Measurement of horizontal motions in Alaska using very long baseline interferometry

Abstract

We have analyzed 6 years of very long baseline interferometry (VLBI) data beginning in 1984 from a network of sites in Alaska, the Yukon, and the conterminous United States. The analysis of these 53 data sets includes constrained, continuous, piecewise‐linear estimation of the site atmospheres and clocks; application of values and correlations from a self‐consistent Earth orientation series derived from other VLBI data; and simultaneous estimation of site positions and velocities. From a solution that includes 21,007 observations with a fit of 40 ps in delay and 64 fs/s in delay rate, we have determined the horizontal velocities of seven sites with precisions of 0.5–2 mm/yr (1σ standard deviation). The three VLBI sites within the Alaska‐Aleutian subduction boundary zone (Yakataga, Kodiak, and Sand Point) have higher instantaneous velocities relative to eastern North America than the interior sites of Alaska (Fairbanks and Sourdough). The estimated velocity of Yakataga relative to Fairbanks is 37.8±1.3 mm/yr at N27°W±2°. Modeling of the Yakataga data suggests that the observed motion is the result of elastic straining of the overriding plate due to a locked main thrust zone with a component of oblique slip. The estimated velocity of Kodiak relative to Fairbanks is 13.6±0.8 mm/yr at N41°W±3°; this result is lower than the rate predicted on the basis of simple elastic dislocation models. The estimated velocity of Sand Point relative to Fairbanks is 6.4±0.9 mm/yr at N65°W±8°. The component of this velocity in the plate direction, 3.5±1.1 mm/yr, is not consistent with either an aseismic slip model (i.e., no strain accumulation in the overriding plate) or a model in which the site is undergoing compressional strain (10.9 mm/yr at ∼N28°W) due to simple subduction at the predicted orientation of Pacific plate convergence. In a North America‐fixed reference frame, the estimated velocities of Fairbanks (1.5±0.5 mm/yr at S12°E±20°) and Sourdough (3.9±1.0 mm/yr at S13°W±13°) are both southward. These results may imply motion of this region relative to stable North America, but more observations are required to reduce the uncertainties in the azimuths. The time evolution of the estimated site positions from VLBI in Alaska are, in general, consistent with constant velocity. However, in 1988 the Yakataga site showed a significant departure from the position predicted from a constant‐velocity model obtained from the previously estimated position. This departure may be static displacement associated with two major strike‐slip earthquakes that occurred in the Gulf of Alaska between the 1987 and 1988 measurement sessions.