Earth Shakes Without Quakes

Tiny pulsations set off by something other than earthquakes are rippling Earth, according to a new analysis of gravity records reported in tomorrow's issue of Science. The oscillations, akin to the planet ringing like a bell every few minutes, may hint at a subtle, previously unknown interaction between Earth and its atmosphere.

Seismologists first observed a global oscillation after the magnitude 9.5 Chilean earthquake of 1960, the century's largest. That event unleashed seismic waves that ricocheted through the planet about once every hour. Since then, researchers have detected similar oscillations after smaller quakes. They also have speculated that the planet might shake more gently from other events, such as atmospheric storms, cyclic sloshings in the ocean, or deformations within the fluid outer core. Until recently, however, analytical techniques lacked the precision needed to isolate such signals.

Seismologist Naoki Suda of Nagoya University in Japan and co-workers tackled that challenge by scrutinizing records from an international network of gravity meters, which measure tiny changes in acceleration. By "stacking" 10 years of records together to search for recurring patterns, they unearthed signatures of pulsations that ripple across the entire planet every 3 to 8 minutes. The heights of these pulsations at the surface are extraordinarily small: about 10 nanometers, the same as an x-ray's wavelength, Suda says. Their oscillation patterns appear to rule out known earthquakes as sources, he says, adding that the signals are most consistent with random atmospheric disturbances, such as large storm systems.

Suda and his colleagues "did everything they could think of" to rule out contributions from earthquakes, says seismologist Hiroo Kanamori of the California Institute of Technology in Pasadena, who has written an accompanying Research Commentary on the report. "But there's still some chance of leftover earthquake energy," he maintains, such as very slow movements under the ocean floor that don't trigger seismographs. If the sources are perturbations in the atmosphere, Kanamori says, they could very well transfer some energy through the oceans to the crust and below. Geophysicists could gain insights about the planet's inner working by distinguishing among various sources, Kanamori adds--but that will be difficult because the pulsations are so minuscule.