S, P, Love and Rayleigh waves

[Royston]

Sorry if this seems obvious, but I'm a bit confused how it's possible to locate the epicentre of an earthquake using the method of recording the speed of vibrational waves to a seismometer. If the only true test is using an earthquake to acquire the data, how is it possible to do preliminary tests to work out the speed of a wave through earth size masses of compressed, and fluid rock.

 

How did they determine the start of an earthquake, and the speed it takes the wave to hit the seismometer, when (at the time) there was little knowledge of the quantities of certain rock that make up the mass of the earth.

[Anjruu]

What is your question? How the epicenter is determined, or how the method was developed?

[Royston]

Well both really. I can't understand how they determined the velocity of the wave before knowing the location of the source, and with not understanding the quantities of certain rock in the earth...I guess I'm missing something obvious.

 

I think I'm just getting the order of evidence mixed up.

[swansont]

This is a guess, but if you have different stations recording the data you triangulate (actually quadrangulate, probably), and you can do independent tests of the wave speed through different types of rock. Then you go back and look at the seismic data and solve it.

 

Since you know that answer has to be a point, though, that will tell you some information about any assumptions you might have made about composition. If you have extra data, i.e. more stations than the minimum, and your result is overconstrained, you can start to use the data to actually determine some of the rock compositions — instead of assuming the value, it can be an unknown, and you have more equations.

 

(It's probably like determining position with GPS, and having to account for the ionosphere and troposphere delays.)

[Royston]

Got it, thanks a lot Swansont ! That makes a lot of sense.