for National Geographic News
It was one of the greatest natural disasters in European history. The 8.7 earthquake that struck Portugal in 1755 killed at least 60,000 people, and triggered tsunamis (giant waves) that wrecked seaports in Portugal, Spain, and Morocco.
Striking on All Saints' Day (November 1), the earthquake sparked a spirited debate among philosophers about divine justice. In Voltaire's Candide, Pangloss, a doctor, inquires about the cause of the earthquake. The injured Candide begs him for medical help, but Pangloss continues his discourse, which he values more than Candide's distress.
The earthquake's cause remained a mystery because the tectonic activity of the region was not clearly understood. The plate boundary off southern Iberia—the peninsula occupied by Spain and Portugal—is not well defined.
A new study suggests that it happened as a result of subduction—the process of the oceanic lithosphere (the outer solid part of the Earth) diving beneath the continental lithosphere.
The study also shows continued activity in the plate system, prompting fears that another earthquake could hit the region with potentially devastating consequences—although probably not for many years to come.
"This was a big, nasty earthquake … [and the] subduction is still active here," said Marc-Andre Gutscher, a researcher with the European Institute for Marine Studies in Plouzane, France. Gutscher is the author of the article that appears in the current issue of the journal Science.
Subduction
The estimated magnitude of 8.7 makes the 1755 earthquake 20 times stronger than the 1906 San Francisco earthquake.
It generated a 5- to 10-meter-high (16- to 33-foot-high) tsunami wave, which swept across the coasts of Spain, southern Portugal and Morocco, devastating most of the ports and dragging people and debris out to sea.
Shaking was felt in France, Italy, and North Africa. The quake was reported to have lasted for as long as ten minutes.
The disaster was a setback to a powerful trading nation—Lisbon may have been the richest city in Europe at the time—and caused an important shift in the balance of power of the era.
Many modern scientists concluded that southern Iberian tectonics were governed by delamination, the sinking (or peeling off) of the lower portion of thickened continental lithosphere. In this process, which is believed to occur shortly after mountain building, there is no horizontal motion of the plates.
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