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I know the focus of an earthquake is where the earthquake originated from, but what I could never figure out is, how to scientists find out where exactly the focus (and epicenter) are located?

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    $\begingroup$ Triangulation, the same way GPS satellites are used to locate something on the surface of the Earth. You use multiple seismometers (3 or more), and the arrival time differences between them tells you how far away the source point was. There is some complication because of inhomogenaities in the Earth causing scattering and deflection of the seismic waves. $\endgroup$ – Chris Mueller May 2 '14 at 19:37
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    $\begingroup$ I think this is the kind of question where some research effort should be shown. $\endgroup$ – tobias47n9e May 2 '14 at 20:23
  • $\begingroup$ Did you intend to down vote the question or the answer? $\endgroup$ – Mark Rovetta May 2 '14 at 20:40
  • $\begingroup$ Actually downvoted both :) - But now only the answer. Your comment does make sense and the link is very educational. So I hope Jaxon can edit his question to "What are good learning resources to find the epicenter and focus of an earthquake?". Then I would also upvote the question again. $\endgroup$ – tobias47n9e May 3 '14 at 7:28
  • $\begingroup$ @MarkRovetta - I actually mixed up the words in my comment. I upvoted the answer but downvotet the question. $\endgroup$ – tobias47n9e May 3 '14 at 10:07
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Earthquake epicenters are located using triangulation, this is possible once seismograms of the earthquake - coming from at least three locations - have been analyzed properly. Here is a good explanation on a site for seismology students at Michigan Tech which takes its seismogram illustrations from Bolt's textbook on earthquakes (1978). Read this page and you will have a good explanation of how seismologists determine the location of an earthquake epicenter.

How Do I Locate That Earthquake's Epicenter?

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    $\begingroup$ While this answer may theoretically answer the question, it is better to include the essential parts of the answer here, and provide the link for reference. $\endgroup$ – hichris123 May 2 '14 at 23:34
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    $\begingroup$ @hichris123 thanks for engaging by adding your comment. I think the answer I provided (a link) is a good answer to the question, but it is also an elementary question. I found many 'answers' online, I consider this link superior because of its figures and sources. I add nothing by repeating content said somewhere else. If I add anything to the answer, it is I know from my personal experience that no one cared more about the education of undergraduate seismologists than Prof. Bolt and that his figures used in the online reference are very likely correct and the result of careful consideration. $\endgroup$ – Mark Rovetta May 3 '14 at 5:59
  • $\begingroup$ Seems better; you might want to add a bit more (you could even quote the page via a blockquote >!). $\endgroup$ – hichris123 May 5 '14 at 20:33
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The seismometer records the time when the P and S-waves arrive at the recording station. P-waves travel faster through the earth than S-waves and so they arrive at the seismometer station before the S-waves and are recorded by the seismometer first. The difference in arrival time between the two types of seismic wave can be used to calculate the distance of the earthquake's epicenter from the seismometer, as the further away an earthquake is, the greater the lag time between the detection of the S waves relative to the P waves.

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