|GREAT LISBON EARTHQUAKE
Last week I raised a question with Terry concerning the difference between the New Madrid earthquake and the Great Lisbon Earthquake, both of which were not related to subduction events and/or volcanic explosions. The New Madrid earthquake was a mid-plate occurrence while the Great Lisbon Earthquake and tsunami was caused by strong transform fault movement along the boundary of 2 continental plates. In turn, both of these subjects seem to have different tectonic considerations from the balance of the assigned topics for student talks today. Terry thought the question was worth a short report and asked me to prepare same as a companion to Mary Jane’s talk on the New Madrid episode.
Dealing first with the tsunami: why are there so few recorded earthquake-generated tsunamis in the Atlantic Ocean? The only subduction zones around the Atlantic are the Puerto Rico Trench, the Antilles subductioin zone around the eastern Caribbean, and the South Sandwich Trench south of South America. These are smaller and much less active than the subduction zones that circle the Pacific. Relatively mild tsunamis have hit Puerto Rico and the Virgin Islands half a dozen times in recorded history, but by far the most powerfully destructive Atlantic tsunami in recorded history was generated by an earthquake nowhere near a subduction zone.
Tsunamis occur when abrupt deformation of large areas of sea floor results in vertical displacement of a massive volume of overlying water. Said movement can be either up or down. Gorringe Bank is a submerged ridge in the Atlantic 120 miles southwest of Lisbon that has been uplifted by northward transform fault movement of the African Plate against the Eurasian Plate. There certainly is convergence between these plates occurring, but the closest subduction zone is far to the east under Italy. On November 1, 1755 a magnitude 8.6 to 9.0 earthquake at Gorringe Bank flattened most of Lisbon. Contemporary reports state the movement lasted between 31/2 and 6 minutes, causing “gigantic fissures 5 metres wide to appear in the city centre.” About 40 minutes after the earthquake hit, the water that had been sucked out of the harbor returned as the first of 3 successive tsunami waves struck, repeatedly engulfing the destroyed city and rushing up the Tagus River. The catastrophe was not confined to Lisbon where up to 100,000 people were killed. Almost all of the coastal towns of the Algarve were heavily damaged as well as all of the ports of Azores. Shocks from the earthquake were felt throughout Europe as far north as Finland and to the south over much of North Africa. Tsunami waves caused great damage at Cornwall, England and swept the northwest coast of Africa west of Gibraltor.
The Azores –Gibraltar Transform Fault which marks the boundary between the African and Eurasian continental plates runs westward from Gibraltar into the Atlantic. It has a complex and active tectonic behavior. Particularly important, in my mind, is the fact that this faulting was responsible for several recorded earthquakes that hit Lisbon before November 1755. There were 8 recorded quakes in the 14th century, 5 in the 16th, 3 in the 17th century and 2 in the 18th century, all without recorded Tsunamis, before Lisbon’s major catastrophe of 1755. It is not a stretch to posit the idea of partial releases of minor events of pressure release happening over the preceding centuries, without significant sea floor deformation, during a continuing buildup of extreme pressure leading to a final climactic release. There is no way to predict how many centuries may be required for the pressure to rebuild for an episodic recurrence. The Azores-Gibraltor fault has recently revealed, by seismic reflection survey of the ocean floor, a 50 km-long thrust structure Southwest of Cape Vincent with a dip slip throw of more than 1 km. This huge movement likely memorializes the Great Lisbon Earthquake of 1755.
Addendum Summary Not in Verbal R
Lisbon is in close proximity to an active transform fault on the boundary of two continental plates.
Presumably the earthquake was relatively shallow, causing violent shaking at the surface onshore.
In addition to massive deaths and injuries and physical property destruction,
The Great Lisbon Earthquake had long range massive economic, cultural and political impacts on Portugal. One aspect of the response of Prime Minister Sebastiao de Melo (Marquis of Pombal), in addition to the practicalities of reconstruction, is important in the history of Earth Science. He ordered a query sent to all parishes of the country regarding the earthquake and its effects. Questions included:
How long did the earthquake last?
How many aftershocks were felt?
What kind of damage was caused?
Did animals act strangely
What happened in wells and water holes?
The answers are still extant in the national archive. Studying and cross referencing the priests’ accounts have enabled modern scientists to reconstruct the event from a scientific perspective. Since the Marquis of Pombal was the first to attempt an objective scientific description of the broad causes and consequences of an earthquake, he is regarded as a forerunner of modern seismological scientists.