On the ratio of the gravity change rate to the uplift rate in Southeast Alaska

From the intensive GPS survey being carried out in Southeast Alaska (SE-AK) by a group of University Alaska Fairbanks (UAF), it is known that SE-AK shows very rapid uplift rates exceeding 30 mm/yr. The origin of rapid uplift rates is considered to be mainly coursed by the GIA (Glacial Isostatic Adjustment) process due to the deglaciation of the past-ices in the little ice age (LIA), which covered the SE-AK area with thickness up to 1.5 km (Larsen et al., 2004 and 2005). Sato et al. (2010) have reevaluated the viscoelastic and elastic responses to the past and present-day ice changes in SE-AK based on 91 GPS data sets by combining the data previously published in 2005 with additional uplift rates compiled recently. The observed uplift rates are affected by the present-day ice melting (PDIM). On the other hand, observation of gravity on the ground is very sensitive to the mass changes at and around the observation site mainly due to the changes in its attraction part. Therefore, combining the gravity data with the displacement data could be important especially in improving the PDIM model, and it is also useful to improve the accuracy of estimation of viscoelastic parameters evaluated from the observed uplift rates. Based on the gravity data obtained at 6 sites in SE-AK with an absolute gravimeter during three years of 2006-2008, we have estimated the ratio of the gravity change rate to the uplift rate in SE-AK. The gravity effects considered here are due to the effects of three ices of the last glacial maximum, the LIA and the present-day. By using the observed gravity and uplift rates corrected for the PDIM effects, we estimated the viscous rate. We will discuss here the results for comparison between the observed ratios and those estimated from GIA modeling. Our results indicate that the obtained ratios are sensitive to the distance between the gravity sites and the GPS sites, because of the rapid spatial changes in the uplift rates in SE-AK.