Euro satellite ‘heard’ Japanese megaquake in SPACE

The European Space Agency (ESA) is claiming a world first after releasing evidence that its GOCE gravity satellite picked up sound waves produced by the Sendai earthquake of March 2011. The ESA explained that particularly large ‘quakes – like the magnitude 9.0 incident that hit Japan – cause the planet’s surface to vibrate, …

[stuartnz]

Can someone reduce my ignorance?

If I were a character from the HHGTTG series, I'd be know-nothing-bozo the non-wonder dog, so I hope someone will have pity and throw me a scrap of a clue on this one:

I thought sound waves travel faster in a denser medium than in a less dense one, but the illustration shows that wave accelerating from 350 m/s to 1150 m/s when it reaches the much thinner part of the atmosphere. Clearly I was asleep during the relevant lesson, so what am I missing here?

[Anonymous Coward]

Re: Can someone reduce my ignorance?

Damn that's a good question. I fear I may be showing my own ignorance as well, but is there a difference between the speed of propagation of vibration (sound wave) and the speed of a pressure wave displacing air?

[Ralph B]

Re: Can someone reduce my ignorance?

I think you might have come to the wrong place. The Reg commentards are pretty good on technology, but for science questions I'd suggest asking elsewhere.

[Blip]

Re: Can someone reduce my ignorance?

@Ralph B - thanks for the link to the Naked Science Forum.

[Vladimir Plouzhnikov]

Re: Can someone reduce my ignorance?

Actually, the speed of sound in gases is independent from pressure and density and is only a function of temperature.

However, it is still strange where the 1150 m/s is coming from as the the speed of sound falls with decrease in temperature...

[Michael H.F. Wilkinson]

The speed of sound increases with the stiffness of the medium, and decreases with density. For an ideal gas stiffness depends on pressure, which depends of density and temperature. The net result is that the speed of sound increases with temperature. So low temperature leads to low speed of sound. High in the atmosphere, the temperature is low. However, at the altitude of satellites, things are probably more complicated, as ionizing radiation of the sun may heat molecules to 2,000 K or more (and ionize them, changing the properties of the dilute gas), depending on solar activity.

[stuartnz]

Thank you very much for that clear, concise explanation.

[John Smith 19]

An *amazing* result

Air pressure halves roughly every time altitude doubles with a basic cycle of 5.6Km. So at 5.6Km it's roughly 50663Pa.

But 270Km is roughly 48x higher.

So air pressure is very low.

So it's amazing the density is still high enough to transmit the pressure wave and the satellite is sensitive enough to detect it.

Historically gravity survey satellites are a niche specialty but linking it's capabilities to seismology and geology opens up the field quite a bit.

Thumbs up for some very lateral thinking.

[Anonymous Coward]

In space no one can hear you scream

There goes another cherished myth.........

[ecofeco]

Another Thing Found...

Another thing found by another of the Earth sensor satellites (sorry, forgot which one) was that the ground temperature rose above normal just before the earthquake occurred.

Rather obvious once you think about it.

So it appears we can, now, indeed predict earthquakes. At least a few minutes before they happen. And if you happen to have a satellite overhead.

[John Smith 19]

Re: Another Thing Found...

"Another thing found by another of the Earth sensor satellites (sorry, forgot which one) was that the ground temperature rose above normal just before the earthquake occurred.

Rather obvious once you think about it.

So it appears we can, now, indeed predict earthquakes. At least a few minutes before they happen. And if you happen to have a satellite overhead."

For large populations that's not really going to be enough. However if the area can be narrowed then locally mounted IR sensors (or probes stuck in the ground) can trigger an alarm. It's a limited capability but may make a difference long term.

[MrZoolook]

I'm glad Reg have an appropriate icon

"The ESA explained that particularly large ‘quakes – like the magnitude 9.0 incident that hit Japan – cause the planet’s surface to vibrate"