**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?

Questions:

- Why do seismologists
need information form at least three different seismograph
stations to determine the location of an earthquake?

- 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?

- What happens
to the difference in arrival times between P and S waves as
the distance from the earthquake increases?