US Geological Survey
Fact Sheet 014-03
block diagram showing the tectonic environment of southern Alaska

Earthquakes are commonplace throughout much of Alaska. On average there is a magnitude 7 or greater earthquake somewhere in or offshore Alaska every 1 to 2 years and a magnitude 8 or greater quake about every 13 years. These quakes occur as a result of stresses caused by movements of tectonic plates that make up the Earth's outer shell. In this region, the Pacific Plate moves steadily northward at a rate of about 2 inches per year and descends, or "subducts," beneath the North American Plate.

An irregularity on top of the Pacific Plate, known as the Yakutat block (YAK), impedes smooth subduction of the Pacific Plate and has caused a wedged-shaped piece of the North American Plate, the Wrangell Subplate, to break loose and rotate counterclockwise. The western Alaska Range, which includes Mount McKinley, the highest peak in North America, is a zone of compression between the North American Plate and the Wrangell Subplate. The Denali and Totschunda Faults form the northeastern margin of the Wrangell Subplate.

The largest earthquakes in the region (magnitudes 8 and 9) occur along the subduction zone and often generate destructive tsunamis (seismic sea waves). These include the second largest quake ever recorded worldwide, the 1964 magnitude 9.2 Prince William Sound earthquake, which killed more than 100 Alaskans. Generally smaller but still powerful quakes (magnitudes 6 to 8), such as the November 2002 Denali Fault earthquake, occur inland along faults like the Denali, Totschunda, and Castle Mountain Faults.

Photograph showing the 2002 Denali Fault rupture This photo shows the 2002 Denali Fault rupture about 1.5 miles west of the Trans-Alaska Oil Pipeline. The ground on the left side of the cracks moved away from the viewer about 15 feet, causing this wide gap to open.
index map of Alaska and California showing three different strike-slip earthquakes

The 2002 Denali Fault earthquake occurred on a "strike-slip" fault (in which the two sides slip horizontally past each other), like the San Andreas Fault in California. This Alaska quake was similar in magnitude (M) and in length of fault ruptured to three major quakes on the San Andreas Fault in the past few centuries.

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