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We have already seen how we can get an estimate of the size of the Earth's core using seismic waves, but how can you determine the location of an earthquake to begin with? Although earthquakes occur at varying depths under the surface, most are only a few kilometers deep. The point on the surface directly above the quake is called the epicenter. . Seismologists can locate the epicenter if they can determine the distance of the quake from three different locations, such as points A, B, and C on the accompanying map. This is called triangulation because it utilizes three angles. First off, determining the distance of a quake epicenter from a particular seismic recording station depends on the two types of seismic waves, P and S. The P waves move faster than the S waves. For instance, through granite, P waves move about 5.5 kilometers per second (k/s), whereas the slower S waves move only about 3 k/s through granite. ( In this example we will make the simplifying assumption that the seismic waves are traveling exclusively through granite, although in real life this would be unlikely and calculations would be more difficult and less precise. )
Imagine that at station A a P wave is detected and the S wave follows 42.8 seconds later. Since the S wave is 2.5 k/s slower than the P wave, difference in speed multiplied by the time difference will give the distance to the source. Thus, the earthquake epicenter is 107 km away from station A (42.8 times 2.5 = 107). Although we can determine the distance, we still don't know the direction, which is why we need data from the other stations.
(Click on the image to change) First we draw a circle of radius 107 km around station A. From station B we find that the difference in arrival times of the P and S waves is 63.2 seconds, which yields a distance of 158 km. Now we draw a circle of radius 158 km around station B.
(Click on the image to change) You will notice that the two circles overlap at two places, one of which is out of the frame at the top. We can conclude that the epicenter is at one of the two overlapping points. To find a more exact location, we need data from another station, such as station C.
(Click on the image to change) At C the P and S waves are 76 seconds apart, yielding a distance of some 190 km. After drawing the circle at that radius about station C, we find that it intersects the other two circles at one point, marked with a small red circle. This is the approximate epicenter of the earthquake.
| Berkeley | 21 seconds | 190 km |
| Jamestown | 20.4 seconds | 188 km |
| Mineral | 12.9 | 105 km |
Note that the speeds do not conform to our example above because this is a real-world example in which the speeds vary according to the type of material through which they are passing. The speeds of earthquake waves in this case are derived from past experience in the area.
Based on this data, draw your arcs at the appropriate distances to determine an approximate location of the epicenter. When you have finished (but not until then!), check your answer by clicking here: EPICENTER .
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