Geodetic measurements for monitoring rapid crustal uplift in southeastern Alaska caused by the recent deglaciation

Glaciers at high latitudes are considered to be extremely sensitive to climate change and thus monitoring of glaciers is a clue to evaluate the future effect of global warming and the related phenomena. Ice mass changes also produce a time-variable surface load and give us useful data to investigate subsurface structure of the earth, especially to constrain the flow characteristics of the mantle. Larsen et al. [EPSL05] have extensively studied on vertical crustal movement in SE Alaska to reveal the world's fastest glacial isostatic uplifting, which can be attributed to the response associated with deglaciation. Displacement data, however, can only be used to constrain the sum of the elastic response to present-day ice melting (PDIM) and the viscoelastic one to past changes in ice. A Japan-US joint research project, ISEA (International geodetic research project in SouthEast Alaska), was initiated in 2005 to add new geodetic data and to refine the viscoelastic model derived by the previous studies. Absolute gravity data have been acquired at the five sites in the stdudy area using a Micro-g LaCoste absolute gravimeter, FG5#111. At each site data were collected over a 48~62 hour period. The long-term variation in absolute gravity at 2 stations, HNSG and BRM, where the measurements were performed in 1987 by Sasagawa et al. [JGR89] demonstrates rapid gravity decrease with rates of -4.4 micro-gal/yr, and -3.0 micro-gal/yr, respectively, and can be attributed to uplifting and mass-redstribution. ISEA supplements pre-existing continuous GPS (CGPS) stations operated by the U.S. Coast Guard (USCG) and the UNAVCO (Plate Boundary Observatory, PBO) and improves the spatial coverage in and around Glacier Bay. The time series of the site coordinates obtained for Queen Inlet (QUIC), which locates close to a zone of maximum uplift, shows obvious uplifting, even though there are long- term gaps because of an antenna cable trouble in 2006 and power outage in 2008 causing rather obscure temporal variation. Taking the differences between the vertical coordinates in July-October in 2006 and those in 2008, the uplifting rate for this period ranges about 10-20 mm/yr, which is much smaller than the previous study [Larsen et al., EPSL05]. It may be related to heavy snow accumulation in the winter of 2006-2007 around the study area and left for the further investigation.