How could scientists use seismograph station data to locate the epicenter of an earthquake?

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1. 

   joesixpack on September 4, 2009 at 9:03 pm

   An earthquake generates two types of waves. a surface wave (the so-called "S-wave") and the compression, or sub-surface wave (the P-wave" ). These waves propagate at different speeds, so that the farther away from the epicenter of the quake a seismograph is, the longer between the arrival of each wave. So with one seismograph, we can determine how far away the epicenter is. If we draw a circle on a map of that radius, we now know that it was somewhere on that circumference. If we have a second seismograph, we can draw a second circle which will intersect at two points with the first circle. Now we know that it was at one of those two points. If there is a third seismograph, we can draw a third circle which will intersect at one of those two intersections, and we will have our location.