Jökull - 01.12.1994, Qupperneq 27

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Time (hours)
Fig. 5 Predicted variations in vertical point posi-
tions relative to point Núpafjall for regional
points in the SISZ and on the Reykjanes peninsu-
la due to the ocean loading effect, and the ob-
served variation. The calculated results are from
ionosphere-free, ambiguity-free solutions for
two-hour discrete data windows. Error bars
shown are lo unscaled formal errors.
5. mynd. Reglulegirferlar: útreiknuð frávik íhæð
einstakra mœlipunkta miðað við Núpafjall, efþau
stafa eingöngu af áhrifum sjávarfalla. Oreglu-
legir ferlar: tveggja klukkustunda meðaltals-
frávik GPS-mœlinganna, með óvissu (lóðrétt
strik).
lated point coordinates are sub-centimetre for the
horizontal components and approximately 1 cm in
the vertical. Point repeatabilities after network ad-
justment were sub-centimetre in the horizontal
and mostly sub-centimetre in the vertical.
3. Ocean loading effects on relative coordinates for
lines up to 75 km long in South Iceland are an
order of magnitude smaller than those of other
error sources for typical GPS surveys such as
ours. The ocean loading effect therefore does not
need to be modelled for such surveys. The impor-
tance of this effect would increase, however, for
longer lines and surveys of higher accuracy.
ACKNOWLEDGMENTS
We would like to thank Benedikt Halldórsson,
Bruce Julian, Kate Julian, Rex Allen, Mary O'Neil,
Guðmundur Baldursson and Guðjón Sigurðsson,
without whose help the data could not have been col-
lected. Gunnar Thorbergsson, Orkustofnun, supplied
the new ground control points. Kosuke Heki assisted
with the processing of the data as did Michelle Hofton
and Simon Williams. NERC provided the field equip-
ment (loan 338/1290) and the fieldwork wa's funded
by NERC grant GR9/134 and a USGS Gilbert Fel-
lowship. KMH was funded by a Ph.D. studentship
from the Dept. Education Northem Ireland.
REFERENCES
Björnsson, A., K. Saemundsson, P. Einarsson, E.
Tryggvason and K. Gronvold 1977. Current rifting
episode in North Iceland. Nature 266. 318-323.
Einarsson, P. 1988. On the propagation velocity of the
eastem rift zone in Iceland. Paper presented at the
Symposium on volcanic activity in Iceland. Reykja-
vik, Iceland.
Farrel, W. E. 1972. Deformation of the earth by surface
loads. Rev. Geophys. 10. 761-797.
Foulger, G.R., C-H Jahn, G. Seeber, P. Einarsson, B.R.
Julian and K. Heki 1992. Post rifting stress relax-
ation at the accretionary plate boundary in Iceland,
measured using the Global Positioning System.
Nature 358. 488-490.
Foulger, G., G. Beutler, R. Bilham, P. Einarsson, S.
Fankhauser, W. Gurtner, U. Hugentobler, W. J.
Morgan, M. Rothacher, G. Thorbergsson, and U.
Wild 1993. The Iceland 1986 GPS geodetic survey:
Tectonic goals and data processing results. Bull.
JÖKULL, No. 44
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