N.Z. Version, 1998
This is one of a set of planned activities that explore the refraction of seismic waves. Within the earth's crust and mantle, material becomes increasingly dense and rigid at greater depths. (In this set of activities, we keep things simple by considering only waves traveling through the earth's mantle, although the same principles apply to compressional waves traveling through the core.) These changes lead to an increase in speed of both compressional and shear waves as they travel through increasing depths. In this activity, students construct and interpret graphs of travel time versus angular distance in order to determine that seismic waves do change speed.
Students can learn that seismic waves change speed as they travel through various depths of the earth by looking at a graph of travel times for P and S waves versus the angular distance between an event's epicenter and the station. They may either look at such a graph in a text or the Learning Library or construct one themselves from arrival times provided by the simplified version of the Wave Identifier program. Students may run the Wave Identifier program several times, using various angles between the hypocenter and station and noting the arrival times of the P and S waves. An easy way to vary the angle is to enter (0,0) for the hypocenter's coordinates, and to enter zero for the station's latitude and vary its longitude.
Students need to realize that this is not a graph of any one seismic wave's travel over time, but the total travel times of various seismic waves over their entire trips from epicenter to station. This is a departure from the graphs of one object's motion over time with which students may be familiar. The slope at any point, then, gives the average speed of a seismic wave that travels that angular distance to reach the earth's surface. The changing slope indicates that the average speed of a seismic wave that travels further before reaching the earth's surface is greater than the average speed of a wave that reaches the surface closer to the epicenter.
Looking at a cross-section of the earth shows that the waves that travel further with greater average speeds travel deeper into the earth than the waves that travel smaller angular distances with smaller average speeds. This raises the question: Do seismic waves travel faster as they get deeper inside the earth, and if so, why? The related activity on seismic waves and Snell's Law can help students explore this question.
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