Reduce earthquake damage: Estimate your risk

Estimate your risk

Earthquakes are a risk that we accept as part of living in Alaska. We face many other risks in our lives, and we routinely take precautions to reduce our losses from them. For example, we wear seat belts to reduce the risk of injury during automobile accidents. This is an action that most people have come to accept as a reasonable precaution.

Earthquake hazards can also be reduced significantly by individuals, businesses, and governments taking appropriate action. The basic actions described in these pages are reasonable precautions that should be taken by all residents of Alaska. Other actions--such as strengthening or replacing a dangerous building or even choosing to live in a safer building or in a safer part of your city--may involve significant expense and some disruption. Yet, damage to buildings and other structures is the primary cause of death, injury, and financial loss during a large earthquake.

To decide how much action is required to reduce earthquake hazards, you must estimate your risk. Earthquake risk varies from location to location, from structure to structure, from person to person

Is there a risk of serious injury or even death for occupants of a specific building?

What would be the cost of repairing or replacing a building after a large earthquake?

What would be the cost of not being able to use a building after a large earthquake?

What are the odds that time and money spent on action today will prove cost-effective within your lifetime and within the lifetimes of existing structures?

If a structure will be replaced by normal development within 10 years, is strengthening it to resist earthquake damage cost-effective?

Is such strengthening required by a governmental agency, is it legally reasonable, or is it morally necessary?

These are difficult questions. The sections on the following pages are designed to help you assess your risk from earthquakes and determine how much action is appropriate for you. We can live more safely with earthquakes by understanding the risks and by taking reasonable precautions.

What causes damage during an earthquake?

1. Duration of shaking. Duration depends on how the fault breaks during the earthquake. The strongest shaking during the 1964 earthquake lasted 3 to 4 minutes. During a magnitude 7 earthquake, the shaking may last 30 to 40 seconds. The longer buildings shake, the greater the damage.

2. Strength of shaking. Many damaging earthquakes occur within 15 miles of the Earth's surface. In this case, shaking decreases rapidly with increasing distance from the fault that produced the earthquake. In Alaska, these are most common in central and southeastern Alaska. Deeper earthquakes are common beneath southern Alaska and the Aleutian Islands. Because of their greater depth, the shaking directly above such shocks is reduced, and the shaking decreases gradually with increasing distance from the epicenter of the earthquake.

3. Type of soil. Shaking is increased in soft, thick, wet soils. In certain soils the ground surface may settle or slide. Damage is reduced in buildings located on bedrock.

4. Type of building. Some existing buildings are not resistant enough to the side-to-side and up-and-down shaking common during earthquakes.