An important tectonic boundary, the Fairweather-Queen Charlotte fault system, runs along the western coast of the park. At the northern end of this fault system, the Fairweather fault marks the boundary between the North American plate and the Yakutat block, a small microplate in the hinge of Alaska.
The Fairweather fault is similar to the San Andreas fault in California in that it has the same sense of motion – if standing on one side of the fault looking at the other side, the land on the other side is moving to the right. This kind of horizontal motion to the right defines the boundary as transform and fault as right lateral strike-slip.
Although the plates are sliding past each other at the seemingly slow pace of about 50 millimeters per year, the impact of this movement in a single event can be tremendous. During the last century alone this fault system has generated six massive, magnitude 7 or greater earthquakes. One such earthquake that occurred in 1958, triggered a 1700 foot high tsunami in Lituya Bay. This magnitude 7.8 earthquake caused as much as 21 feet of horizontal movement and up to 3 feet of vertical movement and was felt as far south as Seattle, Washington.
The Fairweather fault is visible on land for about 170 miles from Cross Sound to its junction with the St. Elias fault near Yakutat Bay. The trace of the Fairweather fault is marked by a topographic trench that along some segments of the fault is over a half mile wide.