Jökull - 01.01.2010, Side 117

Reviewed research article
Mapping of Holocene surface ruptures in the South Iceland
Seismic Zone
Páll Einarsson
Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland
palli@raunvis.hi.is
Abstract — The South Iceland Seismic Zone is a transform zone marking the southern boundary of the Hreppar
microplate. It is the source area of some of the most destructive earthquakes in Iceland’s history. The surface
formations of the zone are ground moraines, alluvial plains and Postglacial lava flows, and show widespread
evidence of Holocene faulting. The fractured area is 15 km wide and 70 km long. A project to map by GPS-
instruments all recognizable Holocene fault structures in this zone is described here. A large majority of
fractures strike NNE to NE and form left-stepping, en echelon arrays with a northerly trend. They are associated
with right-lateral faulting at depth. Right-stepping arrays also exist, apparently associated with faulting on
conjugate faults with ENE strike, but they are an order of magnitude less frequent and mostly of a secondary
nature. Other fault trends also occur, but are rare. Push-up structures are prominent in association with the
en echelon arrays, sometimes reaching heights of several meters. Fractures active during a few of the large,
historical earthquakes in this region have been identified and traced, e.g. the 1630, 1784, 1896, and 1912
events. The fractures are found within narrow, N-S trending zones crossing the seismic zone. Thus the large-
scale, left- lateral transform motion across the plate boundary is accommodated by right-lateral slip on a series
of transverse faults arranged side by side within the zone and by slight rotation of the blocks between them,
a process sometimes called "bookshelf tectonism". Fractures formed during the earthquakes of June 17 and
21 (Mw=6.5) in 2000 and May 29 in 2008 (Mw=6.3) follow this pattern and confirm this general model of
faulting along the transform zone. The size of push-up structures gives a clear indication of relative sizes of the
earthquakes. The push-ups formed in 1630 and 1912 are an order of magnitude larger than the ones formed in
the 2000 and 2008 earthquakes.
INTRODUCTION
The mid-Atlantic plate boundary separating the Eura-
sia and North-America plates crosses Iceland from
the SW at the tip of the Reykjanes Peninsula to the
NE where it joins the Kolbeinsey Ridge north of
Iceland. The crust here is abnormally thick due to
the enhanced magmatism of the Iceland hotspot (e.g.
Bjarnason et al., 1993a, Menke and Levin, 1995).
Furthermore, the movements of the two major plates
relative to the hotspot lead to frequent ridge jumps as
the plate boundary seeks to relocate itself above the
center of the underlying mantle plume (e.g. Einars-
son, 1986, 1991, Pálmason and Sæmundsson, 1974).
The thick crust and the unstable configuration of the
plate boundary makes the structure of the bound-
ary segments more complex in Iceland than else-
where along the Mid-Atlantic Ridge. Most of the seg-
ments are oblique to the plate spreading vector and in
some areas spreading is divided between two or more
branches of the boundary. Each branch of the plate
boundary has its own characteristics with respect to
obliqueness, volcanism, type of faulting and pattern of
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