Satellite Laser Ranging Observations of Unsteady Tectonic Motion

Plate velocity models derived from space techniques are based on the assumption that the velocity is uniform during the observation period. The actual motion of individual sites, however, can exhibit deviations in velocity that may be of tectonic interest. As a means of identifying nonsteady motion, we utilize an analysis method that directly determines monthly estimates of three-dimensional Satellite Laser Ranging (SLR) station locations from observations of the LAGEOS orbit. A minimum constraint on the motion is applied to a pair of well- determined tracking stations at Greenbelt in Maryland and Maui in Hawaii. The time-averaged horizontal components of station velocities are used to compute relative rotation poles, and these may be directly compared to those from prevailing geophysical models, which are based on million-year time scales. The deviations from this time-averaged model can also be examined for evidence of non-steady behavior over the nearly 15 years of SLR observations, and this can be found at some stations along plate boundaries. In particular, at the station Monument Peak, located close to the San Andreas fault system, there is a difference of about 5 mm/yr between the pre-1991 and post-1991 velocity averages that is well above the formal uncertainty. The NNE direction of this difference could be interpreted as shortening across the San Andreas fault system. If a tectonic signal can be confirmed by comparison with other observables in the region collected over the same time span, it would demonstrate the value of these data in constraining the long-term variations in plate boundary deformation. The deviation from time- averaged motion at each station also allows us to bound the formal error estimates of the uniform velocity model, and to assess any preferred direction for significant variations in the motion.