Jökull - 01.01.2010, Blaðsíða 118

P. Einarsson
fault systems (Einarsson, 2008). In this paper the tec-
tonic fault pattern in one of these branches, the South
Iceland Seismic Zone (Figure 1), will be described.
This branch is a transform zone and takes up the trans-
form motion between the oblique Reykjanes Penin-
sula rift and the Eastern Volcanic Zone. A long-term
project has been conducted in this zone since 1977 to
map all surface faulting structures. This paper is based
on results of that project.
TECTONIC FRAMEWORK OF THE
SOUTH ICELAND SEISMIC ZONE
The complexity of the Icelandic plate boundary is par-
ticularly apparent in South Iceland where a microplate
has been defined between two sub-parallel rift zones,
the Western and the Eastern Volcanic Zones (Einars-
son, 2008, Sinton et al., 2005, Einarsson et al., 2006).
This microplate, the Hreppar Microplate, is consid-
ered to have formed when the Eastern Volcanic Zone
began propagating away from the center of the hotspot
at about 3 Ma. This crustal block is gradually be-
ing transferred from the Eurasia Plate to the North
America Plate as the Eastern Volcanic Zone takes over
from the Western Volcanic Zone as the main rift in
South Iceland. The spreading rates across the Western
and Eastern Volcanic Zones are estimated from GPS-
measurements to be 1–5 mm/a and 14–19 mm/a, re-
spectively, varying along the zones (LaFemina et al.,
2005). The rate increases southwards along the west-
ern zone but decreases southwards along the eastern
zone. This leads to rotation of the microplate with the
pole of rotation slightly north of the end of the West-
ern Volcanic Zone in West-central Iceland (Einarsson,
2008). The total spreading rate between the two ma-
jor plates is consistent with the rate estimated from the
pole of rotation of the Nuvel-1A model of DeMets et
al. (1994) for the Eurasia and North America Plates,
about 19 mm/a (Geirsson et al., 2006, Árnadóttir et
al., 2009).
The South Iceland Seismic Zone is the source area
of most destructive earthquakes in Historic Time in
Iceland (Einarsson et al., 1981, Tryggvason et al.,
1958, Thoroddsen, 1899, 1905). This zone is a 70 km
long branch of the plate boundary, has an E-W ori-
entation and separates the Hreppar Microplate from
the Eurasia Plate. The relative plate movement across
the zone is left-lateral transcurrent motion, at a rate
of about 15 mm/a. Volcanism in this zone is mini-
mal so one must assume only a small amount of di-
vergence across the zone. The only expression of
recent volcanic activity is the late Pleistocene activ-
ity at Hestfjall (Kjartansson, 1943) and the Gríms-
nes volcanic system, a group of monogenetic lava and
cinder cones at the northern boundary of the zone
(Jakobsson, 1966, Sinton et al., 2005) from the early
Holocene.
The South Iceland Seismic Zone as defined by
destruction areas of historical earthquakes, Holocene
surface ruptures and instrumentally determined epi-
centers (Einarsson et al., 1981, Stefánsson et al.,
1993) is 10–15 km wide. Destruction areas of in-
dividual earthquakes and surface faulting indicated
rather early on that each event is associated with fault-
ing on N-S striking planes, perpendicular to the main
zone (Einarsson et al., 1981, Einarsson and Eiríks-
son, 1982a,b). The over-all left-lateral transform mo-
tion along the zone, i.e. between the Hreppar Mi-
croplate to the north and the Eurasia Plate to the
south, thus appears to be accommodated by right-
lateral faulting on many parallel, transverse faults
and counter-clockwise rotation of the blocks between
them, "bookshelf faulting" (Einarsson et al., 1981,
Sigmundsson et al., 1995).
Earthquakes in South Iceland tend to occur in ma-
jor sequences in which most of the zone is affected.
These sequences last from a few days to a few years.
Each sequence typically begins with a magnitude 6.5–
7 event in the eastern part of the zone, followed by
similar or smaller events farther west. Sequences of
this type occurred in 1896, 1784, 1732–1734, 1630–
1633, 1389–1391, 1339 and 1294. Apart from the his-
toric gap between 1391 and 1630, the sequences thus
occur at intervals that range between 45 and 112 years
(Einarsson et al., 1981), and it has been argued that a
complete stress release of the whole zone is accom-
plished in about 140 years (Stefánsson and Halldórs-
son, 1988). The long time since the last sequence led
to a long-term forecast published in 1985 (Einarsson,
1985), later refined by Stefánsson et al. (1993), of a
118 JÖKULL No. 60