Gravity signals could detect earthquakes at the speed of light | Science


Two minutes after the world’s largest tectonic plate shook off the coast of Japan, the country’s meteorological agency issued its latest warning to around 50 million people: an 8.1 magnitude earthquake had generated a tsunami heading towards the shore. But it was only hours after the waves hit that experts measured the true magnitude of the Tohoku earthquake of March 11, 2011. In the end, it reached a magnitude of 9, releasing more than 22 times the forecasts by energy experts and killing at least 18,000 people, some in areas that never received the alert. Now scientists have found a way to get more accurate size estimates faster, using computer algorithms to identify the trail of gravitational waves shooting out of the fault at the speed of light.

“It’s a brand new [way to recognize] large-magnitude earthquakes,” says Richard Allen, a seismologist at the University of California, Berkeley, who was not involved in the study. “If we were to implement this algorithm, we would be that much more confident that it is a very large earthquake, and we could broadcast this alert to a much larger area sooner.”

Scientists typically detect earthquakes by monitoring ground vibrations, or seismic waves, with devices called seismometers. The amount of advance warning they can provide depends on the distance between the earthquake and the seismometers, and the speed of the seismic waves, which travel at less than 6 kilometers per second. Networks in Japan, Mexico and California provide seconds or even minutes of advance warning, and the approach works well for relatively small tremors. But beyond magnitude 7, seismic waves can saturate seismometers. This makes the most destructive earthquakes, such as the Tohoku earthquake in Japan, the most difficult to identify, Allen says.

Recently, researchers involved in the hunt for gravitational waves – ripples in spacetime created by the movement of massive objects – realized that these gravity signals, moving at the speed of light, could also be used to monitor earthquakes. “The idea is that as soon as the mass moves anywhere, the gravitational field changes, and … everything feels it”, explains Bernard Whiting, a University of Florida physicist who worked on the Laser Interferometer Gravitational Wave Observatory. “What was amazing was that the signal would be present even in the seismometers.”

Sure enough, in 2016, Whiting and his colleagues reported that ordinary seismometers could detect these gravity signals. Earthquakes cause large changes in mass; these changes give off gravitational effects that distort both the existing gravitational fields and the ground beneath the seismometers. By measuring the difference between the two, the scientists concluded that they could create a new kind of earthquake early warning system. Gravitational signals appear on seismometers before the first seismic waves arrive, in a traditionally ignored part of the seismogram. By combining signals from dozens of seismometers on top of each other, scientists can identify patterns to interpret the size and location of large events, says Whiting.

Now, Andrea Licciardi, a postdoc at the Université Côte d’Azur, and her colleagues have built a machine learning algorithm to do this pattern recognition. They trained the model on hundreds of thousands of simulated earthquakes before testing it on the real Tohoku dataset. The model accurately predicted the earthquake’s magnitude in about 50 seconds, faster than other state-of-the-art early warning systems, researchers report today in Nature.

“It’s more than the germ of an idea, they’ve shown it’s doable,” says Whiting. “What we showed was proof of principle. What they show is proof of implementation.

Gravity signals are too weak to be used to detect earthquakes below magnitude 8.3 with current technology, and the system is unlikely to provide much more advanced warning in seismic areas that are already blanketed. seismometers. But it could provide more reliable estimates of the size of large-magnitude earthquakes, which is crucial especially for predicting tsunamis, which often take an extra 10 or 15 minutes to arrive, Allen says. With this technique, seismologists in Japan could have accurately determined the magnitude of Tohoku and issued appropriate alerts “1 or 2 minutes after the start of the earthquake”, explains Jean-Paul Ampuero, a seismologist also at the Université Côte d’Azur. ‘Azur and co-author of the paper. “In 2011, it took hours. It would have been fantastic.

But the technology is not yet operational: it has not processed the data in real time. The model should be deployed in Japan, but only for earthquakes generated by a specific fault zone likely to generate “big” ones. The algorithm needs to be trained separately for use in different regions, and researchers are currently doing this for seismic networks in Peru and Chile, Licciardi says. Still, “We have a first-generation algorithm…it’s a huge step forward,” Allen said. “Now let’s see if it actually works.”

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