Deformation in the Yakataga seismic gap, Southern Alaska, 1980-1986

A 60-by-40-km trilateration network in the Yakataga seismic gap was surveyed in 1980, 1982, 1984, and 1986 with precise electro-optical distance-measuring equipment to measure strain accumulation. The overall deformation is roughly approximated by a 0.24±0.03 µstrain/yr N32°W±2.4° uniaxial contraction that is uniform in time. However, the spatial distribution of deformation shows some concentration of convergence in the neighborhood of the Chugach-St. Elias fault and of right-lateral shear across the Contact fault. A simple dislocation model of the plate interaction in the Yakataga gap fits the observed deformation reasonably well but seems to require that the motion of the Pacific plate relative to the North American plate be directed more nearly N36°W than N15°W, the generally accepted direction of relative motion for this location. However, the direction of plate motion inferred from the dislocation model depends upon details of the interaction at the plate boundary that may not have been modeled accurately. A nearby but smaller trilateration network at Icy Bay was surveyed in 1982, 1984, and 1986. This network spans the southwest corner of the rupture zone of the 1979 St. Elias earthquake. The deformation at Icy Bay consists of left-lateral shear across a northeast trending zone. The relation of this deformation to strain accumulation in the Yakataga gap, postseismic relaxation associated with the 1979 earthquake, or rebound from the unloading associated with the rapid recession of the Guyot glacier is not understood.