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About Earthquakes

What is an earthquake?


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.

How are earthquakes measured?

Vibrations produced by earthquakes are detected, recorded, and measured by instruments called seismographs. These devices may amplify ground motions beneath the instruments to over 1 million times, transcribing the ground motion into a zig-zag or wiggly trace called a seismogram. From the data expressed in seismograms, the time, epicenter, and focal depth of an earthquake can be determined. Also, estimates can be made of its relative size and amount of energy it released.

The point on the fault where rupture initiates is referred to as the focus or hypocenter of an earthquake. The hypocenter is described by its depth in kilometers, its map location in latitude and longitude, its date and time of occurrence, and its magnitude (a measure of the amount of energy radiated as seismic waves). The term epicenter, which is more commonly used to refer to an earthquake location, is the point on the earth's surface directly above the hypocenter. The description of an epicenter is the same as for a hypocenter except the depth is omitted.

The strength of an earthquake is generally expressed in two ways: magnitude and intensity. The magnitude is a measure that depends on the seismic energy radiated by the earthquake as recorded on seismographs. An earthquake's magnitude is expressed in whole numbers and decimals (e.g., 6.8). The intensity at a specific location is a measure that depends on the effects of the earthquake on people or buildings. Intensity is expressed in Roman numerals or whole numbers (e.g., VI or 6). Although there is only one magnitude for a specific earthquake, there may be many values of intensity (damage) for that earthquake at different sites.