Inversion analysis of static displacement data associated with the Alaska earthquake of 1964.

Abstract

By using a generalized inversion technique and the finite-element method, permanent changes in displacement caused by the Alaska earthquake of 1964 are analyzed to investigate a regional variation in the faulting mechanism of the extensive fracture system. In section 2, the fracture system is modelled by a multiple-fault system composed of four rectangular faults in an elastic half-space. The results of the inversion analysis for the multiple-fault system show that the Alaska earthquake consists of a main faulting of a low-angle underthrust and a subsidiary surface faulting of a somewhat high-angle underthrust with a little left-lateral component. The main fault extends from Kodiak Island through Prince William Sound to Kayak Island with a total length of 600 km, and the dip-angle (δ) and dislocation (D) in the corresponding regions are δ=25°NW and D=19m, δ=7°NW and D=10m, and δ=20°NE and D=8m respectively. For the subsidiary fault, the optimal estimates are δ=31°NW and D=12m. The strike direction (N36°E) is parallel to that of the main fault in the Kodiak Island region. In section 3, setting a vertical section to be perpendicular to the strike direction in the Kodiak Island region, the fracture system is treated in a framework of a plane-strain, finite-element approximation, where each fault surface is simulated by a sequence of dislocated double nodes. From the inversion analysis for the two-dimensional fault model, it was found that the dislocation along the main fault surface has a broad peak of 30m at a depth of 20km and decreases monotonously up to a depth of 60km. The profile in the shallower part is obscure, because of the lack of the data in the Gulf of Alaska. For the subsidiary fault, the dislocation has a peak of 12m close to the earth's surface and decreases steeply at a depth of 10 km. The profiles of the observed displacement fields across the fracture system are well interpreted by these dislocation functions varying with depth.