Compare and contrast the P and S seismic waves.

How to Determine the Distance to the Epicenter of an Earthquake

Aim: How do seismologists determine the distance to an earthquake epicenter?

I. Determining the distance to an earthquake epicenter

A. Read seismogram to determine the P-wave arrival (PA) time and the S-wave arrival (SA) time.

Look at seismogram hand-outs.

How to read a seismogram:

•P-wave is the first to arrive (fastest). Look for first spike in seismogram. S wave is second to arrive. Look for next major spike – usually larger in amplitude (height) than S wave.

•Observe how time is divided on the bottom of the seismogram (hours, minutes, seconds)

02:47:30

1. Record your P-wave arrival time to the nearest second.

2. Record your S-wave arrival time to the nearest second.

3. Find the difference in arrival times (S.A. time subtract P.A. time)

4. Look at p. 11 of your ESRT (Earthquake travel time graph)

a. Vertical axis (Travel Time in minutes) – each minute is divided into three smaller boxes – 20 seconds each.

b. Horizontal Axis – Epicenter Distance (x103 Km) – distance is in thousands of kilometers and each thousand is divided into 5 smaller boxes – 200 km each box.

c. P and S wave curves

Sample Problems

1. How long does it take a P wave to travel 4000 km?

•Look at your E-quake Travel Graph

•Go to 4 on the x-axis (remember it is 4 x 103)

•Follow the 4 line up until it hits the P curve.

•Follow the Time line (y-axis) that the P curve intersects at this distance

•This is your P travel time

•7 minutes

1. How long does it take an S wave to travel 5400 km?2. How far can a P wave travel in 5 minutes?3. How far can an S wave travel in 4 minutes and 20 seconds?

6. Slide your scrap paper (“Electric Slide”) until the top mark is on the S-wave line and the bottom mark is on the P-wave line

simultaneously.

7. Follow that line down and note the distance. This is your distance to the epicenter.

5. Using a piece of scrap paper, mark off your arrival time difference.

Answer the problems in the first chart on your travel Time Curve worksheet.

S.A. = 06:03:00

P.A.= 05:58:201. Always start from the right (seconds)

2. If you cannot subtract the seconds, you need to borrow a minute, which is 60 seconds

3. If you cannot subtract the minutes, you need to borrow an hour, which is 60 minutes.

Answer Part II of your Travel Time Worksheet.