Earthquake damage is typically concentrated in locations that can be identified in advance:
- Areas nearest to fault segments that are likely to move.
- Areas of soft or saturated soils where shaking is increased.
- Areas where the ground may settle or slide.
Failure of the ground during an earthquake can happen in many ways. Faults may break the ground as they slip during an earthquake. Steep or unstable hill slopes may slide. Cracks may open along ridge crests close to the earthquake epicenter. Soft ground may amplify the ground acceleration and settle during shaking. The ground may fail if a sandy layer at shallow depth is saturated with water and flows like a liquid during the earthquake shaking.
Damage during an earthquake results from several factors:
- Size of the earthquake. A large earthquake will cause more damage than a smaller earthquake in the same location. However, a moderate earthquake close to populated areas can produce more damage than a large earthquake centered in a remote area.
- Distance from the earthquake. Strength of shaking generally decreases rapidly with distance from the earthquake. The strong shaking along the fault segment that moved during the earthquake is usually only half as strong at a distance of 8 miles and a sixteenth as strong 50 miles away.
- Length of shaking. The longer the strong shaking lasts, the more damage the earthquake can cause. The duration of shaking depends on how the fault breaks during the earthquake and how large a segment of the fault moves. The strongest shaking during the April 1992 earthquake lasted about 12 seconds. The 1906 San Francisco earthquake produced strong shaking in the Humboldt Bay area of about 40 seconds.
- Type of soil. Loose sand and soil increase shaking compared to hard rock. In addition, if the soil is wet, it may lose its strength and the ground surface may settle or slide.
- Type of building. Certain types of buildings, or buildings not adequately secured to their foundations, are not strong enough to resist the side-to-side shaking common in earthquakes and are much more likely to suffer damage than well designed structures.
Faults are the scars of past earthquakes. They may range in length from a few feet to hundreds of miles. Many faults were created by earthquakes millions of years ago and are no longer active. Active faults have either produced earthquakes in historic times or show geologic evidence of movement in the past 10,000 years. They are the most likely sites of earthquakes in the near future. It may be difficult, however, to determine how recently a fault moved and it is not unusual for a fault to first be recognized after a strong earthquake occurs along it. The Alquist-Priolo Special Study Zones Act requires the mapping of all known active faults in California. Areas within 500 feet on these faults are designated Special Study Zones and development of these zones is regulated.
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Kathy Moley
31 August 1996