TIDE-GAUGE MEASUREMENTS OF UPLIFT ALONG THE SOUTH COAST OF ALASKA

Annual mean sea levels along the south coast of Alaska are used to measure uplift along the Alaska-Aleutian subduction zone. Oceanographic effects are removed from the observed annual mean sea levels by subtracting a correction that is proportional to the sea level fluctuations observed in southeast Alaska. That correction is effective in reducing fluctuations in the observed, annual mean sea level as far west as the tip of Alaska peninsula. Additional corrections to remove the eustatic rise in sea level and the apparent fall in sea level due to postglacial isostatic rebound of the land are introduced. This corrected sea level record should provide a measure of tectonic subsidence. In the area affected by the 1964 Alaska earthquake, postseismic uplift occurs where coseismic subsidence was observed, and postseismic subsidence occurs where coseismic uplift was observed. The immediate postseismic response is damped out within the first decade, and the subsequent uplift rates appear to be steady over the 1974-1989 interval. However, some of those rates seem to be too high to be sustained over the approximately 1000 year earthquake recurrence interval appropriate to this area if the interseismic deformation is only to recover the coseismic displacement. Thus a long-term (approximately 100 years) relaxation in uplift rates is postulated. The immediate (time constant approximately 5 years) postseismic relaxation is attributed to postseismic slip on the plate interface directly downdip from the coseismic rupture. The long-term (time constant approximately 100 years) relaxation is attributed to flow in the asthenosphere.