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For Teachers


Locating the Quake

Where did the quake hit?

If seismic waves from many kilometers away shake the ground underneath your feet, can you tell where they came from?  As long as you can read some seismograms, you can locate the epicenter of an earthquake.

At the end of Race of the Waves, you learned that a seismogram receives Primary (P) waves first, and then it receives Secondary (S) waves.  P waves travel through rock faster than S waves.  Because of this speed difference, the more time P and S waves have to race away from an earthquake, the farther ahead the P waves will be.  In fact, if you measure how many minutes the S waves are behind the P waves, you can estimate how long they have been traveling and how far away they came from.  This is how you can locate the epicenter of an earthquake.

Look again at the graph you created in Race of the Waves.  Does it look like this one?

You need this graph to answer the following questions.

Reading the P and S wave graph

1.  Create a data table with three columns labeled LOCATION, DIFFERENCE IN P AND S WAVE ARRIVAL TIMES, DISTANCE TO EPICENTER.

2.  In the location column write the cities Tokyo, Japan; Sydney, Australia; Hawaii, USA; and California, USA.

Determine on your seismogram precisely how many minutes the first S waves arrived after the first P wave arrived. This is the time delay for S and P waves.  Record this information in the second column of the table.

4.  Using the graph you created in the activity How Far Did the Waves Race?, determine how far each city is from the epicenter of the earthquake.  Record this information in column three of your data table.

Your New Challenge: Locate the Earthquake...

Step 1: Check your time delay.
Look at your data table prepared during the activity How Far Did the Waves Race?  How many minutes apart are the P and S waves for...

a) Tokyo?
b) Sydney?
c) Hawaii?

In order to continue this activity, you need to check the minutes you wrote down for these three locations (the time delay between P and S waves). Here are some hints:

a) Tokyo: The S wave arrived 4.2 minutes after the P wave.
b) Sydney: The S wave arrived almost 6 min after the P wave. (Get a more precise answer.)
c) Hawaii: The S wave arrived almost 10 min after the P wave. (Get a more precise answer.)

Step 2: Find the distance.
You now know how many minutes apart the P waves and the S waves were from each other.  Using the P and S wave time graph, you determined how far in kilometers from each city the earthquake actually occurred.  According to your best estimate, how far from the actual quake was each city?

To help you check your answers, here are some hints:

a) Tokyo is approximately 3100 km from the actual quake.
b) Sydney is between 4000 and 5000 km from the actual quake.
c) Hawaii is almost 9000 km from the actual quake.

Step 3: Pinpoint the epicenter on a map.
Here comes the best part: Where was the epicenter of the actual earthquake?  To answer this question, you need your three answers from Step 2 above. You also need a map of the Pacific Ocean that has a scale in kilometers.

a) On this map, locate and label Tokyo, Japan. Remember that Tokyo was 3100 km from the actual quake. On your map, how far is 3100 km? Set your compass at a radius equal to the distance from Tokyo, Japan to the earthquake epicenter (approximately 3100 km).  Draw a circle with the radius determined on your copy of the map.

HINT: Draw your circles carefully.  You may need to draw some parts of the circles off the map.

b) Now draw a second circle around Sydney. Make this second circle big enough to represent the distance from Sydney to the epicenter. Again, use the map scale to determine how large to draw the circle.

c) Now draw a third circle around Hawaii. Make this third circle as big as the distance from Hawaii to the epicenter. Use the map scale again.

d) Where do the three circles cross? If they meet very close to one point, you have probably done your job correctly. Can you name the location of this epicenter?


  1. Why do seismologists need information form at least three different seismograph stations to determine the location of an earthquake?
  2. About how far is the epicenter that you found from Anchorage, Alaska?  What would the difference in arrival times of the P waves and S waves be for a recording station in Anchorage?
  3. What happens to the difference in arrival times between P and S waves as the distance from the earthquake increases?



Last updated on June 2, 2007 2002