Continuous uplift near the seaward edge of the Prince William Sound megathrust: Middleton Island, Alaska

Middleton Island, located at the seaward edge of the continental shelf 50 km from the base of the inner wall of the Aleutian Trench, affords an opportunity to make land-based measurements of uplift near the toe of the Prince William Sound megathrust, site of the 1964, M = 9.2, Alaska earthquake. Leveling surveys (1973-1993) on Middleton Island indicate roughly uniform tilting (similar to 1 mu rad/a down to the northwest) of the island, and GPS surveys (1993-2012) show an uplift rate of 14 mm/a of the island relative to fixed North America. The data are consistent with a combined (coseismic and postseismic) uplift (in meters) due to the 1964 earthquake as a function of time tau (years after the earthquake) u(tau) = (3.5 + 1.21 log(10)[1 + 1.67 tau]) H(tau) where 3.5 is the coseismic uplift and H(tau) is 0 for tau < 0 and 1 otherwise. The current uplift on Middleton Island is attributed to continuous slip on a fault splaying off from the megathrust, and the long-term uplift is the superposition of the effects of past earthquakes, each earthquake being similar to the 1964 event. Then, the predicted uplift at time t due to a sequence of earthquakes at times t(i) would be Sigma(i) u(t - t(i)). From studies of strandlines associated with the uplifted terraces on Middleton Island, Plafker et al. (1992) estimated the occurrence times of the last six earthquakes and measured the present-day elevations of those strandlines. The predicted uplift is in rough agreement with those measurements. About half of the predicted uplift is due to postseismic relaxation from previous earthquakes.