Jakarta, Indonesia, 9 August 2006: Scientists funded by NASA, the US National Science Foundation and the Ohio Supercomputer Centre have used satellite data from NASA’s two gravity recovery and climate experiment (GRACE) satellites for the first time to detect changes in the Earth’s surface caused by a massive earthquake.
The research paints a clearer picture of how the Earth changed after the December 2004 Sumatra-Andaman earthquake, the 9.1-magnitude temblor that set off a devastating tsunami across the Indian Ocean in December 2004 disrupted the earth enough to change gravity and to deflect satellites passing hundreds of miles above.
Two identical satellites, collectively known as the GRACE, travel one behind the other in a polar orbit separated by about 130 miles. By recording small changes in the distance between them when their orbits are deflected, the satellites provide data used to calculate variations in the Earth’s gravitational field.
Centered off the west coast of Northern Sumatra, the event followed the slipping of two continental plates along a massive fault under the seafloor. The slippage occurred along 1,207 kilometers of the line where the Indian plate slides under the Burma Plate, a process called Subduction. The quake raised the seafloor in the region by several meters for thousands of square kilometers.
“The earthquake changed the gravity in that part of the world in two ways that we were able to detect,” said Shin-Chan Han, an Ohio State research scientist. First, he said, the quake triggered the massive uplift of the seafloor, changing the geometry of the region and altering previous Global Positioning System (GPS) satellite measurements from the area.
Second, the density of the rock under the seafloor was changed after the slippage, and an increase or decrease in density produces a detectable gravity change, Han said. The gravity at the Earth’s surface decreased by as much as about 0.0000015%, meaning that a 150-pound person would experience a weight loss of about one-25,000th of an ounce. In other places, where the force of the earthquake compressed rocks, gravity increased by a similar amount.
The force of gravity is changing in other areas of the earth, too. In Hudson Bay, Canada, which was crushed downward by the weight of ice during the last ice age, the ground is still rebounding upward. That change in gravity adds about one-400,000th of an ounce to the weight of a 150-pound person every year.