Earthquakes jostle matter within the crust, thereby modifying local gravity. But because of imprecise, unstable instruments, scientists could monitor these changes only with the aid of long records before and after quakes. Japanese investigators have devised a much quicker method. They monitored how superconducting balls respond to gravity while trapped floating in ultrastable magnetic fields generated by superconducting coils. After accounting for the separate pulls of the sun, moon, air, ocean and the earth's rotation, the researchers detected a permanent increase in gravity roughly a half billionth that of the earth's pull southeast off the coast of Hokkaido, the epicenter of the magnitude 8 Tokachi-oki quake in 2003. The results, in the October 15 Science, agree with theoretical predictions, suggesting that superconducting gravimeters can help satellites chart the earth's gravity to map changes in polar ice cap thickness, seawater levels, atmospheric density and planetary geology.
This article was originally published with the title "Shaky Gravity Boost" in Scientific American 292, 1, 28 (January 2005)