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Great Portugal Quake May Have a Sequel, Study Says

Stefan Lovgren
for National Geographic News
August 30, 2004
 
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.

But recent seismic images and seafloor bathymetry (the measurement of seawater depth) instead suggest that subduction is taking place in the region, causing compressive stress to accumulate along the interface between the tectonic plates, which leads to earthquakes.

"The African plate is moving [by] four millimeters [16-hundredths of an inch] per year to the west-northwest against Iberia," Gutscher said. "At the same time, a flap of the African plate is sinking into the mantle (the layer beneath the upper crust of the Earth)."

Gulf of Cádiz

Gutscher says there are several pieces of evidence that tectonic activity is ongoing.

The geometry of the accretionary wedge (sediments are scraped off a tectonic plate when it collides with another) show thrust faults. (A thrust fault is a fault where one side rides up over the other.) Also, marine seismic data show active folding and thrusting of the youngest sediments.

Active mud volcanoes in the Gulf of Cádiz off the southwestern coast of Spain also indicate continued tectonic activity. Such volcanoes are common to accretionary wedges in other active subduction zones.

At least one scientist, however, cautions that Gutscher may be looking at the wrong region.

"He jumps to focus on the Gulf of Cádiz … [even though] nearly all papers published to date have the 1755 quake epicenter located west of Portugal on the seafloor," said Alastair Dawson, a professor of geology at Coventry University in England.

Dawson argues that the 1755 earthquake must have struck offshore to produce such a high tsunami. "The explanation is the same … a quake originating due to collision of African and European plates," he said.

Storing Energy

If the active subduction zone off southern Iberia produced the 1755 earthquake, a comparable quake could strike in the future, according to Gutscher.

He believes the subduction zone off southern Iberia is active but that the seismogenic fault zone is locked, meaning a portion of the plate interface is locked, while further down the boundary the plate is slowly creeping along.

This plate motion accumulates year after year—Gutscher likens it to cocking the cord of a crossbow—until the energy stored is released suddenly, causing an earthquake.

"Imagine picking up the state of Massachusetts, and displacing it by 10 meters [11 yards] in about one minute," he said. "That's what causes the disaster."

A quake in Morocco earlier this year was related to the tectonic activity of the region, though that event was not a subduction-type earthquake.

Predicting earthquakes is almost impossible, but Gutscher warns that great earthquakes could return periodically to the region, possibly at 1,000-to-2,000-year intervals.

"Earth scientists will never be able to predict the precise day or month when a quake will occur," he said. "But we can try to determine the approximate repeat times for great events, and we can try to determine the expected magnitude of the event, based on accumulated yet unreleased slip and based on the surface area of the fault zone."

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