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Japanese Earthquake

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795278.  Fri Mar 11, 2011 10:32 pm Reply with quote

The tsunami apparently missed Australia thankfully. We didn't really get anything when the Indonesian earthquake hit a few years back either, so we also have escaped - this time.

795296.  Sat Mar 12, 2011 2:00 am Reply with quote

Hang on...

Nuclear incident,

As if Japan didn't have enough on their plate.

795322.  Sat Mar 12, 2011 5:16 am Reply with quote

otyikondo wrote:
Can someone explain the physical process of an earthquake happening? As it feels on the ground?

It seems from several accounts today that people initially shrugged, and said: "Oh, it's just another trembler", and it was only when it had lasted, say, above 30 seconds and had become rather stronger during that time that they realised it was a big one.

I'm not sure WHY, but I've always sort of assumed the physics would be: RUPTURE - BIG SHAKE - followed by progressively smaller shaking until it stops. It seems this is NOT AT ALL what happens, but the shaking starts, gets bigger and bigger (if it does; at which point, well into the earthquake, people recognise that this is for real).

This seismogram indicates clearly why people simply shrugged at first: initially there was nothing for them to get that worked up about. What I don't quite grasp, and hope someone can explain in simple joined-up writing, is how the release of pressure when a quake hits becomes AMPLIFIED over time, rather than being an instant, catastrophic response gradually dying away.

The reason for that pattern of shaking is because the seismic waves forming an earthquake travel have different speeds of propagation. The initial shaking is due to primary wave transmission, then you have the secondary waves. Finally, there are body waves, which have much lower speeds of transmission, but are by far the most destructive due to their amplitude.
Because all of those waves have different frequencies, and speeds of transmission, they separate out over large distances. That is why you have a gradual build up - because there is a distinct time lag between each of the various seismic waves reaching your site.

You can also have amplification of the seismic waves due to geological and topographical effects.
The geological effects are due to soil and bedrock conditions at your site, which can lead to resonance effects - the 1985 Mexico City Earthquake is an example of this, where resonance effects meant that the peak ground acceleration in Mexico City was higher than at the epicentre - despite the fact that Mexico City was over 300 km away from the epicentre.
The topographical effects relate to surface features, such as hills or valleys, which can act to locally amplify or scatter the seismic waves. However, both of those effects are areas of current research, and are still not fully understood.

795325.  Sat Mar 12, 2011 5:21 am Reply with quote

Thank you, Mr Brunel - that's a satisfying answer. Even an seismic ignoramus like me can understand that explanation. :)

795328.  Sat Mar 12, 2011 5:55 am Reply with quote

Lots of angry reactions of Dutch tv viewers who were majorly upset when the NOS showed a dog who couldn't outrun the tidal wave.

There's more footage becoming available of the absolutely mindboggling surge of water, debris, cars etc. Boats floating in the midde of town centres... You watch it happen, but it's still surreal.

795523.  Sun Mar 13, 2011 1:07 am Reply with quote

Also, apparently, the Earth's axis has shifted.

795529.  Sun Mar 13, 2011 3:49 am Reply with quote

Yes, it did that after the Indonesian earthquake of 2004 and also the Chilean earthquake. (this one wasn't updated after the Japan earthquake but is still very interesting).

Ian Dunn
795530.  Sun Mar 13, 2011 4:30 am Reply with quote

I'd just like to point out that if there a nuclear power plant does explose, please don't make jokes about. QI is already got into enough trouble over that as it is.

795532.  Sun Mar 13, 2011 5:39 am Reply with quote

They're saying a death toll in the region of 10,000 - how can it be that low with entire towns "erased".

Ian Dunn wrote:
QI is already got into enough trouble over that as it is.
What are you like? Who would be that dumb?

Ah. Perhaps you should tweet his Fryness and the elves and editors then, not us?

795535.  Sun Mar 13, 2011 5:59 am Reply with quote

Which reminds me of

Last edited by 'yorz on Sun Mar 13, 2011 1:01 pm; edited 1 time in total

Ian Dunn
795546.  Sun Mar 13, 2011 6:58 am Reply with quote

Posital wrote:
Ian Dunn wrote:
QI is already got into enough trouble over that as it is.
What are you like? Who would be that dumb?

Ah. Perhaps you should tweet his Fryness and the elves and editors then, not us?

The elves are here, aren't they?

hassan el kebir
795641.  Sun Mar 13, 2011 10:56 am Reply with quote

I like Hokusai, thanks 'yorz ;-)

795656.  Sun Mar 13, 2011 11:33 am Reply with quote

Afwan :)

795669.  Sun Mar 13, 2011 12:36 pm Reply with quote

hassan el kebir wrote:
I like Hokusai, thanks 'yorz ;-)

Seconded. Mind you, I think I prefer Eishi, as his work is so romantic.

795694.  Sun Mar 13, 2011 1:15 pm Reply with quote

I don't wish to be pedantic, but the Hokusai is not a tsunami, however much people associate it with the idea.

On a quite different topic, it is a sobering thought that the worrying incident at the Japanese nuclear power plant has been classified as a 4 on the INES (International Nuclear Event Scale) scale, which goes up to seven.

Between 1955 and 1979 there were FIVE such INES4 incidents at Sellafield. The fire at Windscale in 1957 was an INES5 incident.


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