Tectonic plate sliding backward toward Mexico City

Tectonic plate sliding backward toward Mexico City


www.chinaview.cn 2007-08-03 21:13:54 Print

BEIJING, Aug. 3 (Xinhuanet) -- An vast offshore tectonic plate that has reversed course and is sliding backward has geologists worried that a devastating earthquake may strike Mexico City.

The tectonic plate had been sliding toward Mexico City at a rate of 1 inch per year, as recorded by Global Positioning System measuring stations near Acapulco and Guerrero, which is about 175 miles southwest of Mexico City. That movement was normal, as predicted by theories of how Earth's crustal plates should move.

At subduction zones, like this one, an oceanic plate usually slides beneath a continental plate, and now and then major temblors occur. Suddenly, in the latter half of 2006, the plate began moving the other way and quadrupled its speed, scientists announced today.

A magnitude-8.1 event centered north of Guerrero killed 10,000 people in Mexico City in 1985.
In 1911, a magnitude-7.6 quake was the last major shaker centered in Guerrero. Scientists think strain on this local fault has been building and the area may be due for a much larger temblor, perhaps in the magnitude range of 8.1 to 8.4, said Kristine Larson, a University of Colorado at Boulder engineering professor.

"A very large earthquake in Guerrero would produce seismic waves that would travel quickly to the Mexican capital, and since Mexico City is built on water-saturated lakebed deposits that amplify seismic energy, the results would be catastrophic," said Larson.

The reversal of the fault is itself an important discovery. Similar reversals have been noted at other subduction zones, such as the highly active Cascadia region off the coast of Washington and Oregon. But this is the largest such backward event so far detected.

"Before GPS, we thought the ground moved at a constant speed between earthquakes," Larson said. "The recognition of these transient events where the plate reverses direction is arguably the most important geophysical discovery that has stemmed from the introduction of GPS measurements."

(Agencies)



Editor: Gareth Dodd