Article issued September 25, 2001

Understanding Earthquakes

What is an Earthquake?
Let's first take a look at the earth. The earth is made up of 4 main layers-the inner core, the outer core, the mantle and the crust. The crust is where we live, on the surface of the earth. Below the crust lies the mantle, which is made up of solids, liquids and gases.

The lithosphere is made up of the crust and the upper most layer of the mantle, and is divided up into 12 major plates. As the plates of the lithosphere shift, weak spots, or "faults" develop. When this shifting has built up over long periods of time, the crust of the earth weakens and an earthquake occurs. The place where the crust is weakened is called a "fault".

Sometimes the movement of plates is slow, with great pressure accumulating over time. Other times, plates become locked together and when pressure has reached a certain point, the plates are released and an earthquake happens. If this earthquake happens in a populated area, the effects on mankind can be devastating.

Earthquake Terms
Information provided courtesy of the USGS

Faults
As described above, a fault is a weak spot, or fracture (break) in the earth's crust. There are three main types of faults. A normal fault occurs as a result of pulling tension that develops as one plate slips away from another plate (Diagram 2, "B"). A thrust (reverse) fault occurs as a result of the compression of plates pushing towards each other (Diagram 2, "C"). A strike-slip (lateral) fault occurs as a result of two adjacent places sliding next to each other in opposite directions. (Diagram 2, "A").

Focus and Focal Depth
The focal depth of an earthquake is the depth from the Earth's surface to the region where an earthquake's energy originates (the focus). Earthquakes with focal depths from the surface to about 70 kilometers (43.5 miles) are classified as shallow. Earthquakes with focal depths from 70 to 300 kilometers (43.5 to 186 miles) are classified as intermediate. The focus of deep earthquakes may reach depths of more than 700 kilometers (435 miles). The focuses of most earthquakes are concentrated in the crust and upper mantle. The depth to the center of the Earth's core is about 6,370 kilometers (3,960 miles), so event the deepest earthquakes originate in relatively shallow parts of the Earth's interior.

Epicenter
The epicenter of an earthquake is the point on the Earth's surface directly above the focus. The location of an earthquake is commonly described by the geographic position of its epicenter and by its focal depth.

Tsunamis
Earthquakes beneath the ocean floor sometimes generate immense sea waves or tsunamis (Japan's dread "huge wave"). These waves travel across the ocean at speeds as great as 960 kilometers per hour (597 miles per hour) and may be 15 meters (49 feet) high or higher by the time they reach the shore.

Seismology
The vibrations produced by earthquakes are detected, recorded, and measured by instruments call seismographs. The zig-zag line made by a seismograph, called a "seismogram," reflects the changing intensity of the vibrations by responding to the motion of the ground surface beneath the instrument. From the data expressed in seismograms, scientists can determine the time, the epicenter, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released.

Vibrations
The two general types of vibrations produced by earthquakes are surface waves, which travel along the Earth's surface, and body waves, which travel through the Earth. Surface waves usually have the strongest vibrations and probably cause most of the damage done by earthquakes.

Richter Scale
The severity of an earthquake can be expressed in several ways. The magnitude of an earthquake, usually expressed by the Richter Scale, is a measure of the amplitude of the seismic waves. The moment magnitude of an earthquake is a measure of the amount of energy released - an amount that can be estimated from seismograph readings. The intensity, as expressed by the Modified Mercalli Scale, is a subjective measure that describes how strong a shock was felt at a particular location.

The Richter Scale, named after Dr. Charles F. Richter of the California Institute of Technology, is the best known scale for measuring the magnitude of earthquakes. The scale is logarithmic so that a recording of 7, for example, indicates a disturbance with ground motion 10 times as large as a recording of 6. A quake of magnitude 2 is the smallest quake normally felt by people. Earthquakes with a Richter value of 6 or more are commonly considered major; great earthquakes have magnitude of 8 or more on the Richter scale.

Earthquakes in History
Information provided courtesy of the USGS

The earliest earthquake for which we have descriptive information occurred in China in 1177 B.C. The Chinese earthquake catalog describes several dozen large earthquakes in China during the next few thousand years.

Earthquakes in Europe are mentioned as early as 580 B.C., but the earliest for which we have some descriptive information occurred in the mid-16th century.

The earliest known earthquakes in the Americas were in Mexico in the late 14th century and in Peru in 1471, but descriptions of the effects were not well documented.

By the 17th century, descriptions of the effects of earthquakes were being published around the world - although these accounts were often exaggerated or distorted.

The most widely felt earthquakes in the recorded history of North America were a series that occurred in 1811-1812 near New Madrid, Missouri. A great earthquake, whose magnitude is estimated to be about 8, occurred on the morning of December 16, 1811. Another great earthquake occurred on January 23, 1812, and a third, the strongest yet, on February 7, 1812. Aftershocks were nearly continuous between these great earthquakes and continued for months afterwards. These earthquakes were felt by people as far away as Boston and Denver. Because the most intense effects were in a sparsely populated region, the destruction of human life and property was slight.

The San Francisco earthquakes of 1906 was one of the most destructive in the recorded history of North America - the earthquake and the fire that followed killed nearly 700 people and left the city in ruins.

The Alaska earthquake of March 27, 1964, was of greater magnitude than the San Francisco earthquake; it released perhaps twice as much energy and was felt over an area of almost 500,000 square miles.

The ground motion near the epicenter was so violent that the tops of some trees were snapped off. One hundred and fourteen people (some as far away as California) died as a result of this earthquake, but loss of life and property would have been far greater had Alaska been more densely populated.

Earthquake Hazards and Protection

The USGS provides an excellent earthquake preparedness Web site. Here are some highlights:

Planning for an Earthquake

1. Make sure each member of your family knows what to do no matter where they are when EQs occur:
   • Establish a mtg place where you can all reunite afterward
   • Find out about EQ plans developed by children's school or day care
   • Remember transportation may be disrupted, keep some emergency supplies--food, liquids, and comfortable shoes, for example--at work

2. KNOW where you gas, electric and water main shutoffs are and how to turn them off if there is a leak or electrical short. Make sure older members of the family can shut off utilities

3. LOCATE your nearest fire and police stations and emergency medical facility

4. TALK to your neighbors--how could they help you, or you them after an EQ

5. TAKE Red Cross First Aid and CPR Training Course.

1. If you are INDOORS--STAY THERE! (Get under a desk or table and hang on to it, or move into a hallway or get against an inside wall. STAY CLEAR of windows, fireplaces, and heavy furniture or appliances. GET OUT of the kitchen, which is a dangerous place (things can fall on you). DON'T run downstairs or rush outside while the bldg is shaking or while there is danger of falling and hurting yourself or being hit by falling glass or debris.

2. If you are OUTSIDE-- get into the OPEN, away from bldgs, power lines, chimneys, and anything else that might fall on you.

3. If you are DRIVING--stop, but carefully. Move your car as far out of traffic as possible. DO NOT stop on or under a bridge or overpass or under trees, light posts, power lines, or signs. STAY INSIDE your car until the shaking stops. When you RESUME driving watch for breaks in the pavement, fallen rocks, and bumps in the road at bridge approaches.

4. If you are in a MOUNTAINOUS AREA--watch out for falling rock, landslides, trees, and other debris that could be loosened by quakes.

Learn More!
General Resources about Earthquakes

Online Resources

• USGS Earthquakes for Kids
  This extensive site has resources available by grade level, teachers' resources, lesson plans, links, preparedness pages and much more! A great starting place for teaching about earthquakes.

• Earthquakes: General Interest Publication
  This USGS publication takes a look at earthquakes in history, where and how they occur, and gives a general overview of how they are measured.

• USGS Earthquake Hazards Program
  This site offers monitoring of earthquakes, offers teaching resources, and offers information about earthquake preparedness.

• San Francisco Exploratorium: Earthquakes
  This page has a valuable set of links to important earthquake learning Web sites. Also look under "Activities" for original Exploratorium lessons and themes.

• Earthquake Shake
  Find out what a seismograph is, as well as other earthquake facts, from The Tech Museum of Innovation.

• Earthpulse Center
  This simulation enables students to monitor earthquakes and to gain an understanding of this natural phenomenon.

• Center for Earthquake Research and Information
  This site includes earthquake facts and fiction, the latest earthquake information, and seismology information.

• Earthquake Myths and Folklore
  Learn how various cultures have explained earthquakes throughout history!

• What is Richter Magnitude?
  With a "short answer" and a "long answer" to that question, students of all ages and abilities can learn about the Richter Scale.

• Earthquakes - How Shall We Tell the Children?
  Earthquakes from one Christian perspective.

• McMorrow, Catherine, Marchesi, Sthepen (Illustrator), Quake! (Step into Reading, Step 4, paper), Random House

• Editors of Klutz, Disaster Science, Klutz, Inc

• Watt, Fiona, Earthquakes and Volcanoes (Usborne Understanding Geography), E D C Publications

• VanCleave, Janice Pratt, Janice VanCleave's Earthquakes: Mind-boggling Experiments You Can Turn Into Science Fair Projects, John Wiley & Sons

• Chippendale, Lisa A., The San Francisco Earthquake of 1906 (Great Disasters and Their Reforms), Chelsea House Pub

• Givon, Hannah Gelman, Uttal, David (Illustrator), We Shake in a Quake, Ten Speed Press

• Moreau, Roger, Natural Disaster Mazes, Sterling Publications

LESSON 1:
Seismic Slinky

Concepts:
You will learn about seismic wave types and earthquakes

Lesson:
This Exploratorium Seismic Slinky lesson teaches younger students about various wave types through a simple activity with a slinky. It explains the different types of waves that create different types of earthquakes.

Additional Resources:
Try some other experiments from the Exploratorium!

• Liquefaction
  Find out what happens when filled earth gets shaken up by an earthquake with this simple experiment with a block of wood, water and sand.

• Highway Seismograph
  As you ride down a highway, you can use your extended arm to model the operation of a seismograph.

LESSON 2:
Make a Earthquake Preparedness Plan

Concepts:
You will learn about research, earthquakes and outline writing.

Lesson:
Have students write an earthquake preparedness plan. Whether or not you live in an earthquake zone, students can learn through research and preparation! As a community service, older students can post their plan on the Web, if you have access to a server.

Make sure students include the following in their plan:

1. Emergency supplies and where they will be kept
2. Plan of action-what should the family do? Where should they go?
3. What NOT to do
4. What to do AFTER the Earthquake

Additional Resources:

• USGS Preparedness FAQ
• Putting Down Roots in Earthquake Country

LESSON 3:
Earthquake Vocabulary and Games

Concepts:
You will learn about vocabulary terms, earthquakes, and earth science.

Lesson:
After learning about earthquakes, test the children's vocabulary. They can study vocabulary words by printing out the glossary on the USGS Earthquake Glossary. Then, use this worksheet and have children fill in the definitions. Use the glossary to check answers. As an extension activity, have students' practice spelling the words and later use the words as part of a test. You can use this worksheet or writing paper to test students.

Bonus! Try this Volcano Crossword and Wordsearch!

Additional Resources:

• USGS Java Wordsearches!

• Printable Earthquake Crossword Puzzle

LESSON 4:
Make Your Own Shake Table!

Concepts:
You will learn about measuring and earth science.

Lesson:
Do you want to see how the structures you've designed will stand up in an earthquake? One way you can do this is by testing them on a shake table or seismic simulator. Follow the directions from one of the resources below to make a shake table.

Try building the following structures:

A Water Tower: Use toothpicks and gumdrops or marshmallows to build a structure that can then hold an eight-ounce weight. Place a piece of cardboard with your weighted object on top of the structure. How long can your weighted structure be sustained on your shake table?

A Skyscraper: Use toothpicks and gumdrops or marshmallows to build the tallest structure possible that you believe can be sustained by your shake table. How long did it last?

Additional Resources:

• Make Your Own Shake Table

• Build a Shake Table

• Shake Table

• Table-Top Earthquakes

• Designing Structures To Perform Well During an Earthquake

LESSON 5:
Earthshaking Lab Lesson Plan

Concepts:
You will learn about physics and earth science.

Lesson:
Earthshaking Lab Lesson Plan was developed by John Lahr. He writes, "Earthquakes can provide a useful context for teaching or reviewing many basic physics concepts, such as sliding and static friction, forms of energy and conversion from one form to another, and the elastic properties of materials. Conducting the following lesson plan provides an opportunity for students to work cooperatively together, develop and test a hypothesis, make measurements, and write a short report on the results with graphs. "

Additional Resources:

• Table-Top Earthquakes

LESSON 6:
Virtual Earthquake

Concepts:
You will learn about physics, math and earth science.

Lesson:
The Virtual Earthquake is an interactive Web-based program designed to introduce you to the concepts of how an earthquake EPICENTER is located and how the RICHTER MAGNITUDE of an earthquake is determined. Upon completion of this activity students will be given the opportunity to receive a personalized Certificate as a "Virtual Seismologist."

LESSON 7:
Earthquake Webquests!

Concepts:
You will learn about earthquakes, research, history.

Lesson:
Following is a series of Earthquake Webquests available online:

Can You Predict the Next EarthQuake?
You and the members of your team have been asked to predict the next earthquake in your city. The city will be making plans for evacuations and preparing citizens based on your prediction. Your best work will be required. Research the history of earthquakes in your city, recent quakes, how earthquakes are predicted, the plate tectonics affecting the region and present your prediction to the city council.

Earthquakes in Illinois?
In the winter of 1811-12, the central Mississippi Valley was struck by three of the most powerful earthquakes in U.S. history. Even today, this region has more earthquakes than any other part of the United States east of the Rocky Mountains. However many people in the central Mississippi Valley, which includes Illinois, are not aware that there is a threat of an earthquake in this area. As an employee of the Safety Division of the Illinois Geological Survey, your boss, Mrs. Aftershock, has asked you to help write a informational article. The purpose of this article is to inform the public about the threat of earthquakes in the central Mississippi Valley with the hope of reducing the loss of life and property in future shocks. (for middle school students)

Musical Plates
Musical Plates is an Internet-based multidisciplinary project which will enrich a student's learning experience through "unique and compelling" applications of instructional technology. In particular, this project taps into some of the exciting new applications of the Internet in education by having students access real time earthquake data, interact with experts online, and publish their own work to the project web site. Students will tap into these fantastic resources all in the context of an authentic real world problem which they will need to solve using science and mathematics. (for high school students)

LESSON 8:
How to Build a Model Illustrating Sea-Floor Spreading and Subduction

Concepts:
You will learn about mathematics, measurement, geology and earth sciences.

Lesson:
This report describes how to build a model of the outer 300 km (180 miles) of the Earth that can be used to develop a better understanding of the principal features of plate tectonics, including sea-floor spreading, the pattern of magnetic stripes frozen into the sea floor, transform faulting, thrust faulting, subduction, and volcanism.

LESSON 9:
Shake Rattle and Roll

Concepts:
You will learn about research, comparing, graphs, mathematics, design, map reading, physics.

Lesson:
In this lesson youngsters explore basic principles of structural design and material strength to discover effective ways to construct buildings that can withstand earthquakes. They then design a series of experiments to explore the relationship between the type of ground a structure is built on and the degree of damage it is likely to sustain in an earthquake. This lesson uses simple materials for fun, effective learning.

Article by Patricia Carnabuci, HLN Curriculum Development
Article © Homeschool Learning Network, All Rights Reserved.