Effects of Present-Day Ice Melting on the Geodetic Measurements in Southeast Alaska

It is known that the southeast Alaska (SE-AK) is undergoing a rapid land uplift, which is considered to be mainly due to the effect of melting of past ice, especially in the last two hundred years after the little ice age (LIA). The crustal deformation caused by the post-glacial rebound (PGR) has been clearly detected by GPS and tidal gauge measurements and modeled (Larsen et al., 2004 and 2005). On the other hand, it is considered also that the observed uplift rate is affected by the present-day ice melting (PDIM), which is considered to be the effect of recent global warming (Larsen et al., 2005; Sato et al., 2006). The displacement measurements provide us useful information to evaluate the ice-melting rate and to discuss the viscosity of the earth. However, usually, it is difficult to separate the uplift rate due to the long- term viscous response of the earth by only using displacement observations, because the two effects (i.e. the elastic and viscous deformations) are mixed in the observed data. Related to this problem, Wahr et al. (1995) demonstrated a method to separate the viscous contribution from the observed data by collocating position and gravity measurements. Considered this, since 2006, we, a joint team of Japanese and U.S. researchers are carrying out the absolute gravity (AG) measurements once a year adding to the temporal and continuous GPS observations in SE-AK. Combining the AG measurements and GPS measurements is useful because the attraction part of gravity measurement is sensitive to a mass change of the present-day ice melting, while the past-ices should have no effect to the attraction part of the observed gravity change. In this context of the discussion, precise numerical estimation of the PDIM effect is important (Sato et al., 2007). Based on the two kinds of DEM (Digital Elevation Model), i.e. one is from the 2000 Shuttle Radar Topography Mission (SRTM) and other is that from air photo dating data which were obtained in the period of 1948-1987, Larsen et al. (2007) estimate the volume changes in SE-AK and adjoining area of Canada. Their results show that the glacier surface elevations lowered by a rate of about -1.1 m/yr as an average over the area of 14,580 square km glacier-covered area. According to the Farrell"fs method (1972) and using this thinning data, we have evaluated the effects of PDIM on our AG and GPS measurements in SE-AK. The gravity effect consists of two parts (i.e. the attraction and the effect of elastic deformation). Different from the elastic part, the computation results for the attraction part are sensitive to the assumed mean thickness of the glaciers and the elevation of glacier mass. Therefore, although the computation is still preliminary one, our results indicate that, the maximum gravity effect for our 6 AG sites is -1.4 micro Gal/yr as sum of the two parts mentioned above. Compared with the observed gravity rate of -5 micro Gal/yr, we may say that the PDIM effect is not negligible. On the other hand, for the vertical displacement, it is estimated at the order of +2.4 mm/yr at the most. Therefore, compared the observed maximum uplift rate of +30 mm/yr, the magnitude of the PDIM effect is not so large.