Use the S-P graph to the left and the estimates you made for the S-P time intervals for the three seismograms (shown again for your convenience) to complete the table below. The horizontal grid is in one second intervals.

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You can now determine the distance from each seismic recording station to the earthquake's epicenter using the known times of travel of the S and P waves.

Examine the graph to the right, a graph of seismic wave travel times. There are three curves on the graph: The upper curve shows S wave travel-time graphed versus distance, the center one shows P wave travel time versus distance, and the lower one shows the variation in distance with the difference of the S and P travel times. It takes an S wave approximately 70 seconds to travel 300 kilometers.

For practice, how long does it take the P wave to travel this same distance?

For the rest of this exercise you won't be needing the individual S and P curves, only the S-P curve.

To determine the epicentral distance, we need greater resolution on graph. We will use the graph below, which shows an expanded part of the S-P curve.

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