Gravity signals rapidly show true size of giant quakes

From BBC - December 1, 2017

Researchers have developed a new approach to estimate the true size of very large earthquakes.

At present, scientists use seismic waves from a rupture to work out the scale of the event.

But a new analysis of the Tohoku earthquake in Japan in 2011 shows that changes in gravity can give more rapid information.

This method could have accurately estimated that magnitude 9 tremor in minutes, not the hours actually taken.

As Japan's largest recorded earthquake, the Tohoku event is probably best remembered for the huge tsunami it unleashed.

As well as killing around 12,000 people it triggered the meltdown of three nuclear reactors at the Fukushima Daiichi power plant.

Experts believe that the quake was caused by the rupture of a stretch of the subduction zone associated with the Japan Trench, which separates the Eurasian Plate from the downward-sliding Pacific Plate.

Initial estimates of magnitude by Japan's meteorological agency indicated it was a 7.9 tremor.Three hours later, this was corrected to 8.8. Eventually the Tohoku quake was upgraded to 9.1 magnitude.

These estimates are based on seismic waves generated by the earthquake which travel through the Earth and are recorded on seismometers all over the world.

Scientists look for what are termed P-waves as the fastest way to deduce what is happening in an event. These travel at around 7-8km per second.

However, French researchers have been looking at the possibility of using small shifts in gravity to estimate scale.

Large tremors can cause perturbations in the Earth's gravitational field - and as these signals travel at the speed of light, they open up the possibility of much more rapid detection of quake magnitude.

Up to now, gravitational signals have been difficult to detect above the seismic noise, but the French research team developed their model to analyse the Tohoku event using data recorded with broadband seismometers located some 1,000 to 2,000km from the quake.

"As soon as you have the location and origin time of the earthquake, you know when the seismic waves will arrive at good quality stations inland," said lead author Dr Martin Valle, from the Paris Institute of Earth Physics.

"And if you look to see a gravity signal before the arrival time of the seismic waves, if you can see this, it really means you have a large event. If you cannot see it you will be in a magnitude 8.5 or below."

According to the authors, this method of analysis would have reliably detected that the Tohoku quake was greater than magnitude 9 within three minutes of the origin time.

This could have had major implications especially for the tsunami that followed it.

Using gravity signals for early warning?


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