Relation of ongoing deformation rates to the subduction zone process in southern Alaska

The rate and orientation of ongoing strain associated with subduction of the Pacific plate and the accretion of the Yakutat terrane to southern Alaska has been estimated at 13 sites from Global Positioning System measurements made in June 1993 and 1995. Along the Gulf of Alaska coast near Cape Yakataga, the average rate of deformation, relative to Fairbanks, was approximate to 38 mm/yr at N32 degrees W. Further inland, above the region where the dip of the downgoing Pacific plate changes from about 10 degrees to > 30 degrees, the deformation rate was approximate to 12 mm/yr at N26 degrees W. In the Sourdough/Paxson area, the deformation rate drops to 2-5 mm/yr and suggests a low short-term deformation rate across the Denali fault. Elastic straining of the overriding plate due to back-slip on a main thrust zone with an average dip of about 10 degrees can account for the overall rate and distribution of short-term compressional strain across south central Alaska. Above the transitional region between unstable and stable sliding we suggest that strain associated with approximate to 15 mm/yr of right-lateral strike-slip occurs also. If the strain accumulated since the two 1899 earthquakes (both M-W = 8.1) from the offshore Pamplona fault zone to south of the Border Ranges fault (down-dip width approximate to 100 km) was seismically released on a single fault it would correspond to a M = 8.1 earthquake.