Jökull - 01.12.1994, Side 38

(International Earth Rotation Service (IERS), 1993), a
few tens of meters from our GPS bench mark. The re-
sults, given in the geocentric coordinates ITRF92
(IERS Terrestrial Reference Frame) are X =
2679650.361 m, Y = -727916.552 m, Z =
5722807.152 m with standard deviations of 0.011 m,
0.006 m and 0.020 m, respectively. It is not known
when this point can be reoccupied. A reoccupation in
10 years should resolve the uplift rate to ± 2 mm/yr.
Existing levelling profiles around Vatnajökull in-
clude a line of the Road Department along the main
road in Homafjörður and profiles within the neovol-
canic zone SW of Vatnajökull, surveyed by the Na-
tional Energy Authority (NEA) in 1959. The Horna-
fjörður line was remeasured and augmented in 1992
by a group from the Technical University of Stock-
holm (Lotfinia et al. 1993). The points on the shore of
Lake Langisjór used by Sigmundsson and Einarsson
(1992) to detect tilt changes at the SW-edge of Vatna-
jökull are a part of the NEA levelling network, but
other parts of it have not been relevelled.
DISCUSSION
The measurements reported here are intended to
give a base for future measurements of crustal move-
ments. The main geodynamic process expected to
lead to crustal movements in this intraplate region is
isostatic adjustment to changing ice load of the Vatna-
jökull glacier. The movements are mainly vertical and
the expected rates are of the order of 5-15 mm per
year. The repeatability of the 1991 GPS measure-
ments are around 20 mm in the vertical component.
Repeatabilities better than 10 mm have been obtained
in similar measurements in 1992 and 1993 (Sig-
mundsson et al., 1994; Camitz et al., 1994). Remea-
surements of the GPS network near Vatnajökull may
therefore be expected to give significant results in 5-
Table 4. Gravity observations
4. tafla. Þyngdarmœlingar
1) Gravity difference (epoch 1992.6)
relative to value at Pt 701 as mea-
sured at location of instrument, i.e.
not reduced to height of benchmark.
2) Absolute value of gravity at location
of benchmark; reference is Pt 5273
(Djúpivogur Hotel) measured in
1968 (Thorbergsson et ai, 1990)
and assumed to have remained sta-
ble.
3) Approximate height of gravity sen-
sor above benchmark (bolt or nail).
The sensor is assumed to have been
8.5 ±0.5 cm above the plate edge. g
and Ag are given for this height and
can be approximately reduced to
benchmark height with the normal
free air gradient (-308.55 pGal/m).
Consider however, that the true gra-
dient may locally differ consider-
ably from its normal value.
4) Numbers mean: fraction of degree
north of 64°N and degree and frac-
tion west of-14°E.
PNr Agu gz> mGal 982 000 + Ah2) cm Approx. location Lat-64°N Lon+14° Name E4>
701 0 271.843 13. .297 -1.201 Bergárbrú
601 +2.735 274.578 14.5 .283 -1.066
603 -2.286 269.557 14.5 .290 -1.101
604 -0.595 271.248 13. .288 -1.149 Hagi
605 -2.215 269.628 12. .288 -1.170 Skarðshólar
609 -1.654 270.189 13.5 .290 -1.185 Vegamót 99/1
702 -2.187 269.656 13. .304 -1.217 Seljavellir
802 -0.629 271.214 14. .316 -1.240 Fornustekkar
806 -7.498 264.345 14.5 .335 -1.244 Skriðuhóll
810 -7.455 264.388 11. .324 -1.251 Bjarnanes
1000 -6.813 265.030 -110. .267 -1.198 HOFN, Höfn
1001 -8.108 263.735 12. .328 -1.251 STAP, Stapaklettur
1002 -29.177 242.666 14.5 .418 -1.394 HOFF, Hoffellsjökull
1004 -11.892 259.951 14. .359 -1.347 GILD, Gildrasker
1005 -18.453 253.390 13.5 .265 -1.515 Hólmsárbrú
1006 -32.933 238.910 14. .304 -1.656 HEIN, Heinar
1007 -22.435 249.408 14.5 .234 -1.493 FLAT, Flatey
1008 -29.326 242.517 12.5 .208 -1.723 UPPS, Uppsalir
1009 -24.622 247.221 13. .118 -2.076 HROL, Hrollaugshólar
1010 +12.806 284.649 13.5 .242 -0.979 Stokksnes
1011 -3.434 268.409 12. .423 -0.881 Stafafell
1012 +20.222 292.065 12. .403 -1.542 HVAL, Hvalnes
1013 -11.235 260.608 15.5 .55 -0.62 Hofsárbrú
5219 -1.062 270.781 14. .305 -1.237 Hornafjörður Airport, old
5273 +12.907 284.750 15.5 .657 -0.280 Djúpivogur Hotel
7307 -0.476 271.367 14. .303 -1.223 Hornafjörður Airport, new
8436 +11.248 283.091 11.2 .647 -0.284 DJUP, Djúpivogur
9004 -3.499 268.344 -107. .294 -1.185 Hólar
9012 -1.763 270.080 -102.5 .288 -1.132 Dynjandi
36
JÖKULL, No. 44