GPS signals could monitor tsunamis in transit

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London, Sep 15 (ANI): By measuring the signals of GPS satellites, scientists could monitor tsunamis as they sweep across the ocean.

Researchers hope measuring atmospheric waves will improve early warning of big tsunamis such as the one generated by a February earthquake in Chile.

In the most detailed study to date of the effect, scientists have shown that even though open ocean tsunami waves are only a few centimetres high, they are powerful enough to create atmospheric vibrations extending all the way to the ionosphere, 300 kilometres up in the atmosphere.

The finding could hugely improve tsunami early-warning systems.

In the study, a team of French geophysicists was able to use these ionospheric effects to trace the progress of three recent tsunamis, including the one triggered by the 27 February earthquake in Chile, which had a magnitude of 8.8.

The researchers showed that the strength of the ionospheric effects increased with the height of the wave.

The maximum height of that tsunami, which swept across the Pacific, was only 10 centimetres in mid-ocean, but low-lying tsunami waves can be more than 100 kilometres long.

During a tsunami, hundreds of square kilometres of ocean rise and fall, nearly in unison.

This produces a rhythmic movement in the atmosphere, generating a vertically propagating wave known as an internal gravity wave.

The thinning air causes the wave to spread out vertically and the air movements become larger.

"At around 300-350 kilometres of altitude, the atmospheric wave has been amplified by a factor of 10,000 or more. This means that a 10-centimetre tsunami wave at ocean level will induce atmospheric displacement reaching 1 kilometre," Nature quoted Lucie Rolland, a graduate student at the Paris Institute of Geophysics, as saying.

Rolland says that the collision of this wave with the ionosphere an upper layer of the atmosphere in which incoming solar radiation has ionized atmospheric gases - compresses it by as much as 10 percent.

That produces a corresponding change in the density of the ionosphere's free electrons. This change is enough to affect the signals from Global Positioning System (GPS) satellites - making it appear on GPS receivers as if surface locations are fluctuating by a few centimetres, in rhythm with the passage of tsunami-induced waves through the upper atmosphere.

By looking at data from numerous GPS stations in Hawaii, the scientists were able to filter out other, localized, fluctuations in the ionosphere.

And by looking at signals from GPS satellites in different locations, they were able to map the progress of the ionospheric electron 'wave' as it raced across the sky, about 10 minutes behind the tsunami.

Ultimately, the goal is improved monitoring of tsunamis in transit.

Because the technique allows waves to be seen hundreds of kilometres before they hit land, the study concludes that remote sensing "could be particularly decisive for tsunami warning and damage limitations".

The study has been published in Geophysical Research Letters. (ANI)

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