David probes the causes of earthquakes.
Segment Length - 7:59
- What is an earthquake? Where do most earthquakes occur in the world?
- How do you measure earthquakes and what determines how much damage they
Picture it. You're sitting on the sofa watching another fun-packed episode of Newton's
Apple when out of nowhere, you hear a dull roar. The room starts to shake, pictures
fall off the wall, and for a few seconds it seems you've lost your sense of balance.
Suddenly, all is quiet. Was it an explosion? Did a truck crash into your house?
Finally, the announcement comes over the television--you have just survived an
Could this really happen to you? If you live in California, it happens fairly often.
But what if you live in places like North Dakota, Florida, or New York? Are you safe?
Well, don't get too comfortable. Earthquakes can happen in many places, although they
are concentrated in certain areas.
An earthquake occurs when two parts of Earth's lithosphere slide past, away from, or
into each other. According to current theory, Earth's surface is made up of many large
slabs of crust called plates, which ride like giant rafts on semifluid rock below.
Geologists believe that the plates are driven by large convection currents created by
heat generated deep within Earth by the radioactive decay of certain elements. While
most earthquakes happen at plate boundaries, some occur in the middle of a plate.
Just as the continents have moved, plate boundaries have also changed. Over the years,
Earth's lithosphere has been split up and put together many times, leaving millions of
scars or faults. Many of these old faults are static, but every so often stresses build
up because of rock movement in the mantle, causing a fault to rupture and an earthquake
Today, geologists use two different scales to measure how strong an earthquake is. The
Richter scale measures the actual size (or amplitude) of the wave generated by a
particular earthquake on a seismograph. This is an indirect measure of the amount of
energy released by the earthquake. A one-point increase on the Richter scale equals a
tenfold amplitude of wave increase, which equals approximately 32 times more earthquake
A second type of scale, the modified Mercalli intensity scale, measures the amount and
type of damage that earthquakes do to buildings and other structures, and their effects
on humans. Because of these differences in measurements, an earthquake with a low
Richter magnitude reading that occurs in a densely populated area like Los Angeles can
actually have a higher Mercalli value or intensity than a high-magnitude quake in a
desolate region like Antarctica.
- What do you think would happen to your community if it were hit by a
Richter magnitude- 8 earthquake?
- What structures would be damaged?
- What evacuation plans could you make?
- How could you earthquake-proof your classroom?
- What are the pros and cons of releasing earthquake predictions to the
convection current very slow movement of rock within Earth's mantle caused by heating
epicenter the point on Earth's surface directly above the focus of an earthquake
fault a crack in rock or soil along which there has been movement caused by stress
focus the point inside Earth which is the source of the earthquake
lithosphere outer solid portion of Earth including the crust and uppermost mantle
mantle the zone inside Earth between the solid outer crust and the inner core
seismograph a device for measuring and recording vibrations from earthquakes
subduction zone an area where one tectonic plate (usually part of the ocean) is going
down under a second plate
- Beatley, T. & Berke, P. (1992/93, Winter) Time to shake up earthquake
planning. Science & Technology, pp. 82-89.
- Bolt, B. (1993) Earthquakes (3d. ed.). New York: W. H. Freeman.
- Coch, N. & Ludman, A. (1991) Physical geology. New York: Macmillan.
- Davidson, K. (1994, May) Predicting earthquakes. Earth Magazine, pp. 56-63.
- Earthquakes. (1989) Educational materials produced by NSTA and FEMA. To order, phone
Robinson, A. (1993) Earth shock. London: Thames & Hudson.
- Ross, K. (1992, Fall) Shake tables in the classroom. Science Teachers Bulletin, pp. 20-
- 3-2-1 Classroom Contact videotape: Earth is change. GPN: (800)
Additional sources of information
National Earthquake Information Service
U.S. Geological Survey
Box 25046-MS 967
Denver Federal Center
Denver, CO 80225
College geology professor
State geologic survey for geological maps
One of the main causes of damage in an earthquake is the collapse of buildings not
strong enough to withstand the shaking. Engineers and architects try to design buildings
rigid enough to withstand the shock, but flexible enough to give a little under the
stress. By building and testing different models, they can "shake down" their ideas and
see which one "stands" the test of time.
- 40 coffee stirrers or cocktail straws
- 40 mini marshmallows
- a metric ruler
- 2 shallow cardboard boxes (the trays used for cases of soda cans work well)
- a pair of scissors
- 10-20 marbles
- 4 short rubber bands
- Before building your models, you must first build a shake tray. Place one cardboard
box on a table and, with the scissors, cut the bottom out of the second box so that it
fits inside the first box with a 2-cm clearance around each side. Place the marbles in
the first box and rest the cut piece of cardboard on top of them. Use the stapler to
attach one rubber band to each inside corners of the first box and then to the corners
of the cardboard insert. The rubber bands should be taut, but not overstretched. To
start the tray shaking, pull the insert toward one side of the box and let it go.
- Using the marshmallows and straws (or stirrers) as building elements, assemble a
structure that measures at least 50 cm high.
- Place the structure on the middle of the shake tray and see how it stands up to
your quake. Try building several different designs to see if one particular shape
stands up better than the rest.
- Hold a design competition with your friends. See who can build an earthquake-proof
structure using the least amount of material.
- Try varying the amount of time and the strength of the shaking by how hard you
pull on the insert and how tight you stretch the rubber bands.
- What structural shapes seem to survive quakes best? Can you think of any
existing buildings that use this type of design?
- What type of earthquake motion was your shake tray simulating? Are there other
motions in a quake? How might you duplicate them?
- Do you think that it is possible to build an earthquake-proof structure? Why or why
- How does the amount of shaking time affect building damage?
- How does the strength of the shaking affect building damage?
Adapted with permission from activity by Katharyn Ross, National Center for Earthquake
Engineering Research, State University of New York at Buffalo.
Conduct an earthquake drill for your family or school. What are the proper steps to
take? Where should you go for cover? What materials should you store in advance?
Make a model of Earth's insides to see how plate tectonics work. The next time you cook
oatmeal for breakfast, try placing some crackers on top of it while it simmers.
Scatter them around and watch how they move. Is there any pattern to your continental
Make a plate tectonic model of Earth by tracing the present-day outlines of continents
from a map onto a piece of heavy construction paper. Cut out the different land masses
and see if you can put them back together again. If you try really hard, you might be
able to reunite Gondwanaland.
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