Earthquake shakes up gravity Oct 14, 2004 Geophysicists in Japan
have detected a change in the Earth's gravitational field caused
by an earthquake for the first time. Yuichi Imanishi of the
University of Tokyo and colleagues used an array of
superconducting gravimeters to detect an increase of less than 1
micro-Galileo (10-8 metres per second per second) near the
epicentre of the Tokachi-oki earthquake, which occurred in
September 2003. The results agree with theoretical predictions
and could help interpret measurements of the Earth's gravity
made by satellites (Science 306 476). Earthquakes occur because
pressure builds up as sections of the Earth's crust move
relative to one another. These sections, known as continental
plates, meet at "fault lines" and the resulting movement of mass
can lead to changes in the local gravity. Imanishi and co-
workers made gravity observations at three locations, Esashi,
Matsushiro and Kyoto, located approximately 373, 771 and 1040
kilometres away -- as measured in a straight