Earthquakes in the United States occur most frequently west of the Rocky Mountains. But there are 39 states with a cumulative population of more than 70 million which are potential targets for earthquakes.
During an earthquake, the "solid" earth moves like the deck of a ship. The actual movement of the ground, however, is seldom the direct cause of death or injury. Most casualties result from falling objects and debris because the shocks can shake, damage, or demolish buildings. Earthquakes may also trigger landslides, cause fires, and generate huge ocean waves called tsunamis.
- Building collapse or damage, such as toppling chimneys, falling bricks from wall facings and roof parapets, collapsing walls, falling ceiling plaster, light fixtures, and pictures.
- Flying glass from broken windows. (This danger may be greater in modern, high-rise structures.)
- Overturned bookcases, wall units, and other furniture.
- Fires from broken chimneys, and ruptured gas and electrical lines. The danger may be aggravated by a lack of water caused by broken mains.
- Fallen power lines.
- Drastic human actions resulting from fear.
Earthquakes have been a part of life in Montana almost since the beginning of written history. The geologic history of western Montana, as recorded in the mountain ranges and broad valleys, indicates that earthquakes have occurred here for millions of years. Earthquakes accompanied the formation of the Rocky Mountains and will continue to be part of the mountainous region of western Montana. Because we cannot predict or avoid earthquakes, we must understand and learn to live with these forces of nature.
A zone of earthquake activity runs through western Montana. This seismic zone is known as the Intermountain seismic belt. It extends from northwestern Montana southward to southern Utah (Smith and Arabasz, 1991). Near Yellowstone National Park, a branch of the Intermountain seismic belt extends west into east-central Idaho. This westward branch is known as the Centennial tectonic belt (Stickney and Bartholomew, 1987). Earthquakes within these two seismic zones typically occur three to tem miles below the surface of the earth.
Most of the known active faults in Montana are located within the Centennial tectonic belt; however several active faults are also located within the Intermountain seismic belt. Although active faults are located within the seismically active zones, Montana earthquakes cannot be correlated with specific faults that are visible at the surface of the earth except for earthquakes over magnitude 7.0. This paradox seems to hold true throughout the Intermountain seismic belt. Apparently, small- to moderate-magnitude earthquakes in this region occur at depth (3 to 10 miles below the surface) on small, discontinuous faults that do not extend to the surface. These hidden faults, however, were responsible for damaging earthquakes in the Clarkston Valley (1925) and at Helena (1935).
- Check for potential fire risks. Defective wiring and leaky gas connections are very dangerous in the event of an earthquake. Bolt down or provide other strong support for water heaters and gas appliances. Use flexible connections wherever possible.
- Know where and how to shut off electricity, gas, and water at main switches and valves.
- Check with your local utilities office for instructions.
- Be sure you have a flashlight, a battery powered radio, and spare batteries on hand in case power is cut off.
- Place large and heavy objects on lower shelves. Securely fasten shelves to walls. Brace or anchor high or top-heavy objects.
- Bottled goods, glass, china, and other breakables should likewise not be stored in high places or left where they can freely slide on shelves.
- Overhead lighting fixtures such as chandeliers should be made fast. A little wiring or anchoring will usually take care of these risks.
- Deep plaster cracks should be investigated. Such cracks, especially on ceilings, could result in large pieces of heavy plaster falling and causing injury.
- If you are outdoors, stay outdoors; if indoors, stay indoors. In earthquakes most injuries occur as people are entering or leaving buildings.
- If indoors, take cover under a heavy desk, table, bench, or in doorways, or against inside walls. Stay away from glass. Don't use candles, matches, or other open flames either during or after the tremor because of possible gas leaks. Douse all fires.
- If in a high-rise building, get under a desk or similar heavy furniture. Do not dash for exits, since stairways may be broken and jammed with people. Never use elevators, since power may fail.
- If in crowded store, do not dash for a doorway since hundreds may have the same idea. If you must leave the building, choose your exit as carefully as possible.
- If outdoors, move away from buildings and utility wires. The greatest danger from falling debris is just outside doorways and close to outer walls. Once in the open, stay there until the shaking stops.
- If in a moving car, stop as quickly as safety permits, but stay in the vehicle. A car may jiggle violently on its springs during an earthquake, but it is a good place to stay until the shaking stops. When you drive on, watch for hazards created by the earthquake, such as fallen or falling objects, downed electric wires, damaged bridges, or broken undermined roadways.
- Be prepared for additional earthquake shocks called "aftershocks". Although most of these are smaller than the main shock, some may be large enough to cause additional damage.
- Check for injuries. Do not attempt to move seriously injured persons unless they are in immediate danger of further injury.
- Turn on your radio or television to get the latest emergency bulletins and instructions from local authorities.
- Check utilities. Earth movement may have broken gas, electrical and water lines. If you smell gas, open windows and shut off the main gas valve. Then leave the building and report gas leakage to authorities. Do not re-enter the building until a utility official says it is safe. If electrical wiring is shorting out, shut off current at the main box. If water pipes are damaged, shut off the supply at the main valve. Emergency water may be obtained from such sources as hot water tanks, toilet tanks, and melted ice cubes.
- Check to see that sewage lines are intact before permitting continued flushing of toilets.
- Check chimneys for cracks and damage. Unnoticed damage could lead to a fire. The initial check should be made from a distance. Approach chimneys with great caution.
- Do not touch downed power lines or objects touched by downed lines.
- Immediately clean up spilled medicines, drugs, and other potentially harmful materials.
- Do not eat or drink anything from open containers near shattered glass.
- If power is off, check your freezer and plan meals to use foods which will spoil quickly.
- Stay out of severely damaged buildings. Aftershocks can shake them down.