GRACE Spies Sumatra Earthquake

In a demonstration of the GRACE satellites' sensitivity to minute changes in Earth's mass, the science team measured the deformation of the Earth's crust caused by the December 2004 Sumatra earthquake. That quake changed Earth's gravity by one part in a billion.

An important objective of GRACE science research is to better capture gravity changes that result from changes in Earth’s mantle and lithosphere during earthquakes.

"As many great earthquakes with moment magnitudes greater than 8.0 occur at the site of subducting oceanic slabs, much of the associated co-seismic and post-seismic solid Earth deformation occurs beneath the sea," said Erik Ivins, senior research scientist at NASA's Jet Propulsion Laboratory. "Using satellite gravity data that detect shifts in mass, whether they occur at the surface, or deep in the solid Earth, we can estimate both the duration and spatial extent of the flow in the mantle after an earthquake by measuring the movement of mantle and crustal material beneath the ocean. This forms a unique data set, as there are no alternative observations that so directly measure this deformation beneath the sea."

Another significant application of the data is in measuring changes on land and in ocean basins due to glacial isostatic adjustment (GIA); the changes observed as regions previously covered by thick ice sheets rebound after the ice melts away. The "signature" of GIA must be removed from trends in GRACE gravity data so that the data can more clearly tell the story of long-term changes in water storage.

Earth's shifting wobble
The relationship between continental water mass and the east-west wobble in Earth's spin axis. Earth does not always spin on an axis running through its poles. Before about 2000, Earth's spin axis was drifting toward Canada (green arrow, left globe). Before about 2000, Earth's spin axis was drifting toward Canada (green arrow, left globe). JPL scientists calculated the effect of changes in water mass in different regions (center globe) in pulling the direction of drift eastward and speeding the rate (right globe). Credit: NASA/JPL-Caltech

The inferred changes in global water storage can be utilized even to study Earth’s rotation itself. “The redistribution of surface mass perturbs the Earth’s inertia tensor, causing the rotational pole to tilt in the direction of the effective mass deficit. For example, if the Greenland Ice Sheet were the only place to lose the mass, the North (rotational) pole would have headed toward Greenland,” said scientist Surendra Adhikari, part of the Earth Surface and Interior group at JPL. Adhikari and Ivins used GRACE data to solve two mysteries about Earth's rotation.

"Glacial isostatic adjustment (GIA) is a global mantle flow that occurs due to the slow gravitational and stress relaxation following the disappearance of the Last Ice Age 21,000 years ago when sea- evel lowered by about 130 meters (425 feet) and great ice sheets grew over Canada and Scandinavia," Ivins said. "After a warming climate melted the ice sheets, the mantle readjusts into a new state of gravitational equilibrium and this deep flow today produces a strong detectable signal in the satellite gravity change measurements. These GRACE satellite measurements can be used to form trend maps. From these we can discover new aspects of how these former ice sheets were configured, how rapidly they disappeared and how our current understanding of mantle viscosity may, or may not, be consistent with model predictions of the maps retrieved by GRACE analysis."

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