Sensors that measure local gravity can be used to detect earthquakes
Developing technology to forewarn the public could save hundreds to thousands of lives that are claimed by earthquakes each year. Researchers at the University of Tokyo’s Earthquake Research Institute (ERI) have found a new technique that detects subtle gravitational signals that travel just ahead of an earthquake.
Among the large amounts of seismic data produced by the 2011 Tohoku earthquake in eastern Japan, the ERI team discovered critical information. The research was inspired when Professor Shingo Watada read a paper from the Istituto Nazionale di Fisica Nucleare in Italy that suggested gravimeters, sensors which measure the strength of local gravity, could potentially be used to detect earthquakes.
“This got me thinking,” said Professor Watada. “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.”
When seismic waves transmit energy through the Earth, the density of the subsurface material they pass through becomes denser and develops a slightly greater gravitational attraction. Gravity is generated at light speed, which means that sensitive gravimeters can pick up on these changes in density before the seismic waves arrive.
“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,” explained 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 researchers used a range of data collected from seismic instruments located both on land and at sea, and also gathered data from superconducting gravimeters in central Japan.
The signal analysis performed by the team was so reliable that it received a score known as 7-sigma accuracy, which means there is only a one-in-a-trillion chance that a result is incorrect. This will be extremely useful in the calibration of future instruments built specifically to detect earthquakes. Professor Masaki Ando from the Department of Physics has already invented a new 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,” explained ERI Professor 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 researchers envision a network of TOBA instruments distributed throughout seismically active regions. This early warning system could alert people 10 seconds before the first seismic waves arrive, and could mean the difference in life or death for many people.
The study is published in the journal Earth, Planets and Space.