Great Portugal Quake May Have a Sequel, Study Says

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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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