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).

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Greg Anderson

ganderson@ucsd.edu

Fri Feb 28 16:14:29 PST 1997