Activity 4: A Mantle and a Core
Evidence for a separate core and mantle within the Earth comes from P- and S-wave travel times. Before turning to that evidence, look at Fig. 1, which shows the path taken by a seismic wave traveling along a straight line from an epicenter to a distant seismographic station. The distance from the epicenter to the station can be represented by an angle delta.gif (64 bytes) inside the Earth or by an arc of length D on its surface. Seismologists use delta.gif (64 bytes) because it gives a better sense of distance. For example, it’s easy to see that 90° is one-quarter the distance around the Earth.

Fig. 1: Angular relations for seismic travel times.

Figure 2 shows how long it takes for P and S waves to travel from an epicenter to various points on the Earth’s surface. It takes almost 12.5 minutes for P waves to travel to a distance of 80° from the epicenter but 22.5 minutes for S waves. Notice that P waves are not recorded at stations located between 103° and 143° from the epicenter, and S waves are not recorded above 103°.


Fig. 2: Travel times for the two different types of seismic waves.

These regions are called shadow zones, and their existence is an important clue about Earth’s internal structure. The fact that P and S waves are abruptly cut off at 103° tells us that there is a sharp discontinuity in seismic velocity within the Earth. This discontinuity is the boundary between the mantle and the core. But how do velocities differ between the mantle and core? To answer that question, we will use another Java applet. The applet that appears below is useful for creating and testing seismic velocity models.


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