20 Latitude 70 80 60 70 50 60 50 40 Nome 40 30 Mexico City New York 20 Longitude 170 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 1000 2000 3000 4000 5000 6000 500 1500 2500 3500 4500 5500 Scale of miles Figure 4.6 THREE SEISMOGRAMS Seismogram A-New York, NY 1 minute T Seismogram B-Nome, Alaska FIRST P WAVE FIRST SWAVE Seismogram C-Mexico City, Mexico
Seismogram A (New York, NY): The time difference between the P-wave and S-wave arrival is approximately 5 minutes, roughly 2000 kilometers.
Seismogram B (Nome, Alaska): The time difference between the P-wave and S-wave arrival is approximately 6 minutes, roughly 2900 kilometers.
Seismogram C (Mexico City, Mexico): The time difference between the P-wave and S-wave arrival is approximately 4 minutes, roughly 1500 kilometers.
Use Figure 4.6 and a drafting compass to draw a circle around each of the three stations. Make the radius (in miles) of each circle equal to the station's distance from the epicenter as determined above. (Use the scale on the map to set the radius on the drafting compass.) The circles you draw should intersect at approximately one point. This point is the epicenter. If they do not intersect at one point, find a point that is equidistant from the edges of the three circles and use this as the epicenter. Label the epicenter on the map.
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