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LabWrite for Middle School - Earthquake Background
6th Grade SCOS Objective 3.04 Evaluate the forces which shape the lithosphere: Earthquakes
Use the information below and chapter 2 in your textbook (p. 42-75) to answer the
following questions on a separate sheet of paper. You will have thirty minutes to complete
the questions. Be prepared to discuss your answers with the class.
You will need this information to complete the laboratory activity this week.
1. Choose one type of fault. Draw it and explain what it is.
2. Explain what an earthquake is. ______________________________________________________
_________________________________________________________________________________
3. Which type of seismic wave causes the most damage: P waves or S waves? Why? _____________
_________________________________________________________________________________
4. Your teacher(s) will assign you a picture from pages 6-9. Give the probable events that caused the
image you see by identifying the following.
a) location of the picture.
b) tectonic plates involved
c) type of fault causing the earthquake
d) seismic waves involved
e) geology of the area seen in the picture
5. Choose any 7 terms in bold then write definitions for the terms on the back.
Figure 1.
The Earth's surface is covered by a crust. The crust, together with the rigid upper portion of
the mantle that lies just below the crust, makes up the lithosphere. The lithosphere is broken into
15 pieces of varying sizes. These pieces are called plates.
Interaction between plates as they move over the surface of the earth produces Earthquakes. See
figure 2. Plate tectonics is the continual slow movement of the tectonic plates, the outermost part of
the earth. This motion is what causes earthquakes and volcanoes and has created most of the
spectacular scenery around the world.
Figure 2.
An earthquake is caused by a sudden slip on a fault. Stresses in the earth's outer layer push the
sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves
that travel through the earth's crust and cause the shaking that we feel during an earthquake.
A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move
relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur
slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of
kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the
rock on one side of the fault suddenly slips with respect to the other.
Figure 3.
normal fault - a fault in which the block above the fault has moved
downward relative to the block below. Plates are splitting or pulling
apart. This type of faulting is found in the Western United States
Basin and along oceanic ridge systems.
thrust fault (reverse fault) - a fault in which two plates are forced
together. The rocks on one side of the fracture are forced upward
relative to the rocks on the other side of the fault. This type of
faulting is common in areas of compression, such as Japan, the
Himalayan Mountains of India and Tibet.
strike-slip fault - a fault on which the two blocks slide past one
another in opposite directions or at different rates in the same
direction. The San Andreas Fault is an example of a right lateral
fault.
Strike-slip snap: Snap your fingers. You are only able to snap your fingers when the sheer force
directing your fingers to move in opposite directions is greater than the compressional force holding
your fingers together. Just as snapping your fingers releases energy, energy is released as rocks move
past each other along a strike-slip fault.
Earthquakes occur on faults. When an earthquake occurs on one of these faults, the rock on one
side of the fault slips with respect to the other. Plate interaction cause rocks to fail (give way to the
stresses on them). This releases energy that causes vibrations to be sent out from the site of the
earthquake in the form of waves.
Seismic Waves
When an earthquake occurs, it releases energy in the form of waves that radiate from the earthquake
source in all directions. The different types of energy waves shake the ground in different ways and
also travel through the earth at different velocities. See Figure 4.
The P waves, also called primary or compressional waves are the fastest wave, traveling 5.5
km/second (3.3 miles/second or 12,000 miles/hour). They and are the first to arrive at a given
location and can travel through solid and liquid layers of the earth. They
alternately compresses and expands material in the same direction it is traveling. They travel
similar to the way an earthworm travels in a push-pull linear motion. Because they travel in a linear
motion, there is little displacement of Earth's materials. P-waves are the least damaging of all seismic
waves.
Linear motion
The S waves, also called secondary or sheer waves is slower than the P wave, maximum speed is 3
km/second. S waves shake the ground up and down and back and forth perpendicular to the direction
it is traveling. Because S waves do not travel in a linear motion these are the most destructive. S
waves can only pass through solids, and not through the liquid outer core of the Earth's interior. This
is why we can't predict an earthquake.
Figure 4.
During an earthquake you first will feel a swaying or small jerking motion, then a slight pause,
followed by a more intense rolling or jerking motion. The duration of the shaking you feel depends on
the earthquake's magnitude, your distance from the epicenter, and the geology of the ground under
your feet. Shaking at a site with soft sediments, for example, can last 3 times as long as shaking at a
stable bedrock site such as one composed of granite. If the site is in a building, then the height of the
building and type of material it is constructed from are also factors.
For minor earthquakes, ground shaking usually lasts only a few seconds. Strong shaking from a major
earthquake usually lasts less than one minute. For example, shaking in the 1989 magnitude 7.1 Loma
Prieta (San Francisco) earthquake lasted 15 seconds; for the 1906 magnitude 8.3 San Francisco
earthquake it lasted about 40 seconds. Shaking for the 1964 magnitude 9.2 Alaska earthquake,
however, lasted three minutes.
WHERE WAS IT
When an earthquake occurs, one of the first questions is "where was it?" The location may tell us what
fault it was on and where damage (if any) most likely occurred.
Unfortunately, the earth is not transparent and we can't just see or photograph the earthquake
disturbance like meteorologists can photograph clouds. When an earthquake occurs, it generates an
expanding wavefront from the earthquake hypocenter at a speed of several kilometers per second.
Figure 5.
We observe earthquakes with a network of seismometers on the earth's surface.
PICTURES
Picture 1.
Workers examine a cave-in on a road Sept. 26,
2003, after the world's most-powerful earthquake
in 2-1/2 years hit in Kushiro, northern Japan.
(Photo: AP Photo/Kyodo News)
Picture 2.
Residents look at a small metal processing factory
which collapsed in Shironuka, about 13 miles west
of Kushiro on Hokkaido island, northern Japan,
after the earthquake Sept. 26, 2003. The sign
reads: "Ibata Sheet Metal Processing Co. Ltd."
(Photo: AP)
Picture 3.
A fishing boat is beached on the pier of Tokachi
port, on Hokkaido, following the earthquake Sept.
26, 2003, causing a tidal wave to hit the area.
(Photo: AP Photo/Kyodo News)
Picture 4.
Russian rescue team members with their dog
search for survivors under the rubble, in the
ancient city of Bam, where a massive earthquake
hit, about 630 miles southeast of the capital
Tehran, Iran.
(Photo: AP)
Picture 5.
Destroyed bicycles are seen amid the rubble of
the Iranian city of Bam after the earthquake,
Tuesday, Dec. 30, 2003. Some 28,000 bodies
have been pulled from the rubble of the
devastated city.
(Photo: AP)
Picture 6.
A street of demolished buildings in Bam.
(Photo: AP)
The information and images in this worksheet came from:
1. CBS News.com photo essays
http://www.cbsnews.com/stories/2003/12/29/tech/main590561.shtml
2. Science Kit¨ & Boreal¨ Laboratories
3. USGS Earthquake Hazards Program
http://earthquake.usgs.gov/faq/plates.html#6