Japan quake 'heard at edge of space'

The great Tohoku earthquake in Japan two years ago was so big its effects were even felt at the edge of space.

Scientists say the Magnitude 9.0 tremor on 11 March 2011 sent a ripple of sound through the atmosphere that was picked up by the Goce satellite.

Its super-sensitive instrumentation was able to detect the disturbance as it passed through the thin wisps of air still present 255km above the Earth.

The observation is reported in the journal Geophysical Research Letters.

It has long been recognised that major quakes will generate very low-frequency acoustic waves, or infrasound - a type of deep rumble at frequencies below those discernible to the human ear. But no spacecraft in orbit has had the capability to record them, until now.

"We've looked for this signal before with other satellites and haven't seen it, and I think that's because you need an incredibly fine instrument," said Dr Rune Floberghagen from the European Space Agency (Esa).

"Goce's accelerometers are about a hundred times more sensitive than any previous instrumentation and we detected the acoustic wave not once, but twice - passing through it over the Pacific and over Europe," the mission manager told BBC News.

Goce's prime purpose is to map very subtle differences in the pull of gravity across the surface of the Earth caused by the uneven distribution of mass within the planet.

These variations produce almost imperceptible changes in the velocity of the satellite as it flies overhead and which it records with those high-precision accelerometers.

This gravity signal is very weak, however, and that means Goce must fly incredibly low to sense it - so low, in fact, that it actually drags through the top of the atmosphere.

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