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Mass movement

Earth's gravitational field changed measurably in response to the December 2004 tsunami-spawning earthquake west of Sumatra. The Gravity Recovery and Climate Experiment (GRACE) mission's satellites noted changes associated with the temblor—the first such feat, says Byron D. Tapley, director of the Center for Space Research at the University of Texas at Austin. GRACE's data depict an area—southwest of the quake's epicenter—where Earth's gravity decreased slightly (lower patch of dark blue and purple) and a region northeast of the rupture where gravity increased somewhat (lowest red and yellow patch).

Other changes in the image, released by NASA on Dec. 20, 2005, probably reflect movements of water and weather systems unrelated to the quake, says Tapley.

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