Gravity-based earthquake sensors provide earlier warnings

Tuesday, 19 March, 2019

Researchers from the University of Tokyo’s Earthquake Research Institute (ERI) have demonstrated a new earthquake detection method, exploiting tell-tale gravitational signals travelling ahead of tremors, that could be used in future to boost early warning systems and perhaps even save lives.

It all started when ERI Associate Professor Shingo Watada read an interesting physics paper by J Harms from the Istituto Nazionale di Fisica Nucleare in Italy. The paper suggests gravimeters — sensors which measure the strength of local gravity — could theoretically detect earthquakes.

“This got me thinking,” Assoc Prof Watada said. “If we have enough seismic and gravitational data from the time and place a big earthquake hit, we could learn to detect earthquakes with gravimeters as well as seismometers. This could be an important tool for future research of seismic phenomena.”

Earthquakes occur when a point along the edge of a tectonic plate makes a sudden movement. This generates seismic waves which radiate from that point at 6–8 km per second. These waves transmit energy through the Earth and rapidly alter the density of the subsurface material they pass through. Dense material imparts a slightly greater gravitational attraction than less dense material. As gravity propagates at light speed, sensitive gravimeters can pick up these changes in density ahead of the seismic waves’ arrival.

The researchers used a range of seismic data from Japan’s extensive networks of seismic instruments on land and at sea, much of which was generated during the devastating 2011 Tohoku earthquake in eastern Japan. They also used superconducting gravimeters (SGs) in Kamioka, Gifu Prefecture and Matsushiro, Nagano Prefecture, in central Japan. Their work was published in the journal Earth, Planets and Space.

“This is the first time anyone has shown definitive earthquake signals with such a method — others have investigated the idea, yet not found reliable signals,” said ERI postgraduate Masaya Kimura. “Our approach is unique, as we examined a broader range of sensors active during the 2011 earthquake. And we used special processing methods to isolate quiet gravitational signals from the noisy data.”

The team’s signal analysis was found to be extremely reliable, scoring what scientists term a 7-sigma accuracy — meaning there is only a one-in-a-trillion chance a result is incorrect. This fact will be useful in calibration of future instruments built specifically to help detect earthquakes. Assoc Prof Masaki Ando has now invented a novel kind of gravimeter — the torsion bar antenna (TOBA) — which aims to be the first of such instruments.

“SGs and seismometers are not ideal as the sensors within them move together with the instrument, which almost cancels subtle signals from earthquakes,” said ERI Assoc Prof Nobuki Kame. “This is known as an Einstein’s elevator, or the equivalence principle. However, the TOBA will overcome this problem. It senses changes in gravity gradient despite motion. It was originally designed to detect gravitational waves from the big bang, like earthquakes in space, but our purpose is more down to earth.”

The team dreams of a network of TOBA distributed around seismically active regions, an early warning system that could alert people 10 seconds before the first ground-shaking waves arrive from an epicentre 100 km away. This will take time, but the researchers are continually refining models to improve the accuracy of their method for eventual use in the field.

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