A seismometer in orbit

Earth’s interior, oceans, and atmosphere are all mechanically coupled. So when a huge megathrust earthquake struck Japan two years ago and released the energy equivalent of a 100-megaton bomb, it excited seismic waves beneath Earth’s surface, unleashed a devastating tsunami, and sent acoustic waves skyward (see the article by Thorne Lay and Hiroo Kanamori in Physics Today, December 2011, page 33 ). A new study by Raphaël Garcia (Paul Sabatier University) and colleagues reveals that the European Space Agency’s GOCE satellite detected those acoustic waves—the first orbiting satellite to directly do so from an earthquake. Since it was launched in 2009, GOCE has been mapping Earth’s gravity field with a precision of 10⁻¹² m/s² from an orbit of 270 km. Low-frequency infrasound waves reach that height, which is low enough that Earth’s atmosphere exerts significant drag on the satellite. Onboard accelerometers adjust the satellite’s engines to offset the drag and maintain the orbit. As GOCE circled Earth on 11 March 2011, it crossed the infrasound wavefront in two places—first over the Pacific Ocean and then over Europe. Each time, the accelerometers picked up telltale 11% variations in air density and 10⁻⁷ m/s² variations in the satellite’s vertical acceleration. The researchers also modeled the amplitude, timing, and waveform of the atmospheric waves and found that the numerical results compared well to those recorded by GOCE. (R. F. Garcia et al., Geophys. Res. Lett., in press, doi:10.1002/grl.50205 .)