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## How do seismologists locate earthquakes?

One question lots of people ask me is: ``How do you locate an earthquake?'' It turns out that, while the procedure is not entirely straightforward, it is not all that difficult to locate an earthquake. I'll tell you how we locate local earthquakes; distant earthquakes are located using similar methods, but they are a bit more complicated than we need to worry about.

You will recall from the discussion in Section 3 that there are three major kinds of seismic waves: P, S, and surface waves. P waves travel faster through the Earth than do S waves, so P waves arrive before S waves do. If you have a seismogram, and you know how to measure time accurately on it, you can pick the arrival times of the P wave and the S wave. Next, you figure out how far apart these waves arrive, called the S-P time. You can then go to a table of distance as a function of S-P time and work out how far away the earthquake was from your station. If you have three or more stations, you can draw circles on a map, and where the circles meet is the location of your earthquake. Essentially, you are triangulating the earthquake's location.

Go look over here to see an example on earthquake location (with images).