What is an earthquake?
An earthquake is a sudden, rapid shaking of the Earth caused by the breaking and shifting of rock beneath the Earth's surface. For hundreds of millions of years, the forces of plate tectonics have shaped the Earth as the huge plates that form the Earth's surface move slowly over, under, and past each other. Sometimes the movement is gradual. At other times, the plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free causing the ground to shake. Most earthquakes occur at the boundaries where the plates meet; however, some earthquakes occur in the middle of plates.
An earthquake's magnitude is measured using the Richter Scale, developed by Charles F. Richter in 1935. It is a logarithmic measurement of the amount of energy released by an earthquake. Earthquakes with a magnitude of at least 4.5 are strong enough to be recorded by sensitive seismographs all over the world. In the United States several thousand shocks of varying sizes occur annually.
The effects of earthquakes are also measured by the Modified Mercalli Intensity scale. The intensity of a quake is evaluated according to the observed severity of the quake at specific locations. The Mercalli scale rates the intensity on a Roman nu meral scale that ranges from I to XII.
Ground shaking from earthquakes can collapse buildings and bridges; disrupt gas, electric, and phone service; and sometimes trigger landslides, avalanches, flash floods, fires, and huge, destructive ocean waves (tsunamis). Buildings with foundations resting on unconsolidated landfill and other unstable soil, and trailers and homes not tied to their foundations are at risk because they can be shaken off their mountings during an earthquake. When an earthquake occurs in a populated area, it may cause deaths and injuries and extensive property damage.
Ground movement during an earthquake is seldom the direct cause of death or injury. Most earthquake-related injuries result from collapsing walls, flying glass, and falling objects as a result of the ground shaking, or people trying to move more than a few feet during the shaking. Much of the damage in earthquakes is predictable and preventable.
The Northridge, California, earthquake of January 17, 1994, struck a modern urban environment generally designed to withstand the forces of earthquakes. Its economic cost, nevertheless, has been estimated at $20 billion. Fortunately, relatively few lives were lost. Exactly one year later, Kobe, Japan, a densely populated community less prepared for earthquakes than Northridge, was devastated by the most costly earthquake ever to occur. Property losses were projected at $96 billion, and at least 5,378 people were killed. These two earthquakes tested building codes and construction practices, as well as emergency preparedness and response procedures.
Where earthquakes have occurred in the past, they will happen again. Learn whether earthquakes are a risk in your area by contacting your local emergency management office, American Red Cross chapter, state geological survey , or department of natural resources.
Earthquakes strike suddenly, without warning. Earthquakes can occur at any time of the year and at any time of the day or night. On a yearly basis, 70 to 75 damaging earthquakes occur throughout the world. Estimates of losses from a future earthquake in the United States approach $200 billion.
Expect aftershocks. Aftershocks are smaller earthquakes that follow the main shock and can cause further damage to weakened buildings. After-shocks can occur in the first hours, days, weeks, or even months after the quake. Be aware that some earthquakes are actually foreshocks, and a larger earthquake might occur.
All 50 states and all U.S. territories are vulnerable to earthquakes. There are 45 states and territories in the United States at moderate to very high risk from earthquakes, and they are located in every region of the country. California experiences the most frequent damaging earthquakes; however, Alaska experiences the greatest number of large earthquakes — most located in uninhabited areas. The largest earthquakes felt in the United States were along the New Madrid Fault in Missouri, where a three-month long series of quakes from 1811 to 1812 included three quakes larger than a magnitude of 8 on the Richter Scale. These earthquakes were felt over the entire Eastern United States, with Missouri, Tennessee, Kentucky, Indiana, Illinois, Ohio, Alabama, Arkansas, and Mississippi experiencing the strongest ground shaking.
Stop, Drop and Hold On
- The best protection during an earthquake is to get under heavy furniture such as a desk, table, or bench
- The greatest danger exists directly outside buildings, at exits, and alongside exterior walls. Many of the 120 fatalities from the 1933 Long Beach earthquake occurred when people ran outside of buildings only to be killed by falling debris from collapsing walls.
- Ground movement during an earthquake is seldom the direct cause of death or injury. Most earthquake-related casualties result from collapsing walls, flying glass, and falling objects.
The Federal Emergency Management Agency (FEMA) reiterates its long-standing advice for staying as safe as possible during an earthquake. It's easy to remember and even easier to do:
- DROP to the ground
- take COVER by getting under a sturdy table or other piece of furniture
- HOLD ON until the shaking stops.
Following the spread of an Internet/Email rumor that contradicts the advice given by FEMA, the American Red Cross, the U.S. Centers for Disease Control and Prevention and a number of other agencies about the proper actions to take during an earthquake, FEMA has been asked for clarification on its policy. We continue to advocate DROP, COVER and HOLD ON as the safest action when the earth begins to shake.
Research has shown that most injuries in U.S. earthquakes occur when people inside buildings attempt to move to a different location inside the building or try to leave. Quickly seeking a place of safety, such as under a sturdy table or desk, and moving as short a distance as possible to that place of safety, is recommended based on research.
In the 2003 San Simeon, California, earthquake, two people were crushed by falling debris when they ran from the building. Studies of the 1979 El Centro, 1987 Whittier, 1989 Loma Prieta, and 1994 Northridge earthquakes, as well as mounting evidence from earthquakes outside the United States, confirm this pattern of injuries. DROP, COVER, and HOLD ON reduces the likelihood of serious injury from falling objects.
Other recommendations, which are contrary to the DROP, COVER and HOLD ON advice, have been made by individuals with limited expertise and questionable credibility. Practice DROP, COVER and HOLD ON at school, in the office, and other buildings so that when the earth shakes, you'll be ready.
How To Prepare For Earthquakes
Earthquakes strike suddenly, violently and without warning. Identifying potential hazards ahead of time and advance planning can reduce the dangers of serious injury or loss of life from an earthquake.
BEFORE an earthquake occurs ...
- Check for hazards in the home
- Fasten shelves securely to walls.
- Place large or heavy objects on lower shelves.
- Store breakable items such as bottled foods, glass, and china in low, closed cabinets with latches.
- Hang heavy items such as pictures and mirrors away from beds, couches, and anywhere people sit.
- Brace overhead light fixtures.
- Repair defective electrical wiring and leaky gas connections. These are potential fire risks.
- Secure a water heater by strapping it to the wall studs and bolting it to the floor.
- Repair any deep cracks in ceilings or foundations. Get expert advice if there are signs of structural defects.
- Store weed killers, pesticides, and flammable products securely in closed cabinets with latches and on bottom shelves.
- MITIGATION - Mitigation includes any activities that prevent an emergency, reduce the chance of an emergency happening, or lessen the damaging effects of unavoidable emergencies. Investing in preventive mitigation steps now such as repairing deep plaster cracks in ceilings and foundations, anchoring overhead lighting fixtures to the ceiling and following local seismic building standards, will help reduce the impact of earthquakes in the future. For more information on mitigation, contact your local emergency management office.
- Identify safe places in each room.
- Under sturdy furniture such as a heavy desk or table.
- Against an inside wall.
- Away from where glass could shatter around windows, mirrors, pictures, or where heavy bookcases or other heavy furniture could fall over.
- Locate safe places outdoors. In the open, away from buildings, trees, telephone and electrical lines, overpasses, or elevated expressways.
- Make sure all family members know how to respond after an earthquake. Teach all family members how and when to turn off gas, electricity, and water. Teach children how and when to call 9-1-1, police, or fire department and which radio station to tune to for emergency information. Contact your local emergency management office or American Red Cross chapter for more information on earthquakes.
- Have disaster supplies on hand.
- Flashlight and extra batteries
- Portable battery-operated radio and extra batteries
- First aid kit and manual
- Emergency food and water
- Nonelectric can opener
- Essential medicines
- Cash and credit cards
- Sturdy shoes
- Develop an emergency communication plan. In case family members are separated from one another during an earthquake (a real possibility during the day when adults are at work and children are at school), develop a plan for reuniting after the disaster. Ask an out-of-state relative or friend to serve as the "family contact". After a disaster, it's often easier to call long distance. Make sure everyone in the family knows the name, address, and phone number of the contact person.
- Pets are often not allowed into shelters for health and space reasons. Prepare an emergency pen for pets in the home that includes a 3-day supply of dry food and a large container of water.
DURING an earthquake ...
- If indoors:
- Take cover under a piece of heavy furniture or against an inside wall and hold on.
- Stay inside.
- The most dangerous thing to do during the shaking of an earthquake is to try to leave the building because objects can fall on you.
- If outdoors:
- Move into the open, away from buildings, street lights, and utility wires.
- Once in the open, stay there until the shaking stops.
If in a moving vehicle:
- Stop quickly and stay in the vehicle.
- Move to a clear area away from buildings, trees, overpasses, or utility wires.
- Once the shaking has stopped, proceed with caution. Avoid bridges or ramps that might have been damaged by the quake.
AFTER an earthquake ...
- Be prepared for aftershocks. Although smaller than the main shock, aftershocks cause additional damage and maybring weakened structures down. Aftershocks can occur in the first hours, days, weeks, or even months after the quake.
- Help injured or trapped persons. Give first aid where appropriate. Do not move seriously injured persons unless they are in immediate danger of further injury. Call for help when necessary.
- Remember to help your neighbors who may require special assistance--infants, the elderly, and people with disabilities.
- Listen to a battery-operated radio or television for the latest emergency information.
- Stay out of damaged buildings. Return home only when authorities say it is safe:
- Check for gas leaks--If you smell gas or hear blowing or hissing noise, open a window and quickly leave the building. Turn off the gas at the outside main valve if you can and call the gas company from a neighbor's home. If you turn off the gas for any reason, it must be turned back on by a professional.
- Look for electrical system damage--If you see sparks or broken or frayed wires, or if you smell hot insulation, turn off the electricity at the main fuse box or circuit breaker. If you have to step in water to get to the fuse box or circuit breaker, call an electrician first for advice.
- Check for sewage and water lines damage--If you suspect sewage lines are damaged, avoid using the toilets and call a plumber. If water pipes are damaged, contact the water company and avoid using water from the tap. You can obtain safe water by melting ice cubes, or by accessing water inside your hot water heater (after it has cooled down completely!).
- Use the telephone only for emergency calls.
- Clean up spilled medicines, bleaches or gasoline or other flammable liquids immediately. Leave the area if you smell gas or fumes from other chemicals.
- Open closet and cupboard doors cautiously.
- Inspect the entire length of chimneys carefully for damage. Unnoticed damage could lead to a fire.
- The behavior of pets may change dramatically after an earthquake. Normally quiet and friendly cats and dogs may become aggressive or defensive. Watch animals closely. Leash dogs and place them in a fenced yard.