Jökull - 01.01.2010, Side 12

Geirsson et al.
sequence in 1896–1912 (Decriem et al., 2010). Con-
tinued earthquake activity with moderate size events
rupturing N-S faults in the SISZ can thus be expected
in the coming decades. It is therefore important to
continue the deformation studies in southwest Iceland
and further densify the CGPS network in the South
Iceland Seismic Zone as well as further west on the
Reykjanes Peninsula.
Volcanoes and magma movements
The CGPS network has proven very useful for moni-
toring and improving understanding of the volcanoes
in Iceland. In the following sections we review the
deformation episodes observed. Importantly, the net-
work also indicates which volcanoes do not show
signs of magma movement. This is the case with
the Heimaey island in the Vestmann islands, which
erupted last in 1973. There, the site VMEY (Figure 1)
has not shown signs of volcanic deformation since the
station was installed in 2000.
Hekla 2000 eruption and inter-eruptive deformation
Hekla is one of Iceland’s most active volcanoes with
18 documented eruptions for the past 1100 years
(Thordarson and Larsen, 2007). Hekla erupted in
1970, 1980, 1991, and 2000, and recent dry-tilt mea-
surements indicate that an eruption could take place
any time (Sturkell et al., 2006). When the Hekla erup-
tion occurred in February to March 2000, the closest
CGPS station (SOHO) was 50 km away from Hekla,
however, since 2006 a dense CGPS network has been
in operation at the volcano. Geirsson et al. (2006) re-
port up to 5 mm co-eruptive horizontal displacements
of site SOHO for the year 2000 eruption. Co-eruptive
borehole strain, tilt and episodic GPS measurements
indicate that the dike formed in the eruption was prob-
ably mostly within the volcanic edifice, fed by a small
conduit from the magma chamber residing at 11 km
depth (Sturkell et al., 2005). Ofeigsson et al., (in
press) place the magma chamber at 16 km depth, us-
ing InSAR observations. These models predict a co-
eruptive horizontal signal on the order of 2–5 mm, in
general agreement with the observed displacements at
SOHO in 2000.
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Figure 5
Figure 5. Observed horizontal (green arrows) and ver-
tical (white bars) site velocities at Hekla volcano rela-
tive to a stable North America plate, showing a grad-
ual velocity increase over Hekla. The velocity field is
derived as in Figure 1, and site labels are as in Figure
1. – Láréttir færsluhraðar við Heklu miðað við fastan
Norður-Ameríkufleka (grænar örvar) ásamt lóðrétt-
um hröðum (hvítar stikur) sýna hraðaaukningu yfir
svæðið.
Between eruptions, a somewhat complicated de-
formation pattern is observed at Hekla from InSAR,
with maximum uplift of less than 3 mm/yr occurring
at a distance of about 10 km surrounding Hekla and
subsidence in the center relative to the maximum up-
lift (Ofeigsson et al., in press). This signal is inter-
preted as being a broad inflation signal from a deep
magma source with subsidence due to the load of
Hekla volcano itself superimposed (Ofeigsson et al.,
in press; Grapenthin et al., 2010). Despite a short time
series of the CGPS stations at Hekla, we can conclude
that the deformation is steady in time. Crustal widen-
ing is observed across Hekla (Figure 5), which could
be caused by magma recharge and/or plate spreading
because Hekla is at essentially within the Eastern Vol-
canic Zone. The CGPS station closest to the volcano
12 JÖKULL No. 60