High School Earth Science/Staying Safe in Earthquakes< High School Earth Science
Earthquakes are rivaled only by hurricanes in their ability to cause enormous amounts of damage. Earthquake damage comes not only from ground shaking, but also from the fires, landslides, and tsunamis that may result from the shaking. There are ways for communities to prepare for earthquakes by using earthquake-safe construction techniques or retrofitting old structures. Individuals and households can take actions such as securing heavy objects and preparing an emergency kit. Still, despite the best precautions, a massive earthquake can cause enormous numbers of fatalities and damage.
- Describe different types of earthquake damage.
- Describe the features that make a structure more earthquake safe.
- Describe the ways that a person and a household can protect themselves in earthquake country.
Damage from EarthquakesEdit
Earthquakes kill people and damage property. There are a lot of falsehoods about how earthquakes do their damage and what sort of damage they do or can be expected to do. The ground shaking almost never kills or injures people; rarely, if ever, does the ground open up and swallow someone. Fatalities and injuries caused by earthquakes are due to structures falling on people. More damage is done and more people are killed by the fires that usually follow an earthquake than by the earthquake itself.
Damage to people and property depends on an earthquake's magnitude, the distance of the epicenter to population centers, and how long the ground shakes. But human factors are important too. The type of ground structures are built on is an enormous factor in the amount of damage done. Damage also depends on the quality of structures, including what materials are used.
The largest earthquakes are not necessarily the deadliest. Only about 2,000 people died in the 1960 Great Chilean earthquake, which was the largest earthquake ever recorded at magnitude 9.5. The 1556 Shaanxi earthquake in China measured a magnitude 8.0, but is estimated to have killed about 830,000 people. The Great Sumatra - Andaman earthquake of 2004 makes the list of the largest earthquakes and was also one of the deadliest. However, most of the 230,000 fatalities were due to the tsunami that followed the earthquake, not the earthquake itself.
Damage during an earthquake depends on the type of ground under the buildings. Solid bedrock vibrates much less than soft sediments. Buildings on bedrock will only fail if the earthquake is extremely violent. Soft ground, such as sand, silt, or clay, settles when it is shaken and so buildings tilt or fall over, and pipelines, roadways, and other structures break. Sediments that are saturated with water undergo liquefaction during an earthquake and become like quicksand. Soil on a hillside that is shaken loose can become a landslide, which takes houses downhill with it or buries structures at the hill's base.
In earthquake-prone areas, city planners spend a lot of time understanding which locations are the most vulnerable and trying to reduce the hazards. For example, in the San Francisco Bay Area, planners study maps that show how much shaking is expected in various areas for different magnitudes and locations of earthquakes (Figure 7.44). Using this information can allow them to understand and prepare for the hazards. For example, when faced with two possible locations for a new hospital, planners must build on bedrock rather than silt and clay.
Mexico City provides an example of how soft ground can magnify earthquake damage. In 1985, a magnitude 8.1 earthquake struck about 350 kilometers west of the city. The earthquake was caused by subduction of the Cocos Plate beneath the North American Plate. Mexican government records show that the earthquake killed at least 9,000 people, injured 30,000 more, left 100,000 people homeless, destroyed 416 buildings, and seriously damaged 3,000 other buildings. The reason for so much destruction so far from the earthquake’s epicenter is that Mexico City is built on a drained lake bed. Beneath the capital city, the ground is soft silt and clay in a basin made of solid rock. When the earthquake struck, seismic waves bounced back-and-forth off the sides and bottom of the rock basin amplifying the shaking. In addition, the wet clay experienced liquefaction (Figure 7.45). The buildings were not anchored to bedrock as they should have been and so they settled into the muck, causing enormous damage.
Water, sewer and electrical systems were destroyed, resulting in fires. Acapulco, which was much closer to the epicenter but built on bedrock suffered little damage. To prevent Mexico City from being taken by surprise again, the government built an alert system. The next time there is an earthquake in the subduction zone, a signal will be activated and sirens will sound in the city. This will give residents about one minute to prepare for the inevitable earthquake. At the least, this is enough time for most people to get in a secure location.
The population density of a region is also important to the number of casualties and the amount of damage. The 1964 Great Alaska Earthquake, near Anchorage, was the largest earthquake ever recorded in North America and the second largest globally, with a magnitude of 9.2. The earthquake lasted for several minutes, resulted in slip of up to 11.5 meters (38 feet), and affected an area of 100,000 square miles (250,000 square km). Ground liquefaction caused landslides (Figure 7.46).
Because the earthquake occurred at a subduction zone offshore, large tsunami (up to 70 meters (20 feet)) were created. Despite the intensity of the earthquake, only 131 people died, mostly due to the tsunami and property damage was relatively modest, at just over $300 million ($1.8 billion in 2007 U.S. dollars). The reason there was such a small amount of damage for such a large earthquake is that very few people lived in the area at that time (Alaska had only been a state for five years!). A similar earthquake today would cause immeasurably more casualties. The number of people that an earthquake kills or injures is often related to the time of day that it strikes and where it strikes. The most lethal earthquakes strike densely populated cities when people are at work and school. Being at home in bed is usually safer.
The way a building is built—its construction—is a large factor in what happens during an earthquake. Building construction is the reason many more people died in the 1988 Armenia earthquake than the 1989 Loma Prieta, California earthquake. Although the Armenian earthquake was only slightly lower in magnitude, the mud houses that are found throughout the area collapsed. Most buildings in California's earthquake country are designed to be earthquake safe. However even earthquake safe buildings can be damaged by a large earthquake.
Engineers who design earthquake safe buildings must understand seismic waves and how they affect different types of ground. Skyscrapers and other large structures built on soft ground must be anchored to bedrock, even if it is lies hundreds of meters below the ground surface.
The materials used to construct a structure affect its ability to weather an earthquake. The type of material that is best depends on the size of the building. Small structures, like houses, do better if they are constructed of materials that bend and sway such as wood and steel rather than brick, stone, and adobe, which are brittle and will break. Brittle materials are less likely to break if they are reinforced by steel or wood. Larger buildings must sway, but not so much that they touch nearby buildings. Counterweights and diagonal steel beams are used to hold down sway. A completely different approach for large buildings is to place them on rollers so that they move with the ground but do not collapse. Buildings may also be placed on layers of steel and rubber, which absorb the shock of the passing seismic waves. Structures that fail usually do so because they are weak at the connections, such as where the walls meet the foundation. Earthquake safe buildings are well connected. In a multi-story building, the first story must be supported or the structure may collapse (Figure 7.47).
Older structures can be retrofitted to be more earthquake safe. Retrofitting includes adding steel or wood to reinforce a buildings structure and its connections (Figure 7.48). Elevated freeways and bridges can also be retrofitted so that they do not collapse. The goal of retrofitting is different depending on the type of structure being altered. Most structures are retrofit only to a strength that protects human life. More important structures, like bridges, are made to survive intact, but may need extensive repair after the earthquake. Structures that need to be used in an emergency, like hospitals, are retrofit to higher standards so that they will need only superficial repairs after an earthquake. The highest level of protection is a retrofit that will allow a building to survive unaffected. This is very expensive and is only done for buildings that are of great historical or cultural significance.
Of course, one of the biggest problems stemming from earthquakes is fire. Fires start because earthquakes rupture gas and electrical lines. Breaks in water mains compound the problem by making it difficult to fight those fires. One effective way of dealing with this is to zigzag pipes so that they bend and flex when the ground shakes. Straight pipes will break in a quake. In San Francisco, water and gas pipelines are separated by valves so that areas can be isolated if one segment breaks.
Since engineers know what sorts of structures do best in earthquakes, why aren't all structures in earthquakes zones constructed for maximum safety? Of course, the reason is cost. More sturdy structures are much more expensive to build. Since no one knows which structures will be exposed to a large earthquake during their effective lifetimes, communities must decide how safe to make their buildings. They must weigh how great the hazard is, what different building strategies will cost, and how much risk they are willing to take. In poor communities, the choice may come down to spending money on earthquake-safe buildings or funding other priorities, such as a water sanitation project. The choice often comes down to protecting against a known risk versus unknown one; for example, many people in developing nations die each year from drinking and bathing in unclean water.
Protecting Yourself in an EarthquakeEdit
If you live in an earthquake zone, there are many things you can do to protect yourself before, during, and after an earthquake. The two goals are to make sure that the house and its contents are not a hazard and for the household to be ready to live independently for a few days until emergency services are available in full force.
Before the Earthquake:
- Have an engineer evaluate your 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. Move them to low places so that they do not fall.
- 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. A wrench should be placed nearby for doing so.
- Prepare an earthquake kit with at least three days supply of water and food. Include a radio and batteries.
- Place flashlights all over the house so that there is always one available. Place one in the glove box of your car.
- Keep several fire extinguishers around the house to fight the small fires that might break out.
- Be sure to have a first aid kit. Everyone in the household who is capable should know basic first aid and CPR.
- Plan in advance how you will evacuate your property 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, drop to the ground, get beneath a sturdy table or desk, cover your head, and hold on.
- Stay away from windows and mirrors since glass can break and fall on you. Stay away from large furniture that may fall on you.
- If the building is structurally unsound, get outside as fast as possible. Run into an open area away from buildings and power lines that may fall on you.
- If you are in a car, stay in the car and stay away from structures that might collapse like 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 with urgent needs will be trying to get through to emergency services.
- Be prepared to wait for help or instructions. Assist others as necessary.
- 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 tsunamis 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 for an earthquake.
- 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 while emergency services regroup and get to all parts of the region.
- What usually kills or injures people in an earthquake?
- In two earthquakes of the same size, what reasons are there that more people would be killed in a location further from the epicenter than in one nearer the epicenter?
- Describe why Mexico City was so devastated by the far away 8.1 earthquake that struck in 1985. Why did Acapulco, which is located much closer to the quake site, fare so much better?
- What is liquefaction and how does it cause damage in an earthquake?
- Pretend that you live in an old home in an earthquake-prone region. No work has ever been done to prepare your home for an earthquake. What should you do to minimize the harm that will come to yourself and your home?
- What can an architect do to make a skyscraper earthquake safe?
- Which types of buildings deserve the greatest protection from earthquake hazards?
- 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?
- Why do wealthy communities (such as those in California) tend to have greater earthquake protection than poorer communities (such as those in developing nations)?
- What are the two goals of earthquake preparation?
- What should you include in an earthquake kit?
- Under what circumstances should you run outside in an earthquake?
- Clay, silt, and sand saturated with water become like quicksand, lose their strength and behave more like a liquid than a solid. Solids turn to a type of paste, solids thus become weak and unstable not providing proper support.
Points to ConsiderEdit
- Many people think that in a large earthquake California will fall into the ocean and that Arizona and Nevada will be beach front 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?