SUBDUCTION INTERFACE GEOMETRY AND STATIC STRESS TRANSFER DURING MEGATHRUST EARTHQUAKES, UPPER COOK INLET BASIN, ALASKA

A three-dimensional, elastic dislocation model of the 1964 great Alaska earthquake demonstrates that abrupt changes in subduction zone geometry strongly influence coseismic stress transfer into the upper plate during a megathrust event. The model ascertains that the current stress field of upper Cook Inlet Basin, with maximum horizontal stress oriented EW (~45° counterclockwise from the motion vector of the subducting plate), is caused in part by megathrust slip on the underlying complex subduction interface. This model is an alternative to, yet does not exclude, the previously proposed 'escape tectonics model' for explaining the anomalous stress field and observed stress rotations in upper Cook Inlet Basin. The elastic dislocation model also ascertains that rupturing the complex subduction interface causes a localized decrease in fault stability along the overlying Castle Mountain fault that coincides with the western segment of the fault, the only upper crust fault in the greater Anchorage area with unequivocal Holocene surface rupture. The 1964 Alaska earthquake, Mw 9.2, ruptured a 170,000 km² region of the subduction interface beneath south-central Alaska, including a complex region of rapid changes in trend and spacing of Benioff contours as the subduction interface transitions from shallow subduction of the leading edge of an allochthonous continental fragment (the Yakutat terrane) to steeper subduction of the Pacific plate. The complexity is caused by subduction of a transform boundary between the Pacific plate and the thicker, more buoyant Yakutat terrane.

Fault-cored anticlines in the upper Cook Inlet petroleum province and the Castle Mountain fault lie within the region affected by the complex geometry of the underlying megathrust. The faults present a significant seismic hazard to the greater Anchorage area and to the infrastructure and production potential of the Cook Inlet oil and gas province. Megathrust-induced, static stress changes on the faults affect fault stability, advancing the time to rupture on some faults (primarily east-dipping) and decreasing it on others.