Jökull - 01.12.1989, Blaðsíða 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.
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JÖKULL, No. 39, 1989 39