Locating The Epicenters’ Of Earthquakes
2014-15
Background Information: To locate the epicenter, a minimum of three circles are drawn on a map. (This process is called triangulation.) You can determine the distance from a seismograph station using the graph you created in "Difference in Arrival
Times for P- and S- Waves”. This distance is the radius of the circle that you draw around that seismograph station. Once you have done the same for at least two more seismograph stations the circles should meet at the epicenter (distortion and estimation error may cause the circles not to meet exactly). With the advent of video cameras being used in so many places of business, the method of locating the epicenter of an earthquake has evolved. When the primary wave (longitudinal) arrives it moves the earth back and forth like a slinky. Seismologists note the direction that stuff falls when the P-wave hits. Using Newton’s law of inertia, they know that if stuff falls toward the east the earthquake came from the east. Knowing the exact location and compass direction of the falling stuff the seismologists triangulate to locate the epicenter. The magnitude, as measured by a seismogram, is scaled by the Richter scale (open ended). The amount of energy released by an earthquake increases by 31 times for each magnitude number. For example, an earthquake with a magnitude of 6 is 31 times stronger than an earthquake with a magnitude of 5 and 1,000 times stronger than an earthquake with a magnitude of 4. (An earthquake magnitude-4 which occurs over a larger fault plane is going to release more energy than an earthquake magnitude-4 over a smaller fault plane. Recently this has been accounted for by a scale called magnitude moment.)
Purpose: You will be determining the epicenter of an earthquake by the process of triangulation.
Materials: the graph created in "Difference in Arrival Times for P- and S- Waves”, pencil, United States map, compass, textbook,
Procedure:
• Read page 219 and 222-223.
• Determine the distance to each seismograph station for the earthquake from the data table below. Rockville, MD has a difference in arrival time of 2 minutes and 00 seconds. By looking on the "Difference in Arrival Times for P- and S- Waves” graph you will see that the distance to the focus is 1,100 kilometers. Record this in the data table.
• Do the same for all 5 cities.
• Using the scale on the United States map, separate the compass so that the point and the pencil are 1,100 kilometer apart.
• Place the point of the compass on Rockville, MD.
• Draw a circle. This circle shows the distance of 1,100 kilometers from Rockville, MD. Which is how far from Rockville, MD the focus of the earthquake is away.
• Following the same procedure for drawing the circles, do the other cities for this earthquake.
• The epicenter is located where all the circles come closest to meeting. Place an X at that spot to show where you believe the epicenter was.
Analysis: Answer the following questions in completely.
1. What is the location of the epicenter of each of these earthquakes (use a map of the USA on pages 708-9 of the text)?
2. What is the minimum number of seismographs needed to accurately locate the epicenter of an earthquake ?
3. How are you able to locate the epicenter of an earthquake if you know where the focus is ?
4. How is distance of a seismograph from the earthquake related to the arrival time of the body waves (P- and S-) ?
5. What causes the ground to become liquefied ? Give an example where it occurred and why is can be so devastating.
6. What is a fire storm ?
7. What is an aftershock ?
8. What is a tsumani ?
9. The New Madrid earthquake was felt over the northeastern United States. An equally severe earthquake in California, however, would not be felt much beyond California. Explain why ?
10. List and explain the reasons why an earthquake of similar magnitude on the east coast of the United States would cause more severe damage than on the west coast of the United States.

Locating The Epicenters’ Of Earthquakes
Earthquake #1
Seismograph Station Difference in arrival times of P- & S- waves
(minutes and seconds)
Rockville 2:00
Newport
Rapid city
Tucson
McMinnville
4:00
2:30
3:20
0:30
Earthquake #2
Seismograph Station
Rockville
Newport
Rapid city
Tucson
McMinnville
Earthquake #3
Seismograph Station
Difference in arrival times of P- & S- waves
(minutes and seconds)
1:50
4:00
2:50
4:15
2:15
Rockville
Newport
Difference in arrival times of P- & S- waves
(minutes and seconds)
4:20
1:50
Rapid city
Tucson
McMinnville
1:25
1:45
3:30
Distance to epicenter
(kilometers)
2014-15
Distance to epicenter
(kilometers)
Distance to epicenter
(kilometers)

Locating The Epicenters’ Of Earthquakes
2014-15

Locating The Epicenters’ Of Earthquakes
2014-15

Locating The Epicenters’ Of Earthquakes
2014-15