Why and Where Earthquakes Happen in Alaska

Look at a map of Alaska. Note the great bend in the Alaska Range near Mt. McKinley. Mt. McKinley and the entire Alaska Range including the great bend, are the result of a process that has been going on for millions of years, namely: the jamming of a corner of the North Pacific floor into the south-central part of the state.

By a process which has become known as "plate-spreading" during the past twenty years, we now know that the northeast Pacific Ocean floor is actually a layer of the earth's outer shell that is gradually moving to the northwest at the rate of about two inches per year. The bend in the Alaska Range at Mt. McKinley marks its furthest encroachment into Alaska. Mt. McKinley is itself a product of this process as the oceanic and land masses collide. In fact, it may generally be said that every part of Alaska south of the Alaska Range once belonged to the Pacific Ocean.

It is understandable that when the crustal plates collide there is some disruption. The crust is buckled and uplifted, and the Alaska Range is a product of this grinding collision; but so are the Aleutian Islands, which are formed in an entirely different manner.

The long dimple in the sea floor south of the Aleutian Islands (the Aleutian Trench) marks the line along which the impinging Pacific plate is being forced back into the earth. This produces great heat at depth, melting the rocks which flow to the surface and erupt as lava. This lava forms the Aleutian Islands.

This process of plate "subduction" also occurs in south-central Alaska (resulting in volcanoes such as Mts. Iliamna, Redoubt and Spurr), but a large volume is also scooped out on the surface, creating such features as the Alaska Range.

All this does not happen quietly. Earthquakes and volcanic eruptions commonly accompany even minor adjustments in Alaska's geography, and strain that has built up over a hundred years may result in sudden movements of over 20 feet during a major earthquake. During the great 1964 Prince William Sound earthquake, horizontal movements much in excess of this were measured on the Kenai Peninsula.

Alaska has more earthquakes per year than all the other states combined: about 500 potentially "feelable" earthquakes (about magnitude 3.0) versus about 250 for the rest of the nation. Put another way, the earthquakes in Alaska during a typical year can be expected to release the energy equivalent of about a megaton of TNT (1,000,000 tons), while all other states combined produce the energy equivalent of about fifty kilotons (50,000 tons).

While the Aleutian Chain produces a large fraction of the yearly earthquake energy, most of the events involved are felt by a comparatively few people. By far the most damaging earthquakes in Alaska are endured by residents of the coastal regions, particularly in the Cook Inlet area and the southeast. An extension of the south-central seismic zone stretches through the Mat-Su and Mt. McKinley areas northward to Fairbanks.

Curiously, the most earthquake-prone areas of the state are generally the most densely populated.