All Saints' Day catastrophe
At around 9:40 AM on November 1, 1755, a violent shaking began. It was All Saints' Day, and Lisbon's many churches were filled with worshippers. The ground motion, which lasted for three to six minutes, opened fissures up to 5 meters wide in the city center. Large public buildings and some 12,000 homes were demolished. Many survivors fled to the open space of the city's waterfront, only to be met by a catastrophic tsunami about an hour later. Three enormous waves surged up the Tagus estuary, with wave heights reaching up to 20 meters (66 feet) in some areas of Portugal and Spain. To complete the destruction, fires broke out and raged for six days. When it was over, an estimated 60,000 people were dead in Lisbon alone. The tsunami's effects were recorded across the Atlantic in Martinique, Barbados, and Newfoundland, Canada.
The geological crime scene
The source of the immense energy release remains one of seismology's greatest puzzles. The epicenter was in the Atlantic Ocean, southwest of Portugal, along the complex boundary between the African and Eurasian tectonic plates. This boundary, the Azores-Gibraltar Fault Zone, experiences a slow convergence rate of about 4 millimeters per year. Generating a magnitude 8.5 to 9.0 earthquake requires a colossal fault rupture—estimates suggest a combined length of 200 km and width of 80 km, with an average slip of 20 meters.
No single, known fault off the Iberian coast, such as the Gorringe Bank or Marquês de Pombal Fault, is large enough to have produced such an event on its own. This has led scientists to explore several alternative hypotheses. One idea is a complex, cascading rupture across multiple, smaller faults. Another prominent theory proposes the formation of a new subduction zone, where the oceanic plate begins to dive under the continent.
More recent research points to a process called delamination. Using seismic tomography, scientists have detected a section of the lower part of the tectonic plate peeling away and sinking into the deeper mantle. This process, previously thought impossible in this type of old oceanic basin, could create the stresses necessary to trigger a megaquake on the seemingly stable seafloor. The true source remains unconfirmed, hidden beneath the waves.