The crustal uplift determined at the Jakobshavn glacier (West Greenland) using ATM and GPS data

The Greenland ice sheet has experienced record melting in recent years. In order to estimate the ice loss we can make use of the earth's natural elasticity to weigh the ice. Ice bends down the bedrock so when the ice melts away, the bedrock rises measurably in response. Throughout this abstract we present both a predicted and observed crustal uplift for the Jakobshavn glacier using ATM data (Airborne Topographic Mapper) from NASA ATM flights during 1997, 2005 and 2010 supplemented with data provided from continuous Global Positioning System (GPS), measurements made on bedrock between 2005-2010. In order to compute the crustal uplift in response to the ice mass loss of the Jakobshavn area from the GPS stations, the convolution of the gridded thinning rates has been computed with the vertical-displacement Green's function as described in [1]. Several manipulations of data were required in order to achieve a good prediction of the crustal uplift. In this sense the programs Matlab and Geogrid-Gravsoft were used along with some Fortran executable files. Furthermore, the GPS data which presents the difference in uplift is provided processed as a difference of data from the permanent GPS stations KAGA, ILUL and QEQE relative to the AASI station (Figure 1). Also, in order to compare the predicted uplift from ATM data with the observed uplift from GPS data the post-glacial rebound (PGR) rates have been subtracted. The results obtained for the predicted crustal uplift for KAAS is 11.62 mm/yr while the observed value was 16.321 mm/yr, for ILAS 1.74 mm/yr and 1.53 mm/yr, for QEAS -0.189 mm/yr and 1.15 mm/yr. That being a difference of 4.701 mm/yr is found for KAAS, 0.21 mm/yr for ILAS and 1.339 mm/yr for QEAS. The uncertainties associated both with the ATM and GPS results are 0.8 mm/yr for ATM and 0.5 mm/yr for GPS. The total ice mass loss in km3 of water predicted from the ATM data concerning the Jakobshavn area is -88.815 between 2010 and 2005 and -83.599 between 2005 and 1997. It seems fair to state that this differences, between the predicted and observed rates, may also be due to the fact that not all the errors have been taken into account when computing the observed results and also due to the fact that, perhaps, ice is melting in Greenland much faster than predicted. REFERENCES Farrell, W. E. (1972), Deformation of the Earth by surface loads, Rev. Geophys., 10(3), 761– 797, doi:10.1029/RG010i003p0076

Seasonal dynamic thinning at Helheim Glacier

AbstractWe investigate three annual mass-balance cycles on Helheim Glacier in south-east Greenland using TanDEM-X interferometric digital elevation models (DEMs), bedrock GPS measurements, and ice velocity from feature-tracking. The DEMs exhibit seasonal surface elevation cycles at elevations up to 800 m.a.s.l. with amplitudes of up to 19 m, from a maximum in July to a minimum in October or November, concentrated on the fast-flowing areas of the glacier indicating that the elevation changes have a mostly dynamic origin. By modelling the detrended bedrock loading/unloading signal we estimate a mean density for the loss of 671±70 kgm−3 and calculate that total water equivalent volume loss from the active part of the glacier (surface flow speeds >1 m day−1) ranges from 0.5 km3 in 2011 to 1.6 km3 in 2013. A rough ice-flux divergence analysis shows that at lower elevations (<200 m) mass loss by dynamic thinning fully explains seasonal elevation changes. In addition, surface elevations decrease by a greater amount than field observations of surface ablation or surface-energy-balance modelling predict, emphasising the dynamic nature of the mass loss. We conclude, on the basis of ice-front position observations through the time series, that melt-induced acceleration is most likely the main driver of the seasonal dynamic thinning, as opposed to changes triggered by retreat

GPS-analyse af jordskælvet - flyttede Sumatra sig virkelig 36 meter?

Anden juledag 2004 blev Syd&oslash;stasien&nbsp;rystet af et kraftigt jordsk&aelig;lv,&nbsp;der m&aring;lte Mw = 9,3 p&aring; Richterskalaen.&nbsp;Jordsk&aelig;lvet er det n&aelig;stkraftigste&nbsp;registreret siden 1900.&nbsp;Jordsk&aelig;lvet fandt sted ca. 100 km&nbsp;vest for det nordlige Sumatra, hvor&nbsp;den relativt tynde Indo-Australske&nbsp;lithosf&aelig;replade bev&aelig;ger sig ind under&nbsp;den noget tykkere Burmesiske&nbsp;plade