h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

> A horizontal shift showed a magnitude of 7.8. If it had been a vertical shift (measured at the same place), would it have been 7.8? [Rtb] < ...
>> I'm not sure if I understand what you mean. The earthquake magnitude is a measure of the energy released, not distance. [Aurora]<<

No doubt I've got it wrong again...
A hammer blow vertically on a nail (across the grain) to drive it into a wood block
vs
A hammer blow horizontally on a wedge (along the grain) to split the block.
- Measured at the same spot, would the energy expenditure be the same? (I know it could Be the same, but measured the same?)

(I know the difference in a different scenario - that a pffft feels diferent to a thrrrp)

And it now seems that NZ is bit closer to Australia.
Well really a bit wider according to the BBC news article - http://news.bbc.co.uk/1/hi/world/asia-pacific/8162628.stm

@RodtheBrit

I am no geologist so, while I make a series of assumptions, forgive me for stating anything which is obvious to you.

The typical seismometer measures: -
1) displacement in the north-south axis
2) displacement in the east-west axis
3) displacement in the up-down axis

Whatever the nature of the original movement at the fault, the processes of energy transmission and dissipation through the earth's crust will mean there is always a mixture of vertical and horizontal displacements at the measuring station.

Therefore the fairest assessment of energy release in the quake is to measure the areas under the graphs for all three axial motions and add them all together. Since these are sine waves, you would have to calculate the RMS (root mean squared) value. Duration of the quake is presumably as much of a factor as wave amplitude.

With three measuring stations, it should be possible to triangulate the signals and work out the epicentre of the quake.

Once the position is known, the distance from the measuring station to the epicentre can be worked out. The <./>inverse square law</.> can then be used to scale up the signal strength received at any given station to work out the signal strength of the source.

After comparing notes with other stations, it should be possible to come to a consensus about the amount of energy released in the quake and thus publish a magnitude figure to the news agencies.


(At this point, I'm expecting a real geologist to come along and ask me whether I am Mary Queen of Scots*).


EYG
* - this is a private joke between me and anyone else who has heard the relevant Monty Python album

not sure about the first question but i can say that the first quake is not always the largest as pre-quakes often occur and indeed are the best way of prediciting that a large earthquake is coming

EYG: >>Therefore the fairest assessment of energy release in the quake is to measure the areas under the graphs for all three axial motions and add them all together<<

'Add them all together' - yeah, maybe that's the bit I missed.

You might all be interested in the latest big quake in NZ, a 7.1 which happened on the other side of the island this time and in a sandy area rather than a rocky one:

http://www.geonet.org.nz/news/sep-2010-darfield-earthquake/gns-science-response.html

Check out the second photo from the left.

Lots of damage around the epicenre, to nearby Christchurch and a town a bit further north. We're incredibly luck that the quake was at 4.30am which has meant no deaths.

There's much geological significance to this quake as they haven't known much about faults in the area, and this quake has opened a new fault. And because it's sandy, there's been liquification during the quake and so lots of silt, sand and water damage.

They've had a massive number of afterquakes too, and quite big ones:

http://lists.geonet.org.nz/pipermail/eqnews/2010-September/date.html

Rod has some interesting thoughts about why some places got badly damaged and others nearby had no damage at all.

Here's a pic of the 'sand pyramids' that appeared after the quake. I think this is fairly close the the epicentre.

http://drquigs.com/images/rsgallery/original/CHCH3512.jpg

http://drquigs.com/index.php?option=com_frontpage&Itemid=1