– Space-based GPS provides an effective tsunami early warning system

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May 06, 2022

Researchers have developed a new, inexpensive way to detect tsunamis: using existing GPS satellites orbiting the Earth.

(Image: Shutterstock)

The first tsunami waves are usually a few centimeters high, but nevertheless cause a disturbance in the Earth’s upper atmosphere by raising the air and creating an acoustic wave that amplifies as it rises.

This leads to a change in the ionosphere, 300 km above the Earth’s surface, in which the electron density in the area is reduced. This, in turn, affects the radio signals sent from GPS satellites to GPS receivers on the ground, delaying or speeding up different parts of the signal, or changing the direction of the signal, depending on the frequency.

For the new study, published in Natural Hazards and Earth System Sciences, researchers from UCL and Japanese universities developed a new way to detect this drop in electron density from modified GPS signals.

By examining GPS data at the time of the devastating Tohoku-Oki earthquake and tsunami in 2011, they found that a tsunami warning could have been issued with confidence within 15 minutes of the earthquake. that is, at least 10 minutes before the first tsunami. hit the east coast of Japan.

They also found that a warning could have been issued using data from just 5% of Japan’s 1,200 GPS receivers, meaning the method could be used in countries with a less dense GPS network than from Japan.

Professor Serge Guillas (UCL Statistical Science and the Alan Turing Institute), lead author of the paper, said: “Current tsunami warning systems are not as effective as they should be because they often cannot accurately predict the height of a tsunami wave.

“In 2011, the Japanese warning system underestimated the height of the wave. Better warning might have saved lives and reduced the widespread destruction that has occurred, allowing people to travel to higher ground and farther from the sea.

“Our study, a joint effort of statisticians and space scientists, demonstrates a new method of tsunami detection that is inexpensive because it relies on existing GPS networks, and could be implemented globally. worldwide, complementing other tsunami detection methods and improving the accuracy of warning systems.

Lead author and PhD researcher Ryuichi Kanai (UCL Statistical Science and Alan Turing Institute) said: “Our calculations suggest that the size and shape of the wave could be inferred from the disturbance in the ionosphere and therefore the next The research stage will be to investigate this further to see if the method could be used for more accurate predictions of tsunami size and range.

“Based on my experience working for the Japanese government in the past and seeing the damage caused by the tsunami, I believe that if this research comes to fruition, it will surely help save lives.”

The researchers used statistical techniques to reconstruct the electron density depression in the atmosphere from scattered points provided by GPS data, as well as to quantify the uncertainty inherent in the modeling.

The acoustic wave caused by the initial water rise took about seven minutes to reach 300 km high in the ionosphere and the resulting depression in electron density could be detected via satellite signals in 10 to 15 minutes. , the researchers found.

Tsunami waves are low in deep water but can travel at jet speed (up to 800 km/h in deep sea) and when they enter shallower water they slow down, increasing in height.

Many existing tsunami warning systems infer tsunami waves from earthquakes, but this proposed method could be used to predict incoming tsunamis with non-seismic sources, such as landslides and volcanic eruptions.

While some tsunamis reach land in less than 10 minutes, the researchers pointed out that the method could also be used to predict a second or third wave, helping to determine whether a tsunami warning should be canceled or maintained after the first wave. .



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