Determining the true size of a major earthquake is a complex and time-consuming process. For the most powerful quakes, it can take up to half an hour to calculate a reliable magnitude. Now, French researchers have developed a method that could dramatically shorten that timeline—by measuring subtle changes in Earth’s gravitational field that occur almost instantly after a quake begins.
The challenge arises particularly for very large earthquakes—those exceeding magnitude 8 on the Richter scale. Traditional magnitude estimates rely on seismic stations that record ground motion. However, stations located closest to massive quakes often become saturated and cannot accurately capture the full energy released. As a result, scientists must wait for data from more distant stations, delaying precise magnitude calculations.
By analyzing data from the devastating 2011 earthquake in Japan, researchers from CNRS, Institut de Physique du Globe de Paris (IPGP), Université Paris Diderot, and California Institute of Technology (Caltech) have developed a faster approach to estimating earthquake magnitude.
Earthquakes release seismic waves that travel through the Earth at speeds between roughly 3 and 10 kilometers per second. These waves can cause catastrophic damage when they reach populated areas. But seismic waves are not the only signals generated during a quake. The sudden redistribution of mass inside the Earth also produces a disturbance in the planet’s gravitational field.
Instead of waiting for seismic waves to arrive, the researchers focus on detecting these gravity disturbances—sometimes referred to as elastogravity signals. Unlike seismic waves, changes in gravity propagate at the speed of light.
c = 3.0×10^8 m/s
Because gravitational changes travel so quickly, a monitoring station located 1,000 kilometers from the epicenter could detect a gravity signal more than two minutes before the seismic waves arrive. This allows scientists to estimate the size of a major earthquake in approximately three minutes—compared to roughly 30 minutes using conventional techniques.
However, the method currently works best for extremely powerful earthquakes. Detecting gravity changes from quakes below magnitude 8 to 8.5 remains difficult, as the signals are too weak relative to the natural seismic “noise” constantly present within the Earth. Isolating these subtle signals from background disturbances is a major technical challenge that researchers continue to address.
The study, published in the journal Science, could play a crucial role in improving early-warning systems—particularly for tsunamis triggered by large offshore earthquakes. Faster magnitude estimates could translate into earlier alerts and potentially save lives.
Reference:
Vallée et al., Observations and modeling of the elastogravity signals preceding direct seismic waves, Science, December 1, 2017. DOI: 10.1126/science.aao0746.