Determining the Earthquake Distance

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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Determining Distance from S-P

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. Then click the Find Epicenter button.

Station	S-P Interval	Epicentral Distance
Eureka, CA	50 seconds	KM
Elko, NV	72 seconds	KM
Las Vegas, NV	64 seconds	KM

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