Earth’s gravity offers earlier earthquake warnings

Tiny gravity changes can be picked up much faster than seismic waves.

During an earthquake, every extra second of warning can save lives. Earthquakes cause tiny changes to Earth's gravity, and new research suggests these signals can be detected almost instantaneously at the start of an earthquake, offering a much earlier warning than the seismic waves we currently rely on.

Researchers examined gravity data collected in Japan around the time of the 2011 Tohoku-Oki earthquake, which generated the devastating tsunamis leading to the Fukushima Daiichi Nuclear Power Plant disaster. They were looking for a signal that preceded the arrival of seismic waves, and they found one, opening the door for new developments in earthquake early warning systems.

“The gravity signal is almost instantaneous—the speed of light—whereas classical early warning systems are based on the detection of propagating seismic P-waves at ~7-8km/s,” explains the study’s lead author Jean-Paul Montagner, a seismology expert at the Institut de Physique du Globe de Paris. That extra time can make a difference in minimizing the impact of an earthquake. “Any second saved enables us to stop trains, elevators, nuclear plants, warn people, and therefore to save lives.”

The gravity signals are generated when the rupture of an earthquake causes Earth’s mass to shift slightly on either side of the fault line, leading to tiny changes in Earth’s gravity field. “When working with physicists specializing in gravitational waves at the VIRGO detector, we realized that from a theoretical point of view, we should expect an instantaneous gravity signal associated with earthquakes,” says Montagner. It was this realization that lead him and his colleagues to examine gravity data from the 2011 earthquake.

gravity-signals
Three days of gravimetric recording at Kamioka Observatory, starting on March 9, 2011. Montagner et al. 2016, in Nature Communications, 7:13349.

While pinpointing the exact location of earthquakes from gravity signals is difficult, the method is well-suited to measuring their size. To achieve this and provide a reliable early warning system, networks of more accurate and less noisy instruments for measuring gravity signals must first be developed. “So far, it is only an isolated detection for a huge earthquake,” Montagner cautions. But he is confident these advances are on the way: “We can expect in the future to obtain a reliable estimate of magnitude, much faster than with the present systems.”

Featured image courtesy of Joe Parks.