Jökull - 01.12.1989, Page 41

is likely to load the adjacent transform zones to the
point of rupture. A recent example of this is pro-
vided by the Krafla rifting episode in North Iceland
that began in 1975. The íirst rifting event triggered a
magnitude 6.5 (Ms) earthquake in the adjacent
transform zone (Einarsson, 1986). Considerable
crustal movements have been measured geodetically
out to a distance of 50 km from the presently active
part of the rift zone (Wendt and others, 1985).
It is perhaps not surprising that the Holt network
exhibits the greatest changes. It encloses the part of
the South Iceland Seismic Zone that has been the
most seismically active in recent years. Several
earthquakes in the magnitude range 3-4 have
occurred here in the last decade. These events are
likely to be associated with local, time-varying
changes in the regional strain field. Strain pulses
resembling "slowquakes” are frequently recorded by
a strain meter at Saurbær within the Holt network
(Stefánsson and Halldórsson, 1988). It is also possi-
ble that the 1980-81 eruptions of Hekla (Grönvold
and others, 1984) had some effect on the long term
strain field in Holt. Strain pulses of the order of
1 ppm were recorded at several strainmeters in
South Iceland during the outbreak of the Hekla
activity in 1980 (Stefánsson and Halldórsson, 1988).
The Hekla Volcano is situated near the junction
between the transform zone and the Eastem Vol-
canic Zone, at a distance of 35 km from Holt.
ACKNOWLEDGEMENTS
The present work began as an exercise for geo-
physics students at the University of Iceland and has
involved a number of individuals. The program of
remeasurements was supported by the Icelandic Sci-
ence Fund. The instruments were kindly provided by
the Nordic Volcanologic Institute. The paper was
much improved in response to the comments of four
reviewers.
REFERENCES
Einarsson, P. 1985. Jarðskjálftaspár (Earthquake
prediction, in Icelandic). Náttúrufræðingurinn 55,
9-28.
Einarsson, P. 1986. Seismicity along the eastem
margin of the North American Plate. In: The
Geology of North America, Vol. M, The Western
North Atlantic Region, edited by P. R. Vogt and
B. E. Tucholke. Geol. Soc. Am., 99-116.
Einarsson, P. 1988. The South Iceland Seismic
Zone; faulting and structure near a sideways
migrating transform (Abstract). Symposium on
Geophysical and Geological Evidence for Seg-
mentation of Continental and Oceanic Rifts,
Woods Hole, Abstract volume, p. 11.
Einarsson, P., S. Bjömsson, G. Foulger, R. Stefáns-
son, and Th. Skaftadóttir. 1981. Seismicity pat-
tem in the South Iceland Seismic Zone. In:
Earthquake Prediction - An International Review,
edited by D. Simpson and P. G. Richards. Am.
Geophys. Union, Maurice Ewing Series 4, 141-
151.
Einarsson, P. and J. Eiríksson. 1982. Earthquake
fractures in the districts Land and Rangárvellir in
the South Iceland Seismic Zone. Jökull 32, 113-
120.
Erlingsson, S. and P. Einarsson. 1985. Fjarlægðar-
mælinar á jarðskjálftasvæði Suðurlands árið 1984
(Distance measurements in the South Iceland
Seismic Zone 1984, in Icelandic). Science Insti-
tute, Report RH-04-85, Reykjavík, 66 pp.
Grönvold, K., G. Larsen, P. Einarsson, S. Thórarins-
son and K. Sæmundsson. 1984. The Hekla Erup-
tion 1980-81. Bull. Volc. 46, 349-363.
Hauksson, E. and J. G. Goddard. 1981. Radon earth-
quake precursor studies in Iceland. ./. Geophys.
Res. 86, 7037-7054.
Meyer, P. S., H. Sigurdsson and J.-G. Schilling.
1985. Petrological and geochemical variations
along Iceland’s neovolcanic zones. J. Geophys.
Res. 90, 10043-10072.
Óskarsson, N., S. Steinthórsson and G. E. Sigvalda-
son. 1985. Iceland geochemical anomaly: Vol-
canotectonics, chemical fractionation and isotope
evolution of the crust. J. Geophys. Res. 90,
10011-10025.
Stefánsson, R. and P. Halldórsson. 1988. Strain
release and strain build-up in the South Iceland
Seismic Zone. Tectonophys. 152, 267-276.
Talwani, M. and O. Eldholm. 1977. Evolution of the
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