NASA’s two Gravity Recovery and Climate Experiment (GRACE) satellites offer a possible new approach to understanding how earthquakes work. Research recently conducted paints a clearer picture of how the Earth changed following the December 2004 Sumatra-Andaman earthquake, the 9.1-magnitude event in the Indian Ocean which created a deadly tsunami that killed nearly 230,000 people and displaced more than 1 million. Scientists funded by NASA, the U.S. National Science Foundation and the Ohio Supercomputer Center have used satellite data for the first time to detect changes in the Earth’s surface caused by a massive earthquake.

The discovery signifies a new use for the data from NASA’s GRACE satellites and might offer a new approach to understanding how earthquakes work. The satellites have been gathering global gravity measurements since 2002. The identical instruments orbit 300 to 500 kilometers above the planet’s surface and fly 220 kilometers apart. The satellites can detect changes in the density of Earth’s crust or in GPS measurements on the ground and can signal changes in the planet’s gravity at that point.

According to an Aug. 3 release from Ohio State University (OSU). 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," Shin-Chan Han, a research scientist in the OSU School of Earth Sciences, said in a statement. First, the quake triggered the massive uplift of the seafloor, changing the geometry of the region and altering previous GPS measurements of the area. GRACE instruments detected the changes. Second, the density of the rock under the seafloor was altered after the slippage, and an increase or decrease in density produced a detectable gravity change, he said.

Along with colleagues C.K. Shum and Michael Bevis, both professors in the OSU School of Earth Sciences, Han assembled several years of data covering the Indian Ocean region and filtered out seasonal variations. The changing flow of the massive Mekong River, for example, affects area gravity measurements. These annual shifts must be removed from the data to detect changes caused by a quake. The researchers then fed the data into the latest seismic computer model, which illustrated gravity increases on one side of the fault line and decreases on the other.

"With this seismic model, we were able to explain and interpret the GRACE observations," Han said, adding that earthquake models still are evolving. "But the observations can also be used to validate the quality of the model itself," he added, "and therefore improve our knowledge about the solid earth’s dynamics."

The current GRACE technique was applied to understanding the mechanism of "great" earthquakes — those exceeding magnitude 9 — which are rare events. Detecting "major" quakes — magnitude 7 to 8.9 — is being investigated. NASA’s planned extension of the current mission, called GRACE 2, and its enhanced instrumentation should help in that effort.