In the days leading up to some recent moderate-sized earthquakes, instruments nearby have picked up brief low-frequency pulses in Earth’s magnetic field. A few scientists have proposed that such pulses, which seemed to become stronger and more frequent just before the earthquakes occurred, could serve as an early warning sign for impending seismic activity. Now, a team has come up with a model for how these magnetic pulses might be generated, though some critics say they may have a humanmade origin.
Brief fluctuations in Earth’s magnetic field have been detected before many earthquakes in the past 50 years, says Friedemann Freund, a crystallographer at San Jose State University in California. For example, in the weeks before a magnitude-5.4 quake struck about 15 kilometers northeast of San Jose in October 2007, an instrument near the epicenter sensed a number of unusual magnetic pulses, presumably emanating from deep in the Earth. (The largest of them measured 30 nanoteslas, which is about 1/100,000th the typical strength of the planet’s magnetic field measured at Earth’s surface.) Those blips became more frequent as the day of the earthquake approached, Freund says. More recently, prior to several medium- to moderate-sized quakes in Peru, two sensitive magnetometers recorded the same sort of pulses.