Jökull - 01.01.2010, Síða 120

P. Einarsson
FORMER INVESTIGATIONS
Thoroddsen (1899, 1905) collected written documen-
tation of historic earthquakes in Iceland and described
the effects of the 1896 earthquakes, the latest se-
quence in the SISZ before the current episode of
2000–2008. His work contains a wealth of informa-
tion, including numerous descriptions of surface frac-
tures associated with the earlier earthquakes, in many
cases detailed enough to make them identifiable in the
field today. T. Einarsson (1967, 1968) observed and
mapped fractures in two parts of the zone and con-
cluded correctly that they were the result of strike-slip
faulting although he failed to see their significance
within the larger tectonic framework of Iceland.
Einarsson and Eiríksson (1982a,b) reported on the
first results of a systematic effort to map all the rec-
ognizable surface fractures of the seismic zone. The
main faults of the eastern part of the zone were iden-
tified and their structural features described. Further
work was reported by Einarsson et al. (1981).
Bjarnason et al. (1993) studied the fracture system
of the 1912 earthquake in more detail than had been
done earlier by Einarsson and Eiríksson. Further work
on this fracture system concentrating on the push-up
structures was done by Angelier et al. (2004).
Erlendsson and Einarsson (1996) demonstrated
that the system of transverse and parallel strike-slip
faults extended westward along the plate boundary,
beyond the Hengill triple junction. Subsequent work,
e.g. by Clifton et al. (2003), Clifton and Kattenhorn
(2007), Árnadóttir et al. (2004), and Einarsson (2008),
has shown that plate movements across the oblique rift
of Reykjanes Peninsula are partly taken up by book-
shelf faulting.
A group led by J. Angelier and F. Bergerat has
conducted several studies of fractures in the SISZ and
their tectonic significance (e.g. Angelier and Bergerat
(2002), Angelier et al. (2004, 2008), Bergerat and An-
gelier (2000, 2003), Bergerat et al. (1998, 2003).
The importance of recent surface fracturing for
planning and the assessment of seismic hazard has
been recognised and several studies have been made
at the request of the Selfoss municipality (Imsland and
Einarsson, 1995, Imsland et al., 1997, 1998a,b). Fur-
thermore, Þjórsá river, a major source of hydropower
in Iceland, flows across the South Iceland Seismic
Zone. Several power projects are planned on this
river in the near future, requiring detailed information
about the location of active faults and their hydrolog-
ical properties. Several studies have been conducted
to this end in the seismic zone (e.g. Einarsson et al.,
2002, Khodayar and Einarsson, 2002, Khodayar et al.,
2007a,b, 2008). Similarly, the importance of the ac-
tive fractures in conducting hot water to feed geother-
mal areas has been demonstrated (e.g. Khodayar et al.,
2004, 2010).
METHODS
In this paper I summarise the results of a system-
atic effort that began in 1977 to map surface ruptures
within the South Iceland Seismic Zone. Most of these
structures are exposed in Holocene formations and
have therefore been active in the last 10 000 years.
The area was used as a training ground for students
in courses at the University of Iceland, s.a. Tectonics,
to identify active faults and map structures at a large
scale in the field. Field trips were made to the area ev-
ery year since 1977 with groups of 5–20 students, cov-
ering new areas and fracture systems every time. In
the beginning the tools were aerial photographs, tape
measure and compass. After about 1995 differential
GPS-receivers were used. All structures were located
and mapped by walking along and around them. Ac-
curate GPS- maps at a resolution of less than 1 m were
made of all the structures, including the areas that had
been mapped previously by hand.
The usual procedure was to study available aerial
photographs of the area to identify potential fractures
and fault structures. Then the potential sites were vis-
ited and fractures and fracture systems were identi-
fied in the field and traced further. The field inves-
tigation usually revealed a much more extensive sys-
tem than was visible on the photographs. The oppo-
site also happened, that long, linear structures seen
on the photographs turned out to be old bridle paths,
ditches, wind erosion streaks etc. Interviews with the
local inhabitants were frequently useful. They often
had knowledge of fault structures when they were de-
scribed to them, e.g. sinkholes in the fields where trac-
tor wheels tended to get stuck repeatedly, pits where
120 JÖKULL No. 60