water, suddenly retired from the shore and in about 3 min. returned, rising to the height of nearly 4 ft. This flux and reflux continued until 6 a.m. the next day

(above from reference #516)

1761, March 31, 12:05 local time. 37 N, 10 W, magnitude 6.5, intensity IX, tsunami intensity II. (reference #4139)

On the contrary, the tectonic source of 31 March 1761 remains poorly understood. The seismic catalogues present different earthquake locations: 10.00W, 37.00 N (Mezcua and Solares, 1983) or 10.50W, 36.00 N (Oliveira, 1986). Baptista et al. (2006), used macroseismic intensity data and tsunami travel time observations to locate the source circa 13.00W, 34.50 N and estimated the magnitude in 8.5. The source location obtained by Baptista et al. (2006) places the 1761 event southwest of the Southwest Iberian Margin (SWIM) in the outer part of the Gulf of Cadiz. The plate boundary between Eurasia and Africa is not well defined in the SWIM area, as the deformation is distributed over a large area. (reference #12398)

Reference #12955 lists event as questionable