Earthquake!!

An earthquake of magnitude 6.5 on the Richter scale has occurred in southern California. You have been contracted to determine the epicenter for this destructive natural event. Your knowledge earth science, measurement, graphing and Cartesian coordinates will serve as valuable tools as you complete your task.

Materials

• one sheet of paper
• compass
• metric ruler

Procedure

1. Find and mark the center of your paper by folding the paper in half horizontally and vertically.

2. Using the folds as an XY Cartesian coordinate system, mark three seismic stations with the following points:

Station A: X = -1.60 cm, Y = 7.80 cm

Station B: X = 6.38 cm, Y = -8.75 cm

Station C: X = -6.05 cm, Y = -1.01 cm

Scientists know how fast P- and S-waves travel when an earthquake occurs. They can calculate the distance of the epicenter from earthquake by measuring the difference in arrival time of P- and S-waves at their stations. The difference in arrival time of the waves are:

Station A: 238 seconds

Station B: 133 seconds

Station C: 183 seconds

3. Use the following table to record the distance to the epicenter from each station:

Distance to epicenter (km)	Difference in P- and S-wave arrival time (s)
200	40
300	60
400	80
500	100
600	120

4. Convert each distance to cm, so the data can be used on your map. Use the scale 1 cm = 100 km.

5. On your map draw a circle around station A. The radius of the circle is the distance in cm from the epicenter.

6. Repeat the above step for stations B and C.

7. The location of the epicenter of the earthquake is the point where the three circles intersect.

Questions

1. Use your map as a key to determine which California city is at the earthquakes epicenter by placing the origin of your Cartesian coordinate system at Modesto CA with +Y pointing due north.
2. When do scientists need to use this method to find the epicenter?
3. Where is the focus of the earthquake located?
4. How could someone predict the approximate location of an epicenter without a seismograph?