"Most volcanic activity in Alaska is the result of subduction, the process in which one crustal plate slides beneath another. In the Aleutian Arc, the northward-moving Pacific Plate dives beneath the North American Plate at a rate of 6 to 7.5 centimeters (2.5 to 3 inches) per year (see Plate Tectonics). As the Pacific Plate slowly grinds beneath the Alaska portion of the North American Plate, stress and strain are stored and then released in great earthquakes. In fact, about one-quarter of all earthquake energy released on earth is released in Alaska, and three of the ten largest earthquakes since 1900 took place in Alaska. During this century alone, Alaska has had nearly 70 earthquakes of magnitude 7 or greater. These large earthquakes and thousands of smaller ones occur at the boundary of the Pacific and North American plates. They form a narrow curtain of activity which deepens northward from the Aleutian trench and is about 100 kilometers (60 miles) deep underneath the Aleutian Arc volcanoes."
From Nye, C. J., Queen, Katherine, and McCarthy, A. M., 1998, Volcanoes of Alaska: Alaska Division of Geological & Geophysical Surveys Information Circular IC 0038, unpaged, 1 sheet, scale 1:4,000,000