Jökull - 01.12.1989, Síða 34

Distance Changes
in the South Iceland Seismic Zone 1977-1984
SIGURÐUR ERLINGSSON*
and
PÁLL EINARSSON
Science Institute,
University oflceland, Reykjavík
ABSTRACT
A sequence of large earthquakes is expected with
high probability in the South Iceland Seismic Zone
within the next two decades. In response to this
long-term prediction, 19 geodimeter lines were
installed in 1977-1981. Remeasurements in
1983-1984 show that measurable crustal deforma-
tion has occurred. The distance changes are irregu-
lar, however, and are not consistent with a simple
model of gradual accumulation of strain along a
transform type plate boundary. Strain build-up is
apparently not a steady-state process in this seismic
zone. It may even be discontinuous and strongly
affected by external events such as rifting and mag-
matic activity in the volcanic rift zones.
INTRODU CTION
The South Iceland Seismic Zone is a part of the
Mid-Atlantic plate boundary that crosses Iceland
(Einarsson, 1986). It is a transform type plate boun-
dary that connects the obliquely spreading boundary
on the Reykjanes Peninsula to the Eastem Volcanic
Zone of Iceland (Fig. 1). Spreading at this latitude is
taken up by two parallel rift zones, the Western and
the Eastem Volcanic Zone, and it has been sug-
gested on petrological and structural grounds, that
the Eastern Zone is a propagating rift taking over the
* now at the Royal Institute of Technology, Stockholm
rifting from the dying Western Zone (Meyer and
others, 1985; Oskarsson and others, 1985; Einarsson
and Eiríksson, 1982; Einarsson, 1988). The seismic
zone has been the scene of many destructive earth-
quakes in historic times, which in this part of the
world spans the last 1100 years. Maps of destruction
areas, microearthquake epicenters and surface rup-
tures of earthquakes delineate an E-W trending, 10-
15 km wide seismic zone (Einarsson and others,
1981). Geometric configuration of the plate boun-
dary suggests left-lateral transform motion along the
zone. Yet there is no indication of a major E-W
fault in the area. Instead, the earthquakes are asso-
ciated with right-lateral strike-slip along N-S strik-
ing faults or fault zones. This led Einarsson and
Eiríksson (1982) to suggest that the South Iceland
Seismic Zone is a transient feature, migrating south-
wards and sideways in response to the southwards
propagation of the Eastern Volcanic Zone.
Magnetic anomalies indicate that the Reykjanes
and Kolbeinsey Ridges are spreading at a half-rate
of about 1 cm per year (Talwani and Eldholm, 1977;
Vogt and others, 1980). If all the spreading takes
place in the Eastern Volcanic Zone this would imply
that the average displacement rate across the
transform zone is 2 cm per year. The transform dis-
placement is smaller if the Westem Volcanic Zone
takes up some of the spreading. The Hreppar crustal
block is thus moving to the west at a rate of 1-2 cm
per year with respect to the Eurasian plate. The
32 JÖKULL, No. 39, 1989