Signs of gravity could quickly alert large earthquakes and save lives –

Signs of gravity could quickly alert large earthquakes and save lives


Gravity signals running through the ground at the speed of light could help seismologists better manage the size of large, devastating earthquakes shortly after they hit, a study suggests. The small changes in the gravitational field of the Earth, created when the ground changes, arrive at the stations of seismic monitoring long before the seismic waves.

"The good thing we can do with these signals is to have quick information about the magnitude of the earthquake," says Martin Vallée, a seismologist at the Institute of Earth Physics in Paris.

The seismometers in China and South Korea picked up signs of gravity immediately after the Tohoku 9.1 magnitude earthquake that devastated parts of Japan in 2011, report Vallée and colleagues in Science on December 1. The signals appear as small accelerations in the seismic recording equipment, more than a minute before the seismic waves appear.

"We can observe before the seismic waves arrive", says Vallée. "If we do not see anything, we can say that the earthquake that made them was maybe big, but not huge, if we see the signs, it means that we really have a big earthquake."

If seismologists had been monitoring the gravity changes, they might have realized before how big the Tohoku earthquake was. The US Geological Survey UU (USGS) took 40 minutes to update its initial estimate of magnitude 7.9 to 8.8, much closer to the actual size of the earthquake, and 3 hours for the Japan Meteorological Agency to do the same. A small increase in the magnitude of an earthquake means a large change in the energy released by the earthquake and the expected devastation. That information is crucial for emergency responders as they decide what resources to deploy.

"It will be a great contribution if gravitational waves can reduce the time needed to know that a large earthquake is large," says Susan Hough, a USGS seismologist in Pasadena, California.

But there is a lot of work to be done before gravity signals can be considered a reliable tool in the crucial minutes after a major earthquake, says Gavin Hayes, a seismologist at the USGS in Golden, Colorado. "I do not see the game changing," he says.

First wave

The latest work emerged when a group of European and American researchers began to explore how small earthquake vibrations affect gravitational wave detectors, such as the European Virgo and the US laser interferometer. Gravitational wave observatory (LIGO). Many of the scientists also worked on earthquake early warning systems, and began to think if earthquakes created gravitational disturbances and how they could be detected.

The challenge is in picking up the gravitational jolt, which is much weaker than that of seismic waves. In 2016, scientists published a proof-of-concept study that showed that a gravity measuring instrument at the Kamioka Underground Observatory in Japan had detected signs of the 2011 earthquake, which occurred some 500 kilometers away. But others questioned how strong and reliable the signals could be.

In the last document, Vallée and his colleagues report many more observations of signs of gravity immediately after the Tohoku earthquake. The signal was most evident in the monitoring stations between approximately 1,000 and 2,000 kilometers from the epicenter of the earthquake. At that distance, fast signals such as light had enough time to arrive and be clearly recorded before seismic waves flooded them.

Modeling suggests that the method should work to measure tremors of magnitude 8.5 or greater, which are large enough to generate detectable gravity signals. The team is now looking for signals that could have been recorded after other major earthquakes, including the 9.1 magnitude event in Sumatra in 2004 and the magnitude 8.8 event in Chile in 2010.

A few more warning minutes can save lives, especially in coastal areas where people can evacuate before an incoming tsunami, says Jean-Paul Ampuero, a seismologist at the California Institute of Technology in Pasadena. "That's an easy-to-reach fruit."

This article is reproduced with permission and was first published on November 30, 2017.

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