Frequently Asked Questions


What is plate tectonics?

According to the theory of plate tectonics, the earth's crust is broken into a number of large irregularly shaped slabs of rock called "plates." The plates move slowly on the surface of the earth, much like large icebergs in an arctic lake. There are two types of plates: continental and oceanic. They are composed of different materials. Continental plates consist of relatively lightweight minerals such as quartz and feldspar. An example of a continental plate is the North American plate. Oceanic plates consist of heavier and more dense minerals like basalt. An example of an oceanic plate is the Pacific plate.

Alaska is situated in a unique and complex tectonic setting. At the southern edge of the state lies the boundary between the North American and Pacific plates. The Pacific plate is moving northwestward relative to, and being subducted under, the North American plate at an average rate of 5-7 centimeters per year. In southeastern Alaska, the plate boundary is defined by a series of strike-slip faults, known as the Queen Charlotte-Fairweather Fault system. Many of these faults continue through Canada and back into Alaska, creating some of the largest faults in the United States. In the eastern Gulf of Alaska, the plate boundary transitions into a continental collision of the Yakutat block with Alaska. Continuing westward, the Aleutian Megathrust subduction zone defines the remaining portion of the plate boundary in Alaska.

Major Alaska Faults

Where do we have earthquakes in Alaska?

A majority of the earthquakes in Alaska occur along the faults associated with the North American/Pacific plate boundary in south-central Alaska and the Aleutian Islands. However, there are numerous other faults and seismic zones within the state that produce a large quantity of earthquakes as well.

Alaska Seismicity 1898-2002

Depth color key: blue = 0-33km, green = 33-75km, red = 75-125km, yellow = 125+km

What does the magnitude of an earthquake represent?

Magnitude represents the amount of shaking and energy released at the time of the earthquake. The magnitude scale is logarithmic, meaning that the shaking from a magnitude 5 earthquake will be 10 times larger than from a magnitude 4 earthquake at the same distance. However, the total amount of energy released from a magnitude 5 earthquake will be 32 times larger than from a magnitude 4 earthquake. There are many different ways that seismologists measure magnitude, depending on how large and deep an earthquake is and what type of seismometer is recording the shaking. The most common ways to report magnitudes today are:

Md - Duration Magnitude - This magnitude is based on the duration of shaking as measured by the time decay of the amplitude of the seismogram.

Ml - Local Magnitude - The original magnitude relationship defined by Richter and Gutenberg for local earthquakes in 1935. It is based on the maximum amplitude of a seismogram recorded on a Wood-Anderson torsion seismograph. Although these instruments are no longer widely used, Ml values are calculated using modern instruments with appropriate adjustments.

Ms - Surface wave Magnitude - A magnitude for distant earthquakes based on the amplitude of the Rayleigh surface wave.

Mw - Moment Magnitude - Based on the moment of the earthquake, which is equal to the rigidity of the earth times the average amount of slip on the fault times the amount of fault area that slipped.

Mb - Body wave Magnitude - Based on the amplitude of P (compressional) body-waves. This scale is most appropriate for deep earthquakes.

from NEIC - http://earthquake.usgs.gov/recenteqsUS/glossary.htm

What is the difference between magnitude and intensity?

Magnitude measures the amount of energy released at the source of an earthquake and is measured from a seismogram; intensity is the strength of shaking produced by an earthquake at a certain location and is determined from effects on people, structures, and the environment.


Alaska Earthquake Facts

How many earthquakes do we have in Alaska?

Daily = 50-100
Weekly = 400-700
Monthly = 1500-3000
Yearly = ~24000

This accounts for approximately 11% of the world's earthquakes and 52% of all earthquakes in the U.S. Alaska has more seismicity than any other region in North America and is by far the most seismically active state in the U.S.

How big are the earthquakes in Alaska?

Alaska boasts 3 of the 10 largest earthquakes ever recorded in the world, and 10 of the 15 largest earthquakes recorded in the U.S. None of these earthquakes was smaller than M7.9!

How big was the largest earthquake in Alaska?

The largest earthquake recorded in Alaska was a M9.2 that occurred on March 27, 1964 in the Prince William Sound. This was the 2nd largest earthquake ever recorded in world history. The energy released in this earthquake was equivalent to approximately 4 trillion pounds of explosive. The motions on the fault lasted for 4 minutes, rupturing an area about 800 kilometers long and 250 kilometers wide. The average movement on the fault was about 9 meters.

On average, Alaska can expect:

    1        M > 8 every 13 years

  1        M = 7-8 every year

  5        M = 6-7 every year

  20      M = 5-6 every year

  90      M = 4-5 every year

  400    M = 3-4 every year

 2100  M = 2-3 every year


Updated: October 2006