For the first time, scientists have been able to measure a change in the gravity field caused by an earthquake, in this case the large one near Sumatra in December 24, 2004.
Centered off the west coast of northern Sumatra, the event followed the slipping of two continental plates along a massive fault under the sea floor. The slippage occurred along 750 miles 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 miles.
The research, which appeared in the Science Magazine of August 3rd, was led by Ohio State’s Shin-Chan Han along with C. K. Shum, Michael Bevis, Chung-Yen Kuo and Chen Ji from UC Santa Barbara.
“The earthquake changed the gravity in two ways that we were able to detect. First, the quake triggered the massive uplift of the seafloor, changing the geometry of the region, and second, the density of the rock beneath the seafloor was changed after the slippage. An increase or decrease in density produces a detectable gravity change”, Han said.
We report the detection of an earthquake by a space-based measurement. The Gravity Recovery and Climate Experiment (GRACE) satellites observed a ±15-microgalileo gravity change induced by the great December 2004 Sumatra-Andaman earthquake. Coseismic deformation produces sudden changes in the gravity field by vertical displacement of Earth’s layered density structure and by changing the densities of the crust and mantle. GRACE’s sensitivity to the long spatial wavelength of gravity changes resulted in roughly equal contributions of vertical displacement and dilatation effects in the gravity measurements. The GRACE observations provide evidence of crustal dilatation resulting from an undersea earthquake.
August 6th, 2006 | Earth Sciences and Geomatics