In order to locate an earthquake's epicenter, seismology centers compare the arrival times of: -aftershocks -foreshocks -p and l waves -p and s waves

-P and S waves

Explanation:

To locate the epicenter, we need the information on "arrival time of P (primary) and S (secondary waves), at three seismic centers at least. Briefly, we calculate the difference of time in arrival for both waves (P-S), which is usually in seconds. Then we use the traveltime curve chart and look at the expected distance on the given chart (which is in km). Them we plot 3 circles having the same diameters. These three circles meet at one point suggesting the epicenter of an Earthquake.

An example of the traveltime chart is attached which is obtained from the website of "Frank Wattenberg, Department of Mathematics, Montana State University".

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