Vertical velocities

Figure 28 shows the observed vertical velocities at the GPS stations as in Table 6, where the effects of radome installation and equipment changes have been removed. The June 2000 earthquakes did not affect the vertical positions of the stations so the vertical velocities in Tables 6 and 7 are identical. The vertical velocities in Table 8, obtained using data from August 2001 to December 2001, are generally slightly lower than in Table 6, obtained using the whole data set. The largest differences are observed at HLID (11.5 $\pm$ 0.7 mm/yr and 6 $\pm$ 2 mm/yr for Tables 6 and 8 respectively). The vertical velocity at SKRO is not reliable, since the data may be affected by local disturbances not originating in the crust (Section 2).

All stations with significant vertical velocities are moving up relative to REYK. This is supported by results from International Data Centers which include REYK and HOFN in their routine processing [SOPAC (2002)], [MIT (2002)], [JPL (2001)]. The study by [Sella et al. (2002)] reports vertical velocities of -3.4 mm/yr $\pm$ 1.5 mm/yr for REYK and 4.0 mm/yr $\pm$ 2.3 mm/yr for HOFN. A tide gauge record in Reykjavík shows a sea level rise of 2.4 to 3.4 mm/yr [Einarsson (1994)], similar to the global eustatic sea level rise. REYK is sited on the top of an elevator shaft in a three stories high building in the University of Iceland. The building was constructed in the 1970's and it is possible that the building is still moving slightly, although it is unlikely (S. Erlingsson, personal communication 2000).

  

Figure 28: Calculated vertical velocities (in mm/yr) for the permanent GPS stations, as in Table 6, assuming REYK is fixed. Confidence limits are scaled by a factor 2 from Table 6, shown with grey bars around the arrow heads. The vertical velocity at SKRO is possibly disturbed by local movement of the monument.

HOFN is sited just on the SE side of the retreating Vatnajökull glacier (Figure 28). Previous observations and model calculations suggest a present crustal uplift rate of 5-15 mm/yr in the area around the glacier [Sigmundsson (1990)], [Sigmundsson et al. (1992)]. Thus the uplift rate observed at HOFN can be explained by the retreating and thinning of Vatnajökull glacier. There are no retreating glaciers near the stations in the Hengill area and still it is evident from Figure 28 and Tables 6 and 8 that the stations in the Hengill area are moving up relative to REYK at rates not much lower than at HOFN. The time series (Figures 8 to 20) show that the vertical rates seem to be fairly constant except at THEY, SOHO and HVOL where there is a period of approximately 1 year with amplitude 5 mm to 15 mm in the data. The stations THEY, SOHO and HVOL move down during mid-winter to mid-summer and they move up during mid-summer to mid-winter. These could be signs of annual glacial loading, but a longer time series and more detailed studies are needed to verify that.