Jökull - 01.12.1982, Page 118

trending seismic zone. The reason Jor this unusual configur-
ation is poorly understood, but the tentative explanation is
given, that the seismic zone is afracture Jront migrating
southwards in response to a southwards propagation oj the
eastern rift zone oj Iceland.
INTRODUCTION
The South Iceland Seismic Zone is a part of the
Mid-Atlantic plate boundary that crosses the is-
land. It is an E-W trending zone delineated by
destructive, historical earthquakes, that apparently
takes up transform motion between the submarine
Reykjanes Ridge and the Eastern Volcanic Zone in
South Iceland, both of which represent spreading
axes. Features consistent with a transform fault are
the high seismicity, the occurrence of large earth-
quakes, the geometric relatíonship to the spreading
axes, and the typeoffaulting. The zone, on theother
hand, lacks the clear topographic expression char-
acteristic of many oceanic fracture zones, and the
transform motion does not appear to be taken up by
one majorfault.
The South Iceland Seismic Zone has been rela-
tively quiet for the last seven decades, the most
recent destructive earthquake occurred near its
eastern end in 1912. Historical data, however, indi-
cate that destructive earthquakes have occurred at
least 33 times during the last 800 years. Most of
these shocks aífected only limited areas within the
zone, but about once a century a major earthquake
sequence has caused damage along a considerable
part of the 70 km long zone (Einarsson et al. 1981).
The best known of these are the sequences of 1732-
34, 1784, and 1896 (Fig. 1).
Thoroddsen (1899 and 1905) made an extensive
survey of historical data on South Iceland earth-
quakes, and studied the efiects of the 1896 events
himself. Additional studies of historical data were
done by Thorarinsson (Tiyggvason et al. 1958, Thorar-
insson 1967) and S. Björnsson (1975, 1978), who
mapped destruction zones shown in Fig. 1. A pro-
minent and somewhat peculiar feature of the de-
struction zones is that they are elongate in a north-
erly direction, possibly indicating that individual
earthquakes are related to slip along faults striking
transversely to the main earthquake zone. This ob-
servation, among others, provided the incentive to
study available evidence ofsurface faulting during
these earthquakes. Preliminary investígation re-
vealed that earthquake fractures are common
throughout the seismic zone, not only from the most
recent earthquakes, but from older events as well.
Since 1978 student groups at the University of Ice-
land have visited parts of the zone annually to map
faults as a part of a tectonics course. The most
prominent fracture systems in the two easternmost
districts ofthe seismic zone, Land and Rangárvellir,
have now been mapped. This papersummarizes the
main results. More details are given by P. Einarsson
and Eiríksson (1982).
Fig. 2. Map of fractures in the
districts Land and Rangár-
vellir. Major fracture systems
are shown in a simplified way
as bold, unbroken lines.
Mynd 2. Yfirlitskortyfir sprungur
á Landi og Rangárvöllum. Helstu
sprungukerfi eru synd á einfaldan
hátt með heildregnum línum.
114 JÖKULL 32. ÁR