show #1213

David probes the causes of earthquakes. Segment Length - 7:59

Contents

Insights & Connections


Vocabulary


Resources


Main activity


Try this

INSIGHTS & CONNECTIONS

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 earthquake.

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 to occur.

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 energy.

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.

VOCABULARY

convection current very slow movement of rock within Earth's mantle caused by heating and cooling

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

RESOURCES

Additional sources of information

National Earthquake Information Service
U.S. Geological Survey
Box 25046-MS 967
Denver Federal Center
Denver, CO  80225
(800) 525-7848

Community resources

College geology professor

State geologic survey for geological maps

MAIN ACTIVITY

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.

Materials

  1. 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.
  2. Using the marshmallows and straws (or stirrers) as building elements, assemble a structure that measures at least 50 cm high.
  3. 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.
  4. Hold a design competition with your friends. See who can build an earthquake-proof structure using the least amount of material.
  5. 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.

Questions

  1. What structural shapes seem to survive quakes best? Can you think of any existing buildings that use this type of design?
  2. What type of earthquake motion was your shake tray simulating? Are there other motions in a quake? How might you duplicate them?
  3. Do you think that it is possible to build an earthquake-proof structure? Why or why not?
  4. How does the amount of shaking time affect building damage?
  5. How does the strength of the shaking affect building damage?

TRY THIS!

Adapted with permission from activity by Katharyn Ross, National Center for Earthquake Engineering Research, State University of New York at Buffalo.

TRY THIS!

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?

TRY THIS!

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 cracker collisions?

TRY THIS!

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.
Newton's Apple is a production of KTCA Twin Cities Public Television. Made possible by a grant from 3M. Educational materials developed with the National Science Teachers Association.