Modeling Interseismic and Transient Deformation in Southcentral Alaska

The convergent margin of Southern Alaska marks the active tectonic boundary between the North American and Pacific plates. Here we numerically model the response of the Alaskan lithosphere to interseismic, coseismic and postseismic loading in order to interpret the contemporary velocity field from GPS observations. Results suggest that, to first order, the surface velocities can be explained by the combination of interseismic deformation associated with a locked megathrust and postseismic viscous relaxation following large earthquakes, particularly the 1964 M9.2 Great Alaska earthquake. The best fitting model requires a weak mantle wedge sandwiched between a strong crust and the subducting slab. Most of the trenchward directed velocities observed in the GPS data, near the Kenai Peninsula and Kodiak Island as well in interior Alaska north of the Denali Fault, are a viscous relaxation response to the 1964 earthquake. In a few decades we should begin to see these velocities decay and subsequently point northwestwards. Postseismic viscous relaxation associated with large strike slip earthquakes since 1949 on the Queen Charlotte-Fairweather faults only provide a small contribution (~5mm/yr) to the present day GPS velocity field in that region. Our models demonstrate how subduction of the Pacific plate tends to load all the major faults at the margin including the central and eastern segments of the Denali fault and show how the 1964 earthquake and associated postseismic relaxation combined to increase Coulomb stress at the fault segment that ruptured during the 2002 M7.9 Denali earthquake.