The 1761 Lisbon earthquake struck off the coast of the Portugal on 31 March 1761 at 12:05 local time. Estimated to be around 8.5 on the tsunami magnitude scale, the earthquake was the strongest in the region since 1755, and it was felt across many parts of western Europe. Twenty-five people were killed including several people who died from rockfalls in Madeira and collapsed homes at a village near Porto. In Lisbon, the earthquake destroyed several old homes and remaining ruins from the earlier disaster, although no one was killed. The earthquake generated a tsunami that struck the coasts of Portugal, Spain, England, Sicily, the Azores and Barbados, but did not cause damage.
  1761 Lisbon earthquake (North Atlantic) | |
| Local date | 31 March 1761 |
|---|---|
| Local time | 12:05 |
| Magnitude | Mt8.5 |
| Epicenter | 34°30′N 13°00′W / 34.5°N 13.0°W |
| Areas affected | North Atlantic Ocean |
| Tsunami | Yes |
| Casualties | 25 dead |
Tectonic setting
editThe Azores-Gibraltar Fracture Zone (AGFZ) is an east–west trending plate boundary between the Eurasian and Nubian (African) plates between the Azores and Strait of Gibraltar. It accommodates the north-northeast–south-southwest convergence involving both plates. From west to east the plate boundary experiences right-lateral transtension (at Terceira Ridge) to right-lateral strike-slip motion (on the Gloria Fault) to oblique compression around the Gorringe Ridge and Alboran seafloor. [1]About 200 km (120 mi) south of the Gloria Fault, another approximately east–west trending structure also forms part of this plate boundary. This structure has been intepreted to be a reactivated ancient transform fault or an incipient boundary of a microplate.[2]
The exact linearments of the AGFZ, called the Southwest Iberian Margin (SWIM), is uncertain due to multiple complex geologic features. This region is characterised by a broad zone of compression and transpression more than 200 km (120 mi) wide accommodating convergence at a rate of 4–5 mm (0.16–0.20 in) per year. The tectonic convergence is accommodated by northeast–southwest trending thrust faults and west-northwest–east-southeast striking right-lateral strike-slip faults in a region south of the Gorringe Ridge and Gulf of Cádiz.[1]The SWIM is thought to be related to narrow arc subduction beneath the Gibraltar Arc,[1] involving a Jurassic oceanic slab.[3] Whether this subduction is active or not remains debated.[1]
Active faults in the SWIM poses a tsunami and seismic risk to populated regions nearby like Lisbon. The 1755 and 1969 Lisbon earthquakes involved slip on faults in the SWIM.[1] Among these large northeast trending thrust faults are the Coral Patch, Gorringe Bank, Horseshoe and Marques Pombal faults.[4]
Earthquake
editThe earthquake occurred on 31 March 1761 at 12:05 local time in Lisbon. While the exact epicenter location varies, the general area is in the SWIM, off the southwest coast of Portugal. Researchers from the National Geographic Institute of Spain placed the epicenter west of Cape St. Vincent.[a] In 1986, Portugal's Laboratório Nacional de Engenharia Civil published its catalog and listed the epicenter further southwest.[b][2] M. A. Baptista, J. M. Miranda, and J. F. Luis proposed that the earthquake had an epicenter near the Ampère Seamount, southwest of the SWIM and further away from the gulf.[c] They also estimated the tsunami magnitude (Mt) to be around 8.5 based on modelling a tsunami and comparing the simulated heights with the reported tsunami heights and wave period. Their conclusion was based on analyzing the seismic intensity distribution on the Iberian Peninsula and tsunami arrival time observations along the coasts of Portugal, Galicia, Ireland, England and the West Indies.[5] Another tsunami analysis by Martin Wronna, Maria Ana Baptista and Jorge Miguel Miranda further supported an epicenter area around the Ampère and Coral Patch seamounts.[4]
Impact
editTwenty-five deaths were attributed to the earthquake.[6] In Lisbon, the earthquake was felt with a lower seismic intensity than the 1755 earthquake and there was no serious destruction.[7] The shock was felt across all of mainland Portugal, the Azores and Madeira islands, and as far as Morocco, Madrid, Cork, Amsterdam and the Canary Islands.[8]
The earthquake further destroyed many of the leftover building wreckage from the earlier event in Lisbon.[7] Although there were no deaths in the city, several old homes collapsed while cracks appeared in some new homes. During the earthquake, which some witnesses described as lasting between 3 and 5 minutes, about 300 prisoners managed to escape their facility; all but 14 were subsequently captured. The earthquake was also felt strongly at Porto without damage. Some 20 mi (32 km) from the city, at a village, the collapse of 3 or 4 homes left several residents dead. Four people also died in Madeira, including two people on a fishing boat who were killed by rockfalls. Rockfalls were reported across the island and one church was destroyed.[9]
The earthquake was felt in many parts of Europe without damage. In Madrid, the earthquake shook homes, causing residents to evacuate. At Cork, Ireland, shaking was only reported within an area encompassed by the city gates. At Fort Augustus, Scotland, the water level at Loch Ness rose 2 ft (0.61 m) the first time and 30 in (760 mm) the second, and some fishing boats were carried by the motion.[9] This rising and falling of the lake surface continued for 45 minutes.[10]
Tsunami
editThe tsunami observations were detailed in letters. At Lisbon, the sea fuctuated by up to 2.4 m (7 ft 10 in) with a 6 minute period that lasted until night, but the city was not inundated.[4][11] On Terceira and Faial in the Azores, the sea level dropped enough that the quays were left dry.[12] The tsunami also caused sea level changes along the coast of Ayamonte, El Puerto de Santa María, Cádiz and Barcelona in Spain, but no information about their wave heights were detailed.[13]
According to William Borlase, at around 16:00 local time, on Barbados, the sea began to retreat from the shore and returned with a height of 4 ft (1.2 m).[9] A letter by Abraham Mason described the "extraordinary motion" of the sea, where the tide flowed and receded every eight minutes for three hours.[4] According to the NCEI/WDS Global Significant Earthquake Database, a run-up height of 1.2 m (3 ft 11 in) was listed.[14] At around 17:00 local time on Mount's Bay in Cornwall, England, the tide rose 6 ft (1.8 m), retreating and advancing five times for one hour. On Sicily, tsunami waves were observed for 2 hours with heights of 0.6–1.2 m (2 ft 0 in – 3 ft 11 in).[4][9]
See also
edit
Notes
editReferences
edit- 1 2 3 4 5 Correia, Ricardo; Prada, Manel; Sallarès, Valentí; Cameselle, Alejandra L.; Pinheiro, Luis Menezes; Grevemeyer, Ingo; Ranero, César R. (2025). "Deep crustal structure and seismogenic potential of the Southwest Iberian Margin: insights from seismic and gravimetric data". Geophysical Journal International. 242 (3) ggaf246. doi:10.1093/gji/ggaf246. hdl:10261/403976.
- 1 2 Baptista, M. A. (2019). "Tsunamis Along the Azores Gibraltar Plate Boundary". Pure and Applied Geophysics. 117 (4): 1713–1724. Bibcode:2020PApGe.177.1713B. doi:10.1007/s00024-019-02344-8. S2CID 204741694.
- ↑ Parera-Portell, J. A.; Mancilla, F. d. L.; Morales, J.; Stich, D.; Geissler, W. H.; Silveira, G. (2025). "Imaging the Low Dip Onset of the Gibraltar Arc Subduction in the Gulf of Cádiz Using OBS Receiver Functions". Geophysical Research Letters. 52 (23) e2025GL117817. Bibcode:2025GeoRL..5217817P. doi:10.1029/2025GL117817.
- 1 2 3 4 5 Wronna, Martin; Baptista, Maria Ana; Miranda, Jorge Miguel (2019). "Reanalysis of the 1761 transatlantic tsunami". Natural Hazards and Earth System Sciences. 19 (2): 337–352. doi:10.5194/nhess-2018-30. hdl:10400.21/9946.
- ↑ Baptista, M.; Miranda, J.M.; Luis, J. (2006). "In Search of the 31 March 1761 Earthquake and Tsunami Source". Bulletin of the Seismological Society of America. 96 (2): 713–721. Bibcode:2006BuSSA..96..713B. doi:10.1785/0120050111. S2CID 129323896.
- ↑ Solares, J. M. M.; Mezcua, J. (2002). Catalogo sismico de la Peninsula Iberica (880 a.C.-1990) (PDF). Instituto Geografico Nacional.
- 1 2 Cabral, N. (2020). "Revision of the Azorean catalogue of tsunamis". Geological Society, London, Special Publications. 501: 301–325. doi:10.1144/SP501-2019-107.
- ↑ Galindo, Inés; Romero, Carmen; Martín-González, Esther; Vegas, Juana; Sánchez, Nieves (2021). "A Review on Historical Tsunamis in the Canary Islands: Implications for Tsunami Risk Reduction". Geosciences. 11 (5): 222. Bibcode:2021Geosc..11..222G. doi:10.3390/geosciences11050222. hdl:10261/414712.
- 1 2 3 4 Borlase, William (1 January 1761). "LXV. Some account of the extraordinary agitation of the waters in Mount's-bay, and other places, on the 31st of March 1761: In a letter to the Reverend Dr. Charles Lyttelton, Dean of Exeter, from the Reverend William Borlase, M. A. F. R. S". Philosophical Transactions of the Royal Society of London. 52 (52). The Royal Society: 418–433. doi:10.1098/rstl.1761.0067.
- ↑ Davison, C. (1936). Great Earthquakes. London, United Kingdom: Thomas Murty and Co.
At Fort Augustus, at about a p.m., the surface of Loch Ness rose suddenly about 2 ft. and continued to rise and fall for 45 min. According to another account from the same place, the movements began between noon and 1 p.m. and lasted half an hour, the rise in the level being nearly 30 in.
- ↑ National Geophysical Data Center / World Data Service: NCEI/WDS Global Historical Tsunami Database. "Runup Information". NOAA National Centers for Environmental Information. doi:10.7289/V5PN93H7.
- ↑ National Geophysical Data Center / World Data Service: NCEI/WDS Global Historical Tsunami Database. "Runup Information". NOAA National Centers for Environmental Information. doi:10.7289/V5PN93H7.
- ↑ Baptista, M. A.; Miranda, J. M. (2009). "Revision of the Portuguese catalog of tsunamis" (PDF). Natural Hazards and Earth System Sciences. 9 (1): 25–42. Bibcode:2009NHESS...9...25B. doi:10.5194/nhess-9-25-2009.
- ↑ National Geophysical Data Center / World Data Service: NCEI/WDS Global Historical Tsunami Database. "Runup Information". NOAA National Centers for Environmental Information. doi:10.7289/V5PN93H7.