Talk:Seismogram

message: The seimograms in the example image are read from left to right, with the horizontal scale being time, and the vertical scale being the amount of shaking.
On each line the first (leftmost) indication on the seismogram is that of the P-wave (primary wave). These waves are generally not destructive in nature and can serve as an early warning. P waves travel at a faster speed through the earth so they arrive before the destructive S-waves (secondary waves). 
The S wave appears as a large "spike" on the seismogram some time after (a distance to the right) the P wave. 
Because the relative speed of S waves and P waves is known, the time difference (distance from left to right) on the siesmogram between the arrival of the P and S waves can be used to estimate the distance of the seismograph from the epicentre of the earthquake, much like counting the time delay between the flash of lightning and the peal of thunder.
An earthquake produces two types of wave ? 
It's to do with the direction of vibration - compare a slinky and the sea, the water moves up and down as the wave moves along - that is called a transverse wave, while the slinky contracts and expands in the direction of movement as the wave moves along - that is called a longtitudinal wave.
As the shaking from the earthquake leaves the epicentre the shaking in the longtitudinal direction travels faster through the rock, than the transverse shaking. .
There is also a third wave that arrives and is marked on the seismogram - those waves that travelled along the surface (rather than taking a shortcut through the planet).
Seismographs in certain spots around the planet don't get the S-waves, because S-waves can't travel through the planet core, which effectively creates a shadow. P waves and S waves are bent when they travel through a mendium that has a different density (like light is bent when looking ar a stick that is in water and air). So placing seismographs in other areas also get shadows and don't display particular waves .
Also worth noticing that somewhere 1/4 of the way around the world in one direction is also 3/4 in the other direction so a signal travelling from a point will get P wave, Swave, surface wave, repeat of surface wave (around the surface from the other direction).
Also worth noticing that seismographs don't know which direction the shaking came from, they detect which way they are shaking at that spot, and given the time delay for the different types of wave give an estimate of the distance from the epicentre. More than one seismograph a distance apart is needed, in order for seismograms to be compared and the difference in time of the various wave arrivals can be used to calculate the direction of the epicentre.
Ok well that's a rough rough attempt, all off the top of my head, it probably needs citations and stuff but looking in the various linked articles should provide that. And it's been a few years since I needed to know about it, so apologies for any errors. EdwardLane (talk) 14:06, 24 July 2011 (UTC)
thread_title: Rough attempt at 'reading a seismogram'

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