The various earthquake waves are created the moment rocks begin to break, and emanate from the focus outward. Due to the differences in the methods of propagation, they travel at different speeds, and thus reach seismographs at different times. The length of time it takes for one of these waves to reach the seismograph is called its “ travel time ”. Figure 9.40 is an example of a seismograph recording of an earthquake plotted on a P and S wave travel time chart. In this example, the earthquake occurred some 250 km from the recording station. The first wave of the earthquake was recorded about 38 minutes after the earthquake. Because the S wave is slower, it was recorded at the station 70 minutes after the earthquake. FIGURE 9.39 The Propagation of Love Waves The brown curved block moves back and forth with a horizontal motion between the light blue parallel planes. Illustration by Stan Celestian TIME IN MINUTES DISTANCE IN KILOMETERS TO THE EPICENTER 10 20 30 40 50 60 70 80 90 100 110 100 200 300 400 500 600 700 800 EPICENTER TIME ZERO and DISTANCE ZERO FIGURE 9.40 The Relationship Between the Travel Times of the P and S waves. Illustration by Stan Celestian THE PROPAGATION OF LOVE WAVES
However, in the vast majority of cases, the distance to the earthquake is unknown. Knowing the differences in the P and S wave travel times, the distance can be ascertained using this chart. For example, if the P and S wave are separated by 20 minutes, the distance to the earthquake is about 160 km. This is how it is determined using this chart: On the chart of Figure 9.41, a vertical black line is drawn in the left column, just above the blue box labeled EPICENTER. That vertical black line starts at zero minutes and ends at 20 minutes. That 20 minute vertical line is then placed between the S wave line and the P wave line so that it accurately fits the space separating the S wave line and the P wave line (as shown by the yellow arrow). The position on the graph, where the P wave line and the S wave line are separated by 20 minutes, corresponds to a distance to the epicenter of 160 km. What would the distance to the epicenter be if the travel time difference were 47 minutes? Create a vertical line that is 47 minutes long in the TIME IN MINUTES part of the chart. Then move that line to the chart where that still vertical line fits accurately between the P and S wave lines. (The answer is found next to Figure 9.45, Preliminary Earthquake Report, in red.) One seismograph can then be used to determine the distance to an earthquake. If an earthquake is located 160 km from a seismograph, a circle with a 160 km radius is drawn on a map. The epicenter is located somewhere on that circle. Figure 9.42 is a map of the southwestern United States. On the map is a red circle with a radius of 160 km centered on Phoenix, Arizona. From just the seismograph information from Phoenix, all that can be determined is that the epicenter lies somewhere on the perimeter of that red circle. In order to find the exact epicenter, a process of triangulation is required. The
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