Long term gravity change and rapid uplifting caused by glacial isostatic adjustment in southeastern Alaska

HR: 1340h
AN: G43B-1198 [Abstracts]
TI: Long term gravity change and rapid uplifting caused by glacial isostatic adjustment in southeastern Alaska
AU: * Miura, S
EM: miura@aob.geophys.tohoku.ac.jp
AF: Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 9808578, Japan
AU: Sato, T
AF: Graduate School of Science, Tohoku University, Aoba-ku, Sendai, 9808578, Japan
AU: Sun, W
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 1130032, Japan
AU: Sugano, T
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 1130032, Japan
AU: Kaufman, A M
AF: Geophysical Institute, Univ. of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, United States
AU: Freymueller, J T
AF: Geophysical Institute, Univ. of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, United States
AU: Fujimoto, H
AF: Geophysical Institute, Univ. of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, United States
AB: 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 by means of raised shorelines, tide gauge measurements, and GPS to reveal the world¡¦s fastest glacial isostatic uplifting, which can be attributed to the response associated with glacier retreat. 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 sets and to refine the viscoelastic model derived by the previous studies. The outline of the project and some results are presented in this paper. In June, 2006, three kinds of field work were carried out. Absolute gravity (AG) surveys were performed at five sites in and around Glacier Bay using a Micro-g LaCoste absolute gravimeter, FG5#111 (Bilham and Sasagawa, EOS94). Gravity tide (GT) observation using a Scintrex¡¦s CG3M gravimeter was started in the campus of University of Alaska, Southeast (Sato et al., this meeting) to give precise corrections for the effect of ocean tide loading, which are the keys to increase the observation accuracy of AG and GPS. New continuous GPS (CGPS) sites were also established to examine not only the secular uplifting but the possible seasonal variation due to snow loading in the winter and ice loss in the summer. In December, 2006, another gravimeter, LaCoste and Romberg G-type #578, for GT observation with an electrostatic feedback circuit to increase stability of the scale factor (Harrison and Sato, JGR84) was installed at the same place of the CG3M gravimeter. In June, 2007, AG measurements were carried out at all sites of the 2006 survey and two other sites: Blanchard River in the Yukon Territory, Canada, where Sasagawa et al. (JGR89) performed AG measurements in 1987, and Fairbanks. We also conducted relative gravity survey using LaCoste-Romberg gravimeter G248 at some benchmarks, whose gravity values were compiled by Rice (1969). Some details of each fieldwork are presented below. We have derived long-term gravity changes at two sites, HNSG and BRM, where Sasagawa et al. (JGR89) conducted AG measurements in 1987. AG values obtained at HNSG in 2006 and 2007 by this project together with the value in 1987 (Sasagawa et al., JGR89) are used to estimate a regression line, which fits every observations very well. This suggests that the gravity at HNSG has decreased almost constantly for the last 20 years with the rate of -5.4ƒÝGal/yr, which is the world¡¦s largest as those caused by glacier retreat. Even though the AG values were obtained at BRM only twice, in 1987 and 2007, they also demonstrate the decreasing rate of - 3.9ƒÝGal/yr, which is consistent with the difference in the uplifting rate at two GPS sites near the AG sites obtained by Larsen et al. (EPSL05).
DE: 1204 Control surveys
DE: 1217 Time variable gravity (7223, 7230)
DE: 1236 Rheology of the lithosphere and mantle (7218, 8160)
DE: 1240 Satellite geodesy: results (6929, 7215, 7230, 7240)
DE: 1244 Standards and absolute measurements
SC: Geodesy [G]
MN: 2007 Fall Meeting