TIME-DEPENDENT MODELS OF VERTICAL DEFORMATION FOR THE YELLOWSTONE-HEBGEN LAKE REGION, 1923-1987

Results of repeated levelings have been used to determine the patterns of elevation change associated with the Yellowstone caldera, and apparent viscoelastic rebound following the magnitude 7.3 earthquake near Hebgen Lake in 1959. Models with alternative treatments for nonuniform height change were utilized. Area-wide levelings in 1923, 1975-1977, and 1987 in Yellowstone National Park allowed derivation of uplift velocities for the periods before and after 1976. Maximum uplift velocity for the earlier period was 14 mm/yr, causing 90 cm of height change from 1923 to 1984. Recent annual levelings on the east side of the caldera show a cessation of uplift in 1984-85, and the beginning of rapid subsidence; -19 mm/yr from 1985 to 1989. The long-term uplift correlates closely with the caldera rim. After structuring the leveling observations to be insensitive to the 1959 coseismic deformation (-6 m) at Hebgen Lake, the broader spatial pattern of postseismic rebound was resolved, showing uplift of at least 32 cm. As a group, the relevelings converging at nearby West Yellowstone were accomplished at various times, which allowed selection of a best model to fit the temporal character of postseismic motion. The preferred model has a 2-year decay period indicating that uplift stopped by 1970. Supplementary indications of the uplift near Hebgen Lake were obtained by comparing Global Positioning System data to earlier levelings, facilitated by a new precise geoid by D. G. Milbert and W. T. Dewhurst (submitted manuscript, 1990).