Jökull - 01.12.1976, Side 20

activity. The causaí relationship between the
earthquakes and the end of the eruption is not
clear. The earthquakes were shallow and ap-
peared to be of tectonic origin. A possible cause
of the earthquake activity is a change in the
stress field due to a decreased magma pressure
as the eruption approached its end. An E-W
trend vaguely suggested by the distribution of
epicenters may be related with the zone of large
earthquakes that extends from Hekla westward
across southern Iceland.
ACKN OWLEDGEMENTS
The field work was done while both authors
were employed by the National Energy Au-
thority and was carried out by the authors,
Sigfús Björnsson, Sveinn Magnússon, Olgeir
Kristjónsson and others. The field equipment
was originally purchased with grants from the
National Science Foundation (Grant GP-4815)
and the National Academy of Sciences (Grant
No. 8 of the Arthur L. Day Fund).
The data analysis was done at Lamont-
Doherty Geological Observatory of Columbia
University and was supported by grants from
the National Science Foundation (Grant GA-
29405) and the Icelandic Science Fund.
A computer program written by P. L. Ward
was used for the earthquake locations. Ragnar
Stefánsson kindly allowed us access to the seis-
mogram library of the Meteorological Office in
Reykjavili and provided us with his Preliminary
Seismograph Readings.
REFERENCES
Björnsson, Sveinbjörn and Páll Einarsson. 1974:
Seismicity of Iceland. In Kristjánsson, L.
(Ed.): Geodynamics of Iceland and the
North Atlantic Area, 225—239. D. Reidel
Publ. Co. Dordrecht, Holland.
Decker, R. W., Páll Einarsson and P. A. Mohr.
1971: Rifting in Iceland: New Geodetic
Data, Science 173: 530-533.
Francis, T. J. G. 1968: The detailed seismicity
of mid-ocean ridges, Earth Planet. Sci. Let.
4: 39-46.
Minakami, T. 1960: Fundamental research for
predicting volcanic eruptions, Part 1, Bull.
Earthqu. Res. Inst. 38: 497—544.
Pátmason, Gudmundur. 1971: CrustaÍ structure
of Iceland from explosion seismology, Soc.
Sci. Islandica, Rit 40, 187 pp.
Sigvaldason, Guðmundur E. 1974: The petro-
logy of Hekla and the origin of silicic rocks
in Iceland. The Eruption of Hekla 1947—
1948, 75, 1, Soc. Sci. Islandica, Reykjavik.
Thorarinsson, Sigurdur. 1967: The eruptions of
Hekla in historical times, The Eruption of
Hekla 1947-1948, I, p. 1-170, Soc. Sci. Is-
landica.
— 1970: Hekla, a notorious volcano, Almenna
bókafélagid, Reykjavik, 61 pp. and 48 pl.
— and Gudmundur E. Sigvaldason. 1972: The
Hekla eruption of 1970, Bull. Volc. 35:
1-20.
Tómasson, Haukur. 1971: Um sprungumyndun
í Langöldu, Report (in Icelandic), National
Energy Authority, Reykjavik.
Tryggvason, Eysteinn. 1973: Seismicity, earth-
quake swarms and plate boundaries in the
Iceland region, Bull. Seismol. Soc. Amer.
63: 1327-1348.
Utsu, T. 1971: Aftershocks and earthquake sta-
tistics (III) — Analyses of the distribution
of earthquakes in magnitude, time and
space with special consideration to cluster-
ing characteristics of earthquake occurr-
ence, J. Fac. of Science, Hokkaido Uni-
versity Series VII (Geophys.) 3: 379—441.
Ward, P. L. 1971: New interpretation of the
geology of Iceland, Geol. Soc. Amer. Bull.
82: 2991-3012.
— Gudmundur Pálmason and C. L. Drake.
1969: Microearthquakes and the mid-At-
lantic ridge in Iceland, J. Geophys. Res.
74: 665-684.
— and S. Gregersen. 1973: Comparison of
earthquake location determined with data
from a network of stations and small tri-
partite arrays on Kilauea Volcano, Hawaii,
Bull. Seism. Soc. Amer. 63 : 679—711.
Á G R I P
SKJÁLFTAR OG GOS í HEKLU 1970
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18 JÖKULL 26. AR