Náttúrufræðingurinn - 1974, Blaðsíða 60

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NÁTTÚ RU FRÆÐINGURINN
frá norðaustri til suðvesturs, þvert á áðalsprungustefnuna, þá er
ekki unnt að segja neitt um landsig eða ris annars staðar á Þing-
vallasvæðinu. Það hefði verið æskilegt að mæla á annari línu, sem
lögð hefði verið samsíða gjánum, til að ákvarða ef landsig breytist
í þá stefnu, en vegna kostnaðar var það ekki gert.
HEIMILDARIT - REFERENCES
Kjartansson, Guðmundur, 1964: Aldur nokkurra hrauna á Suðurlandi. Náttúru-
fr., 34: 101—113, Reykjavík.
Thoroddsen, Th., 1925: Die Geschichte der islandischen Vulkane. D. Kgl.
Danske Vidensk. Selsk. Skrifter, Naturvidensk. og Mathem. Afd., 8. Række,
IX: 1-458, Kyíbenhavn.
Tryggvason, Eysteinn, 1968: Measurement of surface deformation in Iceland
by precision levelling. Journal of Geophysical Research, 73: 7039—7050.
Washington.
S U M M A R Y
How fast are Þingvellir subsiding?
hy
Eysteinn Tryggvason
Tbe University of Tulsa, Tulsa, Oklalioma, U.S.A.
Precision lcvelling on a nine kilometer long profile, crossing the Thing-
vellir graben in southwest Iceland, has been conclucted in 1966, 1967, 1969,
ancl 1971. Comparison of the levelling results shows continuous subsidence of
the graben relative to areas outside the graben. Maximum subsidence was
obsérved in the eastern part of the graben, east of Vatnskot, whcre the rate
of subsidence was 0.4 to 0.5 millimeters per year relative to the west side of
Almannagjá. At the east end of the profile no measureable subsidence was
observed but at tlic west end, near Brúsastaðir, tlie land was rising at a rate
of 0.4 to 0.5 millimeters per year relative to the same reference point on the
west side of Almannagjá. No displacement was observed on any of the numer-
ous faults or fissures crossed by the levelling profile. Thus a gentle bending
of the crust indicated and the width of the bended zone is greater than the
length of the levelling profile.
Aclinowledgement
This research was supported by grants GP-5365, GA-987, GA-4112, and GA-
24152 from the National Science Foundation, Washington.