Jökull - 01.01.2010, Blaðsíða 18

Geirsson et al.
Hengill transient deformation
The 1994 to 1998 Hengill intrusion and seismic ac-
tivity (Feigl et al., 2000) had an important impact on
starting the network of CGPS measurements for haz-
ards monitoring in Iceland. The stations closest to
the 1994–1998 magma intrusion, HVER and OLKE
(Figure 8), were installed in April and May 1999.
By then the activity had already ceased, and the sites
show no significant signals of the 1994 to 1998 intru-
sion. However, the sites in the area have shown sig-
nificant temporal variation in site velocities (Figure 8)
with gradually increasing velocities towards north and
west and subsidence at increasing rates. In particu-
lar, OLKE subsided and moved more rapidly towards
west during 2006–2010 than before. A new geother-
mal power plant started operating in the Hengill area
in 2006, and the production capability of an older
geothermal power plant drawing fluids from the north-
ern part of the area was increased substantially in
2008. It is tempting to suggest that the increase in
geothermal fluid withdrawal is causing the changes in
the site motions in the Hengill area, however, more
detailed research is required because a wealth of other
geodetic data better suited for spatial constraints of the
deformation source exists for the area.
DISCUSSION
Since the first CGPS station was installed in Reykja-
vík in 1995, the signals captured by the growing
CGPS network have been of a great importance to
the scientific community and civil defense in Iceland.
Data from the network have also been used as a ref-
erence for numerous precision mapping projects. The
processes causing surface deformation in Iceland are
diverse, covering plate motion, earthquakes, erup-
tions, magma movements, glacial rebound, and pres-
sure changes in geothermal systems. CGPS is one of
the available tools to study these processes. The inte-
gration of CGPS data with other geodetic techniques
(e.g. episodic and semi-continuous GPS, InSAR, tilt,
and leveling), seismic, and other data of relevance has
proven well, emphasizing the importance of interdis-
ciplinary cooperation.
We have yet to capture a plate boundary rifting
event by the CGPS network, which seem to occur ev-
ery few hundred years on the plate boundary. The
last rifting episode in Iceland occurred in the Krafla
volcanic system in 1975–1984 (e.g. Björnsson, 1985).
The latest rifting episode in the Eastern Volcanic Zone
occurred 1862–1864 in the Bárðarbunga volcanic sys-
tem (Thorarinsson and Sigvaldason, 1972) but a major
event took place in 1783–1785, when the Lakagígar
crater row formed and around 27 km3 were erupted
(Thordarson and Larsen, 2007). Although much can
be learned from the ongoing Afar rifting episode (e.g.
Ebinger et al., 2010), there are questions specific to
Iceland that need to be addressed by direct observa-
tions, such as improved understanding of the magma
plumbing systems, the propagating EVZ (LaFemina
et al., 2005), and precursors of rifting episodes.
The south Iceland seismic crisis in 2000 and 2008
demonstrated the importance of the CGPS network
and applications of high-rate data sampling. Since
only a part of the accumulated energy has been re-
leased in the SISZ, continuation of the sequence might
be expected. The Húsavík-Flatey fault in northern Ice-
land had its last large earthquake in 1872 and may
be due for another event, although results from the
CGPS network indicate that the Grímsey oblique rift
is presently taking up a majority of the deformation in
the Tjörnes fracture zone.
Some of the most active volcanoes in Iceland are
now being monitored with continuous GPS. However,
most of the EVZ, where the volcanic production in
Iceland is greatest, is poorly instrumented. The same
holds true for many recently active volcanoes such as
Krafla, Askja, and Öræfajökull. The ability of the
CGPS technique to track subsurface magma move-
ments with time, as best demonstrated by the 2009–
2010 Eyjafjallajökull intrusive and eruptive episodes,
has the potential of being pushed further towards real-
time.
There is a global concern for ongoing climate
change, and the retreat of the Icelandic ice-caps cause
the observed rapid uplift of the highlands in cen-
tral Iceland. The CGPS data have been used to con-
strain structural parameters of the Icelandic crust, but
they could as well be used as an indirect measure of
18 JÖKULL No. 60