Jökull - 01.12.1994, Qupperneq 19

First Epoch GPS Survey of the Hengill Triple Junction, SW
Iceland, and the Effect of Ocean Loading.
K.M. Hodgkinson and G.R. Foulger
Dept. Geological Sciences, Univ. Durham, Science Labs, South Rd., Durham, DHl 3LE, U.K.
Abstract - In 1991 a 23 point, 30 x 25 km Global Positioning System (GPS) geodetic network was
established in the Hengill ridge-ridge-transform triple junction, South Iceland. The network was
tied to regional points up to 75 km distant to the west and east. Measurements were made using
Ashtech CIA code receivers during sessions which averaged eight hoursfor baselines of35 km or
less and20 hoursfor the 75 km baselines. Dataprocessing was conducted using the Bernese V3.2
software. The best solution was an ambiguity-fixed solution, which, after network adjustment,
yielded scaled formal errors (accuracies) ofless than 1 cm in the horizontal and about 1 cm in the
verticalfor the majority ofthe points. Theoretical predictions of the effect ofocean loading in S
Iceland suggest that diurnal variations of up to 6 mm in the vertical and 2 mm in the horizontal
components are to be expected for lines up to about 75 km long (Wu-Ling and Morgan, unpubl-
ished computer program). Diurnal variations in the height differences between points were
generally 50-60 mm however, indicating that the ejfects ofother error sourcesfar exceeded those
ofocean loading in the case ofour survey. The ejfect ofocean loading therefore does not need to
be modelledfor surveys ofthe kind we conducted, although the importance ofthis effect would in-
crease with line length andfor surveys ofhigher accuracy.
INTRODU CTION
Iceland is a uniquely large, subaerial exposure of
oceanic crust traversed by up to 700 km of accret-
ionary plate boundary, which contain over 30 spread-
ing segments and two complex fracture zones (Sae-
mundsson, 1986). It offers unparalleled opportunity
to study crustal deformation processes associated
with oceanic crustal extension. These processes in-
clude coseismic, and possible preseismic deforma-
tions associated with large-magnitude earthquakes,
magma chamber inflation and deflation (e.g., Bjöms-
son et al., 1977), dyke injections (e.g., Moller and
Ritter, 1980), post-seismic and post-dyking deforma-
tion (e.g., Foulger et al., 1992; Heki et al„ 1993) and
slow, continuous plate movements and rift deforma-
tion (e.g., Tryggvason, 1982).
Geodetic surveys using the Global Positioning
System (GPS) have been conducted in Iceland almost
annually since the system became usable for field sur-
veying. Large, regional networks encompassing most
of the country were installed and measured in 1986
and 1987, and remeasured and extended in 1989,
1990, 1991, 1992 and 1993 (Foulger et al„ 1987;
1993; Jahn et al„ 1989; Hackman, 1991; Jahn et al„
1994; Sigmundsson et al„ 1992; Foulger et al„ 1992;
Heki etal„ 1993; Sturkell etal., 1994). Deformations
detected by this work include shearing in the South
Iceland Seismic Zone (SISZ) and the Reykjanes
Peninsula, deflation of a magma chamber beneath the
Hekla volcano and post-dyking viscoelastic strain re-
lease in North Iceland.
The Hengill triple junction is the meeting point of
the Reykjanes Peninsula and the Westem Volcanic ac-
cretionary zones and the SISZ. It has a recent history
of volcanism, rifting and destructive earthquake ac-
JÖKULL, No. 44
17