Finding an Epicenter
NAME ______
Background Information:
The epicenter of an earthquake is the point on the surface of the Earth directly above the focus or origin of the earthquake.
Triangulation requires seismic information from three different recording stations. Seismic recording stations are located all over the world. Scientists time how long it takes seismic waves to reach each recording station. Seismic waves will arrive at recording stations that are close to the epicenter before they reach stations farther away from the epicenter. This information is converted into a length measurement to find the distance from the station to the earthquake. Using the recording station as the center, circles are now drawn with the radius equaling the distance from the station to the epicenter. The epicenter lies somewhere along this circle, but we need more information to find out its exact location.
Earthquakes can occur at any point on the circle, however, when three separate circles are drawn, the circles will intersect at one point, which represents the epicenter of the earthquake.
Earthquakes produce seismic waves. Primary or P waves are the fastest type of seismic wave. Secondary, or S waves, travel slower than P waves. The fastest way to determine the distance from a location to the origin of an earthquake is to take the difference in arrival time of the P wave and the S wave in seconds and multiply by 8 km/s.
Purpose: To use earthquake data to determine where the epicenter of the earthquake.
Materials:
Calculator
Drawing compass
Ruler
Pencil
Procedure:
1. An earthquake is detected in Santa Fe, Little Rock, and Austin. Each city has a seismologist who recorded the arrival times of P and S waves.
2. Use the data table to determine the arrival times of P waves and S waves at Santa Fe.
3. Calculate the difference in arrival time of P waves and S waves in seconds and record your calculation in the data table.
4. Calculate the distance from each of the cities to the epicenter using the formula below and record your distances in your data table.
Distance (km) = difference in arrival time of P and S waves (s) X 8 km/s
5. The map at the back of this lab has the scale on the bottom. 1 cm on the map = 75 km. Use the distance you calculated in step 3 to determine the distance on the map. Record your results in your data table.
6. Draw a circle around Santa Fe that has a radius of the distance you calculated in step 5.
7. Repeat for Little Rock and for Austin.
8. The epicenter of the earthquake is located near the point at which the three circles intersect.
Data Table: (10 points)
City / Arrival of P waves / Arrival of S waves / Difference in arrival of P and S waves (s) / Distance from city to epicenter (km) / Distance on map (cm)Santa Fe / 01:23:45 / 1:25:19
Little Rock / 1:24:34 / 1:26:27
Austin / 1:21:09 / 1:21:46
Analysis:
1. What happens to the lag time as the seismograph gets farther from the seismograph? Why? (2 points)
2. Where is the earthquake’s epicenter? (1 point)
3. Use drawings to show why fewer than three seismographs wouldn’t give enough data to determine exactly where the epicenter of an earthquake is. (2 points)
4. Research the region affected by the earthquake in Japan on March 11th, 2011. What is it about the location of the origin of the earthquake and its epicenter that produced such a large tsunami? Discuss the geology (boundaries) of that earthquake. (5 points)
Conclusion:
What did you learn about how seismologists use data to determine the epicenter of an earthquake? (Please write ONE paragraph on a separate piece of paper and attach to this lab.) (10 points)