WHY DO EARTHQUAKES OCCUR IN 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.
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.
|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.|
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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