Reading A Quake

Can You Read a Quake?

An earthquake has just hit. Can you tell where?  You first need to know how to read a seismogram.

Geologists who study earthquakes are called seismologists.  If you were a seismologist, you would receive data from all across the country.  Within minutes after an earthquake, seismographs located in different parts of the country would record the times of arrival of the P waves and S waves.

When an earthquake occurs, vibrations are sent out in all directions from the earthquake's epicenter.  These vibrations are recorded by machines called seismographs.  A good seismograph can detect seismic waves from an earthquake thousands of kilometers away. In this activity, you will learn how to understand and get information from a seismogram. When you know how to read a seismogram, you can determine how strong the waves are (their intensity, amplitude, and energy).

Suppose your seismograph suddenly begins to receive earthquake waves. At the same moment the first waves arrive, you start your stopwatch. This is how each city below measured the waves that they received: They started their stopwatches when the fist seismic wave (Primary wave) arrived.

The information below displays eight seismograms collected from various locations around the world when the same earthquake hit.  All recorded waves for all eight locations came from one earthquake.  The image may be printed by clicking on the link to the right. Download to print Can you read a quake? image

Questions:

1.Look carefully over the eight seismograms from the seismic stations around the world.

a) Which location experienced the greatest shock? How can you tell?

b) Which location is probably closest to the epicenter?

c) Which location is the farthest from the epicenter?

2. A typical earthquake sends out at least three types of waves: Primary (P), Secondary (S), and Surface (L) waves. These waves travel differently in the earth and make different patterns on the seismogram. For example, L waves are normally the last to arrive (take the longest time), but they also cause the most surface damage to a city.

a) Tokyo received all three types of waves. Locate on the Tokyo seismogram the beginning of the P waves. How many minutes did the Tokyo clock read when the first P waves hit Tokyo?

b) How many minutes did the Tokyo clock read when the first S wave hit Tokyo?

c) How many minutes did the Tokyo clock read when the first L wave hit Tokyo?

3.Name the four locations that received all three types of waves (P, S, and L).

4. From the moment the first P wave arrived, the seismogram shows how many minutes passed before the S waves arrived, followed later by the L waves.

a) Which type of wave is the fastest?

b) Which type of wave is the slowest?

c) At which location did the P and S waves arrive closer together -- San Francisco or Hawaii? What does this tell you about the location of the epicenter?

5. At some locations, not all of the three waves arrived.

a) Which locations appear to have received only P and L waves, but no S waves?

b) Which locations appear to have received only L waves?

6. Some locations that are close to each other did not receive the same waves.

a) Did San Francisco receive a P wave? did Los Angeles?

b) Compared to the size of the earth, these two cities are close to each other.

Why do you think the P wave arrived in one of these cities but not in the other?

Make a good hypothesis to explain this observation.

7. Look at the L waves for Kingston, for Jamaica and for Miami, Florida.

a) Which one -- Kingston or Miami -- is probably closer to the epicenter?

How can you tell from the seismograms?

b) Did Kingston receive a P wave? did Miami receive a P wave?

c) These two cities are not very far from each other on the earth.

Why did the P wave arrive in one of these cities but not in the other?

Make a good hypothesis again.

(Remember which city is farther from the epicenter!)

You can determine how far away the epicenter is from your seismograph. In the next activity, Race of the Waves, you will learn how to find this epicenter distance.