NATURAL AND MAN-MADE DEFORMATION AROUND GEOTHERMAL FIELDS ON
THE REYKJANES PENINSULA, SW ICELAND
Marie Keiding1 , Andy Hooper2 , Thóra Árnadóttir1 , Sigurjón Jónsson3 , and Judicael Decriem1
1
Nordic Volcanological Centre, Institute of Earth Sciences, University of Iceland, 2 Delft University of Technology, The
Netherlands, 3 King Abdullah University of Science and Technology (KAUST), Saudi Arabia
ABSTRACT
The Reykjanes Peninsula in southwest Iceland is a
transtensional plate boundary zone characterised by
high seismicity, recent volcanism and high-temperature
geothermal fields. We examine the crustal deformation
on the peninsula using Envisat data from 2003–2009, as
well as GPS data from the same period. Uplift and elevated seismicity is observed in the Krı́suvı́k geothermal
field, reflecting increased pressure in the geothermal system or a slow magma intrusion. The start of production
in a new geothermal power plant in the Reykjanes field
in 2006 causes subsidence of up to 10 cm during the first
two years of production. A change in the style of seismicity is also observed near the geothermal field.
and normal faults, grouped into four en-echelon volcanic
fissure swarms (Figure 1). A number of high-temperature
geothermal fields are present, and geothermal energy has
been harnessed from geothermal fields on the Reykjanes
Peninsula since 1976. Two new power plants started operating in the Reykjanes and Hengill geothermal fields
during 2006, causing localised areas of subsidence. Here,
we examine the deformation related to both natural and
man-made fluid migration on the Reykjanes Peninsula.
Key words: plate boundary; geothermal fields; manmade subsidence; triggered earthquakes.
1.
INTRODUCTION
Interferometric Synthetic Aperture Radar (InSAR) is well
suited for monitoring the deformation associated with
fluid migration at depth as it can provide a high spatial resolution and good constraints on the vertical motion. Both natural and man-made processes cause fluid
migration, and the resulting deformation is often so large
that it obscures the deformation due to tectonic processes
such as plate boundary deformation. Examples of processes involving fluid migration are ground-water extraction [e.g. 1, 2], mining [e.g. 3], geothermal or hydrocarbon production [4, 5, 6], naturally occurring fluctuations
in geothermal and magmatic systems [7, 8], or transient
post-seismic processes [e.g. 9].
Figure 1. Tectonic map of the Reykjanes Peninsula, with
fracture locations from Clifton and Kattenhorn [11]. A
number of high-temperature geothermal areas are located on the peninsula (green hatched areas). The inset
shows the neovolcanic zones in Iceland, and the direction of the 2 cm/yr spreading on the Reykjanes Peninsula
between North America and Eurasia (arrows).
The Mid-Atlantic plate boundary comes onshore on the
Reykjanes Peninsula, where it forms a diffuse plate
boundary zone characterised by high seismicity and recent volcanism. Analysis of recent GPS data indicates
that the plate motion is primarily left-lateral shear and
lesser extension across the plate boundary zone [10]. The
main tectonic features on the peninsula are a large number of NE-trending tension fractures, eruptive fissures
The Reykjanes Peninsula is well suited for a radar based
study because its surface mainly consists of young and
sparsely vegetated lava fields, hence the surface reflectivity is sufficiently high and changes little with time. We
form InSAR images from descending track 138 Envisat
acquisitions during 2003–2008. The images are then processed using the multi-temporal StaMPS/MTI software
[12, 13], which applies both a persistent scatterer and a
_____________________________________________________
Proc. ‘Fringe 2009 Workshop’, Frascati, Italy,
30 November – 4 December 2009 (ESA SP-677, March 2010)
2.
DATA ANALYSIS
Figure 2. Residual mean line-of-sight velocities after removal of bilinear ramps, relative to the mean value during each period in the area near Reykjavı́k (shown with
the small box). The large box in the bottom panel shows
the area of Figure 3.
small baseline approach. The resulting data set comprises
around 2 million pixels in the area shown in Figure 2. The
combination of the persistent scatterer and a small baseline methods improves the spatial sampling, and thereby
increases the resolution of deformation signals and aids a
more reliable phase unwrapping.
3.
REGIONAL DEFORMATION
From the InSAR time series we compute the mean lineof-sight (LOS) velocities for two different periods: one
period spanning 25 Sep 2003–29 Sep 2005 (before the
start of geothermal production in the two new geothermal power plants) and one period spanning 6 Jul 2006–1
May 2008 (after start of production). The estimated LOS
velocities generally increase from north to south, reflecting the left-lateral shear across the plate boundary. A
subtle zone of subsidence is observed along the central
part of the plate boundary zone, probably reflecting subsidence due to the extension across the plate boundary.
During 2006–2008, areas of subsidence develop in the
Reykjanes and Hengill fields, following the start of production in the new geothermal power plants. Moreover,
local uplift is observed in the Krı́suvı́k geothermal field
on the central part of the peninsula. Boreholes have been
drilled for monitoring the Krı́suvı́k geothermal system,
but a geothermal power plant has not been installed in the
field as of Fall 2009.
Figure 3. Unwrapped Envisat interferograms for the central part of the Reykjanes Peninsula, spanning 6 July
2006 – 3 September 2009. The black dots are concurrent
earthquake locations from the SIL seismic catalogue.
4.
THE KRÍSUVÍK AREA
In order to examine the recent deformation in the
Krı́suvı́k area, we process four additional track 138 Envisat images acquired during June–September 2008 and
one image from September 2009 (Figure 3). The recent
images show that the uplift in the Krı́suvı́k geothermal
field, which started in 2007, is on-going and increasing
during 2009. Elevated seismicity is also observed in the
area during 2009, as recorded by the SIL seismic network. The uplift and seismicity is likely to be caused
by increased pressure in the geothermal system, although
there is not yet any pressure data from the boreholes to
confirm this. Another possibility is that we are seeing the
surface expression of a slow magmatic intrusion in the
central part of the Krı́suvı́k fissure swarm.
5.
THE REYKJANES FIELD
For the western part of the Reykjanes Peninsula we compare the descending data with ascending data as well as
GPS (Figure 4). We include an ascending image formed
from two track 173 Envisat acquisitions, and GPS data
from a network of campaign and continuous stations on
the peninsula. The near-vertical and approximately east-
Figure 4. a) Ascending LOS displacements during 18
June 2005–3 May 2008 from two Envisat track 173 acquisitions. b) Descending LOS displacements during 16
June 2005–1 May 2008, estimated from 13 Envisat track
138 acquisitions. c) Near-vertical radar displacements
from addition of ascending and descending LOS displacements. d) Approximately east-west radar displacements obtained by subtracting the ascending from the descending LOS displacements. The coloured circles show
the magnitudes of the GPS displacements projected onto
the unit vectors of the radar displacements. The arrows
show the line-of-sight direction from ground to satellite.
west deformation fields are obtained from the linear combination of the ascending and descending data, and compared with the GPS data by projecting the GPS displacements during the same time period onto the unit vectors
of the radar data. The radar and GPS data usually agree
within the uncertainty of the estimated displacements.
The near-vertical radar and GPS displacements in Figure
4c indicate that the maximum subsidence in the Reykjanes geothermal field is around 10 cm during the first
two years of production. The area of subsidence is clearly
elongated in the NE–SW direction, thus aligning with the
trend of the fractures in the area. The east-west radar displacements in Figure 4d show horizontal motion on the
order of several cm toward the centre of subsidence.
5.1.
Triggered earthquakes
A change in the style of seismicity is observed following the start of production in the Reykjanes geothermal
Figure 5. Close-up on the near-vertical radar displacement field during 2005–2008 (same as in Figure 4c).
Earthquake locations and focal mechanisms from the SIL
seismic catalogue are shown as background events (small
black dots), and distinct swarm events in 2006 (orange),
2007 (red) and 2008 (blue).
field. Short-lived swarms of micro-earthquakes occurred
SE and NW of the area of subsidence, in areas that had
not previously been seismically active. The focal mechanisms indicate that the largest events were typically consistent with normal faulting on NE-trending planes. Modelling of the stress changes due to the contraction within
the reservoir suggests that the earthquakes may be induced by the stress changes caused by the geothermal
fluid extraction [14].
Interestingly, the radar data reveal a subtle discontinuity
in the area NW of the area of subsidence, as shown in
the profile in Figure 5. The discontinuity is consistent
with displacement of approximately 1 cm in a graben-like
structure, and appears between two acquisitions in Fall
2006, a few months after the start of geothermal energy
production.
ACKNOWLEDGMENTS
The Envisat data were provided by the European Space
Agency, and all earthquake locations and focal mechanisms are from the SIL seismic catalogue, courtesy of the
Icelandic Meteorological Office. The first author is supported by a grant from the Eimskip Fund of the University
of Iceland.
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