Finding Epicenters

Introduction: An earthquake may occur anywhere along a fault line. The friction between rock masses rubbing against one another generates shock waves, which travel through the earth. These shock waves (seismic waves) created by the earthquake are radiated in every direction from the focus, the point in the earth where the actual movement takes place. Rocks may skip within a small area or release energy over several miles. Rock slippage extending over 150 miles occurred along the San Andreas Fault during the San Francisco (World Series) earthquake on October 17, 1989. An Earthquake occurs somewhere on this planet every 30 seconds, day after day. Most of these are so weak they go unnoticed without the use of sensitive modern instruments called seismographs.

Seismologists are scientists who study earth movement. They work in seismographic stations throughout the word. When these stations detect earth movement, they compare their records of the time and type of waves and, from that information, determine the epicenter of an earthquake. The epicenter is the point on the earth’s surface directly above the focus. Seismographs are the records taken on a seismograph. When an earthquake occurs, seismograph stations at different areas record the disturbance. The seismograms show both the duration and severity of the shock.

If three widely spaced seismograph stations report their findings, the locations of the epicenter can be determined.

Objective: after you have completed this investigation, you should be able to: 1. Determine the arrival time of P and S waves. 2. Determine the distance from the epicenter. 3. Determine P travel time and origin time. 4. Locate the epicenter of an earthquake.

Materials: Metric Ruler Drawing Compass Pencil Earth Science Reference Tables Procedure:

1. Locate the “Earthquake S-wave and P-wave Travel Time” chart in the Earth Science Reference Tables. 2. Start with the Report Sheet for Earthquake “A”. 3. Determine the arrival times, to the nearest second, of the P and S waves for the first station. 4. Write the arrival times in the Data Table in the Report Sheet. 5. Subtract (S-P) to find the time difference between their arrival. 6. Convert the time difference to distance using the Travel-Time graph from the Earth Science Reference Tables. 7. Using the scale in the map, set your compass for drawing a circle with that distance to the radius. 8. Draw the circle on the map with the exact radius around the first station. 9. Repeat steps 3 through 8 for the other two stations. If you have worked carefully, the three circles should intersect to form a very small triangle. Assume the epicenter is at the center of the triangle. By careful work, keep this triangular area of uncertainty as small as possible. It helps to have a very sharp pencil in the compass. 10. Use the Travel-Time graph in the Earth Science Reference Tables to determine the travel time of the P-waves and record in the Data Table. A. On the X-axis, find the distance. B. Go to the distance line until you hit the P-line. C. Look to the left to find the travel time. Record! 11. Subtract the travel time from the arrival time of the P-wave to find the origin time. Record! 12. Repeat steps 10 and 11 for other stations. 13. The three computed origin times should agree very closely with one another. If they do not, and the three arcs drawn on the map also do not intersect very closely, then you have made an error somewhere and should go back and recalculate everything until you find your error(s).