our feet?

The solid earth? Not quite! An earthquake is the movement of a part of the planets crust. They
often occur along the lines of faults or the edges of crustal plates. There are three kinds of faults
that are found in the crust.

Normal faults are breaks in the crust that are at a slanted angle. The pieces of crust
pull apart or diverge, and one side slips down. The Sandia Mountains in New
Mexico and the Grand Tetons are examples of mountains created along normal faults.

When the plates push or converge into each other, reverse faults may occur. Blocks
of crust are pushed so that one side of the fault is pushed up to hang over the crust on
the other side of the fault. Strong, deep earthquakes may occur along these faults.
Examples of mountains formed in this manner are found in Glacier National Park.

Strike/slip faults occur when plates slide against each other. The most famous fault
of this sort in the United States is the San Andreas Fault in California. Earthquakes
occur when the forces built up are released.

Enormous pressures from Earths mantle push the crustal plates. But they lock against each
other, and sometimes do not move for a very long time. When they do, all this energy is released.
That makes a massive quake.

The energy of an earthquake takes the form of vibrations or seismic waves that carry the energy
from the focus or origin of the earthquake underneath the surface to other parts of the earth.
There are two kinds of waves:


A. Primary waves (P waves) are the fastest waves of energy moving through all areas of
the earth, solid and liquid. They can compress and expand the ground, much like
sound waves compress and spread apart sound waves in the air. You can remember
these as push waves.

B. Secondary waves (S waves) occur after the P waves. They vibrate from side to side as
well as up and down. S waves travel more slowly and cannot move through areas
that are completely liquid. The surface of the crust and structures on it may shake
violently when S waves reach the surface. You can remember these as shake
waves.

Seismologists around the Earth measure the intensity of the seismic waves as they reach the
surface. After studying the data about the seismic waves, scientists can pinpoint the focus of the
earthquake. Eventually scientists hope they can predict earthquakes instead of just studying
them after they occur.
June 11, 2002

SCoPE SC080104 Page 1 of 6 Middle School Science

Earth Changers
Earth and Space Science

Student Resource

Earthquake intensity is measured on two different scales. The Richter scale shows how much
energy an earthquake has, while the Mercalli scale shows how much damage it does. A very
powerful earthquake in a barren desert might do less damage than a less powerful earthquake in a
crowded city. The following table shows how many earthquakes of each magnitude occur in an
average year.

Magnitude
Number of quakes
per year
8.0 and higher
1
7.0-7.9 18
6.0-6.9 120
5.0-5.9 800
4.0-4.9 6200
3.0-3.9 49,000
2.0-2.9 365,000

Make a bar graph, using the Richter scale information above, to show how many earthquakes of
different intensities occur each year.


The Annual Occurrence Of Earthquakes Of Varying Intensities



















_______________________________________________________

June 11, 2002

SCoPE SC080104 Page 2 of 6 Middle School Science

Earth Changers
Earth and Space Science

Student Resource
Making Models to Increase Your Understanding
Procedure:
1. Using the materials as given to you by your teacher construct models of the three types of
faults and the resulting changes in the land. Begin by making three levels of rock. (You can
use any three colors of flat clay.) Put these layers on top of one another. Make the land a little
uneven, like a hill and a valley. Then use frosting to paint on a stream. (Make sure your water
flows downhill.)

Next, use a plastic knife to cut your land cleanly.

Make a cut that slants as it goes down, to show a normal fault. Let part of your land slip
down.

Show a reverse fault by pushing part of your land up to form a mountain. Use your
hands to mold the clay to show how the lifted part might weather over time.

Finally, let one part of your land slide against the other to show a strike/slip fault.

Watch what happens to the stream for each kind of fault. Draw and label each type of fault in
the boxes below. Show the colors of clay and the stream.















2. Draw what happens when a Slinky is used to model a P wave and an S wave.
















June 11, 2002

SCoPE SC080104 Page 3 of 6 Middle School Science

Earth Changers
Earth and Space Science

Student Resource
June 11, 2002

SCoPE SC080104 Page 4 of 6




Quake Hits Michigan!

Lansing, September 2, 1994


Michigan residents were startled yesterday, as the earth
shook beneath them. An earthquake registering 3.4 on the Richter
scale rocked our normally steady state. Geologists from Michigan
State University were the only ones who were not surprised to
feel the tremors.


Kazuya Fujita, an MSU geology professor, explained to
the Lansing State Journal: In Michigan, there are cracks in the
basement rock that run northwest to southeast. The earthquake
occurred along a similar but as yet unmapped fault. When you
apply a stress, it moves a little bit.


What Fujita called a little bit seemed massive to the
residents who grabbed for the nearest stable object. On the
Mercalli scale, which measures damage to structures and living
things, the quake measured a V on a Roman numeral scale from I
to XII. Geologists speculated that the epicenter was over 10 km
underground.

Luckily, fearful Michiganders probably will not quake
again soon. The last one this state experienced measured 4.7 on
the Richter scale, and was centered in Coldwater in 1947, and on
average occur only once every fifty years.





























Middle School Science

Earth Changers
Earth and Space Science

Student Resource
Its Your Turn
After reading the article, pick the best answer to each question below. Circle the letter of the
best response. Then underline or highlight where in the article you found the information for
your response.

1. The Michigan earthquake originated
a. near the ground surface
b. by a fault
c. 10-15 km underground
d. all of the above


2. Cracks in the lowest rock under Michigan run in these directions:


a.





b.
c.
d.








3. During every century in Michigan we get earthquakes about

a. 1
time
b. 2-3 times
c. 5
times
d. 10 times


4. A 1994 Michigan quake registered

a. 8.3 on the Richter scale
b. 3.4 on the Richter scale
c. XII on the Mercalli scale
d. I on the Mercalli scale


5. Scientists gathered helpful earthquake information about this quake from

a. the
weather
reports
b. dogs and cats that behaved strangely before the earthquake
c. people who felt it
d. satellite pictures
June 11, 2002

SCoPE SC080104 Page 5 of 6 Middle School Science

Earth Changers
Earth and Space Science

Student Resource

Locate the Epicenter

When you feel a P wave, an S wave is on its way. Look at this graph of time and distance:




1. If you were 2000 km from the epicenter and felt a P wave, how long would you wait until
the S wave hit?





2. Suppose there were three students on an Internet chat when an earthquake occurred. Each
of them measured the time between the P and the S wave. Can you tell how far each is
from the earthquakes epicenter?
City Delay
Distance
Tokyo 4.2
minutes

Sydney 6
minutes

Hawaii 10
minutes



3. Get a world map. Use a compass to mark off a circle with a radius equal to the distance
youve filled in above.
Where do the circles intersect?




That is the epicenter.





June 11, 2002

SCoPE SC080104 Page 6 of 6