April 23, 2007 — Deep water may be healing the portion of Earth's crust ruptured by the deadly 2004 Sumatra-Andaman earthquake, say Japanese scientists who have detected changes deep in the quake zone with orbiting, gravity-measuring spacecrafts.
The researchers propose that moving water in Earth's mantle — below the crust — explains the relatively quick fading of a "dent" in Earth's gravity caused by the rare mega-thrust earthquake that launched the terrible 2004 Indian Ocean Tsunami. If so, it means water plays a surprisingly big role in the dynamics of Earth's mantle.
A "dent" in gravity is really a change in the geoid, the invisible global contour map of the lumpy variations in gravitational force. Those variations are caused by differences in the mass of the planet in different places.
Even on a big planet, massive objects like chunks of crust or mountains can still exert their own measurable local gravity, however small compared to cumulative gravity of the entire Earth. This can now be measured from space using the two Gravity Recovery and Climate Experiment (GRACE) satellites.
A mountain range, for instance, positions more mass closer to Earth's surface, and so it has a different gravitational tug than a spot in the middle of the ocean, where Earth's crust is thin and all the gravity-making mass is far below.
In the case of the Sumatra-Andaman earthquake, the downward lurch of one crustal plate under another caused the gigantic quake.
"Geoid changes are due to the vertical movement of mass," explained geoscientist Kosuke Heki of the University of Hokkaido in Japan. He and his colleagues published their study on the changes at Sumatra-Andaman rupture zone in the latest issue of Geophysical Research Letters.
