Gravity Aftermath of Deadly Earthquake

Scientists at the University of Texas have used a pair of satellites to measure the seismic deformations produced in the Earth during and after the huge Sumatra-Andaman earthquake of December 2004, the one whose associated tsunami killed hundreds of thousands around the Indian Ocean coastline.

The Gravity Recovery and Climate Change Experiment (GRACE) consists of two Earth-orbiting satellites. The satellite's relative spacing, monitored continuously, can be altered by the shifting gravitational subtleties triggered by the movement of massive objects beneath. This can mean big changes in land water and lakes, sea level changes, polar ice sheet melting, or sea floor changes caused by earthquakes.

Essentially, GRACE maps the gravity field of the Earth's surface before and after an earthquake. In the case of the Sumatra/Andaman quake numerous detectors were at work, but only GRACE could accurately measure and map the offshore rupture over its entire 1800-km extent from space (see GRACE map at http://www.aip.org/png/2007/288.htm).

They can make this map around the time of the quake and then afterwards, as the Earth slows its seismic ringing. One of the researchers, Jianli Chen (chen@csr.utexas.edu) says GRACE's most successful mission so far has been to monitor terrestrial water storage change and polar ice melting, and so it is one of the most important sentinels of potential climate change. (Chen et al., current issue of Geophysical Research Letters; http://www.csr.utexas.edu/personal/chen/)