Earth knocked for a loop

Chilean quake sped up Earth's rotation, tipped planet's axis

The magnitude 8.8 quake that slammed central Chile February 27 knocked the entire planet for a loop — literally. The sudden, large-scale movement of tectonic plates that triggered the quake shifted immense masses of rock a few meters closer to Earth’s core, tilting the planet’s axis a few centimeters and imperceptibly shortening the day, analyses indicate.

SHAKEN, STIRRED, TILTED The magnitude 8.8 quake that struck central Chile February 27, one of the six largest temblors that have occurred since 1900, caused Earth’s rotation to speed up by 1.26 microseconds each day and tilted the planet’s axis about 8 centimeters. US Geological Survey

Disaster struck just after 3:34 a.m. local time, when seismic stresses that had been building for decades, if not centuries, let loose. Rocks along the interface between two tectonic plates slipped past each other a distance of seven to 11 meters, says Jian Lin, a geophysicist at the Woods Hole Oceanographic Institution in Massachusetts.

Those pieces of Earth’s crust — the South American plate and the Nazca plate, a continent-sized slab of seafloor that lies just west of South America — are colliding at an average speed of about 8 centimeters per year. “This is one of the fastest plate convergence rates on Earth,” Lin notes. Rather than moving steadily, the plates can remain locked in place for long intervals and then slide past each other in quick bursts.

The early-morning temblor, which involved slippage along a 400-kilometer stretch of the tectonic interface, is among the strongest ever recorded: Only four quakes since 1900 — including the largest on record, a magnitude 9.5 shock that shook southern Chile in May 1960 — have been larger, Lin says.

“The new quake picked up where the 1960 rupture ended,” he notes.

That 1960 quake shifted stress northward to a part of the tectonic interface that remained locked. The redistribution of stress probably caused this year’s quake to occur earlier than it otherwise would have, says Lin. Such effects aren’t unknown — just three months after a magnitude 9.1 quake rocked northern Sumatra in late December 2004 (SN: 1/8/05, p. 19), a magnitude 8.6 temblor on an adjacent segment of the same subduction zone shook southern portions of the island (SN: 4/2/05, p. 211).

The death toll from the February 27 quake stands at 799, Chile’s National Emergency Office reported March 3. That tally is still climbing but won’t rise to anywhere near the 200,000-plus death toll of the magnitude 7.0 quake that struck Haiti on January 12. This dramatic difference probably stems from several factors, Lin speculates.

First, even though the Chilean temblor released about 500 times more energy than the Haitian quake, it originated deeper within the Earth and farther from densely inhabited regions. And Chile’s long experience with large quakes and the strong, well-enforced building codes that have been developed as a result also saved lives, Lin says. “The contrasts between the aftermaths of these quakes remind us, once again, that ‘earthquakes do not kill people, buildings do,’” he notes.

In a mere six days, the Chile quake has spawned more than 180 aftershocks, including seven above magnitude 6.0. One aftershock, a magnitude 6.9 quake centered about 125 kilometers off the Chilean coast, was almost as large as the Haiti quake of January 12.

The movement of tectonic plates in Chile February 27 has triggered glitches in Earth’s rotation, a new analysis suggests. Sudden subduction of the Nazca plate carried large amounts of mass closer to the center of the Earth — which, conceptually but on a vastly different scale, works like spinning skaters bringing their arms closer to their bodies, says Richard Gross, a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. As a result, Earth’s day is now about 1.26 microseconds shorter than it was before the massive quake, Gross estimates.

And because the quake’s shift in mass occurred deep in the Southern Hemisphere, Earth was slightly tipped off balance — a result similar to a spinning skater bringing in one arm but not the other. The planet’s “figure axis,” the line about which the Earth is balanced, shifted about 8 centimeters, Gross notes.

Earth’s axis is constantly wobbling at various frequencies, with some oscillations measuring several meters and taking months to unfold (SN: 8/12/00, p. 111). Forces driving those cycles, including those resulting from winds and ocean currents, act continually across Earth’s surface and often are about a thousand times larger than those generated during the Chilean quake.

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