Gravity and uplift rates observed in southeast Alaska and their comparison with GIA model predictions

Over 3 years from 2006 to 2008 we conducted absolute gravity (AG) measurements at 6 sites in and around Glacier Bay (GB) in Southeast Alaska (SE-AK). At two of the 6 sites, AG measurements had been carried out in 1987 by a group from IGPP at UCSD. Mean gravity change rates (unit: μGal/yr, 1 μGal = 10^-8 ms^-2) over the 6 sites are estimated to be -4.50 ± 0.76 and -4.30 μ 0.92 by only using our data and also using the 1987 data, respectively. We computed the uplift and gravity rates predicted by ice load models for three different time intervals: Last Glacial Maximum (LGM), Little Ice Age (LIA) and Present-Day (PD). Except for 1-2 examples, the predictions recover the observed rates within the observation errors. We also estimated the viscous portion of the ratio (unit: μGal/mm) of the observed gravity rate to the uplift rate by correcting for the effects of the Present-Day Ice Mass Change (PDIMC). Two PDIMC models are compared, which are called here as UAF05 and UAF07. Mean ratios are estimated to be -0.205 ± 0.089 and -0.183 ± 0.052 for the cases using UAF05 and UAF07, respectively. The predicted mean ratios are -0.166 ± 0.001 and -0.171 ± 0.002 for the cases using both the LGA and LIA ice models and only using the LIA ice model, respectively. We have confirmed that our AG and GPS observations detect the rates and ratios reflecting an early stage of viscoelastic relaxation mainly due to the unloading effects after the LIA.