Lesson Objectives

  • Describe different types of earthquake damage.
  • Describe the features that make a structure earthquake safe.
  • Describe how to protect a person or household in earthquake country.

Introduction

Earthquakes are natural disasters that cause enormous amounts of damage, second only to hurricanes. Earthquake-safe construction techniques, securing heavy objects and preparing an emergency kit are among the precautions people can take to minimize damage.

Damage from Earthquakes

Earthquakes kill people and cause property damage. However, the ground shaking almost never kills people and the ground does not swallow someone up. Structures falling on people injure and kill them. More damage is done and more people are killed by the fires that follow an earthquake than the earthquake itself. Damage depends on earthquake size, but mostly on the quality of structures.
Earthquake effects on buildings are seen in this animation: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/6/SeismicBuilding-Narrated480.mov
What makes an earthquake deadly?
  • Not size. Only about 2,000 people died in the 1960 Great Chilean earthquake, the largest earthquake ever recorded. The Indian Ocean earthquake of 2004 was one of the largest ever, but most of the 230,000 fatalities were caused by the tsunami, not the earthquake itself.
  • Population density. The magnitude 9.2 1964 Great Alaska Earthquake, near Anchorage, resulted in only 131 deaths. At the time few people lived in the area (Figure below).
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A landslide in a neighborhood in Anchorage Alaska after the 1964 Great Alaska earthquake.
  • Ground type. Solid bedrock vibrates less than soft sediments so there is less damage on bedrock. Sediments that are saturated with water undergo liquefaction and become like quicksand (Figure below). Soil on a hillside may become a landslide.
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Liquefaction of sediments in Mexico City caused the collapse of many buildings in the 1985 earthquake.
In earthquake-prone areas, city planners try to reduce hazards. For example, in the San Francisco Bay Area, maps show how much shaking is expected for different ground types (Figure below). This allows planners to more safely locate new hospitals and schools.
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The expected Modified Mercalli Intensity Scale for an earthquake of magnitude 7.1 on the northern portion of the Hayward Fault.

Earthquake-Safe Structures

Construction is a large factor in what happens during an earthquake. For example, many more people died in the 1988 Armenia earthquake where people live in mud houses than the 1989 Loma Prieta, California, earthquake. Most buildings in California’s earthquake country are designed to be earthquake safe.
  • Skyscrapers and other large structures built on soft ground must be anchored to bedrock, even if it lies hundreds of meters below the ground surface.
  • The correct building materials must be used. Houses should bend and sway. Wood and steel are better than brick, stone, and adobe, which are brittle and will break.
  • Larger buildings must sway, but not so much that they touch nearby buildings. Counterweights and diagonal steel beams are used to hold down sway.
  • Large buildings can be placed on rollers so that they move with the ground.
  • Buildings may be placed on layers of steel and rubber to absorb the shock of the waves.
  • Connections, such as where the walls meet the foundation, must be made strong.
  • In a multi-story building, the first story must be well supported (Figure below).
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The first floor of this San Francisco building is collapsing after the 1989 Loma Prieta earthquake.
To make older buildings more earthquake safe, retrofitting with steel or wood can reinforce a building's structure and its connections (Figure below). Elevated freeways and bridges can also be retrofitted so that they do not collapse.
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Steel trusses were built in an x-pattern to retrofit a dormitory at the University of California, Berkeley. The building is very near the Hayward Fault.
Fires often cause more damage than the earthquake. Fires start because seismic waves rupture gas and electrical lines, and breaks in water mains make it difficult to fight the fires (Figure below). Builders zigzag pipes so that they bend and flex when the ground shakes. In San Francisco, water and gas pipelines are separated by valves so that areas can be isolated if one segment breaks.
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In the 1906 San Francisco earthquake, fire was much more destructive than the ground shaking.
Why aren’t all structures in earthquakes zones constructed for maximum safety? Cost, of course. More sturdy structures are much more expensive to build. So communities must weigh how great the hazard is, what different building strategies cost, and make an informed decision.

Protecting Yourself in an Earthquake

There are many things you can do to protect yourself before, during, and after an earthquake.

Before the Earthquake

  • Have an engineer evaluate the house for structural integrity. Make sure the separate pieces—floor, walls, roof, and foundation—are all well attached to each other.
  • Bracket or brace brick chimneys to the roof.
  • Be sure that heavy objects are not stored in high places.
  • Secure water heaters all around and at the top and bottom.
  • Bolt heavy furniture onto walls with bolts, screws, or strap hinges.
  • Replace halogen and incandescent light bulbs with fluorescent bulbs to lessen fire risk.
  • Check to see that gas lines are made of flexible material so that they do not rupture. Any equipment that uses gas should be well secured.
  • Everyone in the household should know how to shut off the gas line.
  • Prepare an earthquake kit with three days supply of water and food, a radio and batteries.
  • Place flashlights all over the house and in the glove box of your car.
  • Keep several fire extinguishers around the house to fight small fires.
  • Be sure to have a first aid kit. Everyone should know basic first aid and CPR.
  • Plan in advance how you will evacuate and where you will go. Do not plan on driving as roadways will likely be damaged.

During the Earthquake

  • If you are in a building, get beneath a sturdy table, cover your head, and hold on.
  • Stay away from windows, mirrors, and large furniture.
  • If the building is structurally unsound, get outside as fast as possible.
  • If you are outside, run to an open area away from buildings and power lines that may fall.
  • If you are in a car, stay in the car and stay away from structures that might collapse, such as overpasses, bridges, or buildings.

After the Earthquake

  • Be aware that aftershocks are likely.
  • Avoid dangerous areas like hillsides that may experience a landslide.
  • Turn off water and power to your home.
  • Use your phone only if there is an emergency. Many people will be trying to get through to emergency services.
  • Be prepared to wait for help or instructions. Assist others as necessary.

Lesson Summary

  • A person standing in an open field in an earthquake will almost certainly be safe. Nearly all earthquake danger is from buildings falling, roadways collapsing, or from the fires and tsunami that come after the shaking stops.
  • Communities can prepare for earthquakes by requiring that buildings be earthquake safe and by educating citizens on how to prepare.
  • Individuals and households can prepare in two ways: by making sure that their house and its contents are not a hazard and by being ready to live independently for a few days.

Review Questions

  1. What usually kills or injures people in an earthquake?
  2. In two earthquakes of the same magnitude what could produce more damage in a location further from the epicenter than in one nearer the epicenter?
  3. Describe why Mexico City was so devastated by the far away 1985 8.1 earthquake.
  4. What is liquefaction and how does it cause damage in an earthquake?
  5. If you live in an old home in an earthquake-prone region, what should you do to minimize the harm that will come to yourself and your home?
  6. What can an architect do to make a skyscraper earthquake safe?
  7. Which types of buildings deserve the greatest protection from earthquake hazards?
  8. Using what you know about elastic strength, will a building better withstand an earthquake if it is built absolutely solid or if it is able to sway? Why?
  9. What are the two goals of earthquake preparation?
  10. What should you include in an earthquake kit?
  11. Under what circumstances should you run outside in an earthquake?

Vocabulary

liquefaction Clay, silt, and sand saturated with water become like quicksand, lose their strength, and behave more like a liquid than a solid.

Points to Consider

  • Many people think that in a large earthquake, California will fall into the ocean and that Arizona and Nevada will be beachfront property. Why is this not true?
  • If you were the mayor of a small city in an earthquake-prone area, what would you like to know before choosing the building site of a new hospital?
  • How are decisions made for determining how much money to spend preparing people and structures for earthquakes?
  • Why do wealthy communities (such as those in California) tend to have greater earthquake protection than poorer communities (such as those in developing nations)?