Huge Earthquakes Can Change the Strength of Gravity Quickly—And Tell Us How Bad Quake Is

When a big earthquake hits, it not only tears the Earth's crust apart and wreaks havoc on buildings in the area: The seismic upheaval also causes changes in the strength of gravity.  Scientists figured out a way that change in gravity could have "told" people on the ground during the 2011 massive earthquake in Japan just how strong the quake was—just three minutes after it began, according to a paper published today in the journal Science.

That's a very different scenario from what actually played out that day. When the earthquake started, it was calculated to be a magnitude 7.9 event. Three hours later, the Japan Meteorological Agency raised it to 8.8. Final measurements determined the earthquake was actually a magnitude 9.1 tremor. The scale of an earthquake increases faster as the magnitudes increase, so that's a very big difference: A 9.1 quake releases 63 times the energy of a 7.9 one. Finding a way to offer earlier warnings about a quake's magnitude can offer vital time for people to prepare for impact.

Scientists studied the gravity changes caused by the earthquake shortly after it took place, finding that Earth's crust had become the tiniest bit thinner, making gravity in that area just a tiny bit weaker—nothing you would notice standing there, of course, but enough to measure from satellites.

11_30_tohoku_japan_earthquake Destruction from the 2011 earthquake in Japan. Nicolas Asfouri/AFP/Getty Images

The team behind the new paper wanted to revisit that idea using ground-based data collectors, the sort of information that could be used as an event unfolded. So they asked whether changes in gravity had been measured on the ground and whether that information could have been quickly processed into an accurate measurement of the earthquake's strength.

And they succeeded, finding a way to correctly classify the 2011 event as a quake with a magnitude of 9 or higher within about three minutes of its beginning. In the paper, they compared that to the fastest current method for evaluating earthquake strength, which typically required at least 20 minutes.

Related: Earthquake Risk and Earth's Slowing Rotation: Why There Won't Be a Huge Spike in Deadly Quakes in 2018

It's only one example, but it could be the first step toward understanding earthquakes as quickly as possible. Currently, the best warning system for earthquakes relies simply on radio communication, it being a little bit faster than waves moving through the Earth's surface. That technique is focused on what time an earthquake will arrive, rather than how strong it will be, but it can still give people an extra minute to prepare.

Beyond those warning systems, earthquakes remain entirely unpredictable—hence scientists' persistent interest in cracking their secrets.

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