Crustal Deformation In Alaska Measured Using The Global Positioning System

Abstract

Repeat observations using the Global Positioning System at sites on the Earth's surface enable the velocity of those sites to be estimated. These velocity estimates can be used to model the processes of the crust's deformation by faulting and folding. The focus of this study is crustal deformation in Alaska and in particular the region of interior Alaska within 300km of Fairbanks, including the Denali fault; the Fairweather fault and Yakutat block in southern Alaska; and the Semidi region of the Aleutian arc. This deformation is driven by the relentless northwestward motion of the Pacific plate relative to North America. The Yakutat block, an allocthonous terrane located in the 'armpit' of southern Alaska is shown to be moving at neither the Pacific Plate rate nor is it attached to North America. Instead it has a velocity parallel to the Fairweather fault, which means that some offshore structure, possibly the Transition Zone, must accommodate some of the Pacific-North American relative motion. The slip on the Fairweather fault is estimated to be 44 +/- 3 mm/yr with a locking depth of 8 +/- 1 km, which implies a recurrence time of ~80 years for an MS 7.9 earthquake. Using a model of southern Alaska block rotation with the Denali fault as the northern boundary, the slip rate on the McKinley segment of the Denali fault is estimated to be ~6--9 mm/yr for a locking depth of 12 km. Moving to the southwest, data from sites in the Semidi segment of the Alaska subduction zone, between the fully-coupled segment to the northeast and the slipping Shumagin segment to the southwest are studied. This region, which sustained a magnitude 8.2 earthquake in 1938, is determined to be highly coupled and accumulating strain. Finally, all of these pieces are connected in a quantitative model for southern Alaska. This model involves three crustal blocks, the Yakutat block, Fairweather block and southern Alaska block, which lie between North America and the Pacific plate and move relative to these major plates.