The Matanuska Valley is located in the forearc terrane of the Alaska-Aleutian volcanic arc. A period of extensive faulting occurred in the western Matanuska Valley during the Neogene. Faulting was a response to intraplate strain caused by subduction of the Pacific plate beneath the convergent margin. The Neogene fault system consisted of three sets of faults: two sets of north-trending strike-slip faults and a set of east-northeast–trending reverse faults. Three fault sets were required to accommodate the general three-dimensional deformation. Crustal shortening in a north-northwest direction was accompanied by lateral and vertical extension.
The direction of maximum horizontal shortening strain has apparently been constant since the Neogene, although there is no evidence for extensive faulting of Quaternary deposits in the area we have studied. The western Matanuska Valley is located only 50 km above the Benioff zone, and the predicted maximum principal shortening direction is north-northwest, on the basis of the slip-vector azimuth between the North American and Pacific plates and the nature of surficial ground displacements during the 1964 (M = 8.4) subduction zone earthquake. Focal mechanisms for earthquakes in the vicinity of the Benioff zone and in the lower part of the North American plate indicate west-northwest extension and north-northeast shortening. This means that the strain field at depths below 20 km may differ significantly from the near-surface strain field.