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). 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.