Jökull - 31.12.2001, Page 45

Reviewed research article
Seismic monitoring during an injection experiment in the
Svartsengi geothermal field, Iceland
Bryndís Brandsdóttir1, Hjalti Franzson2, Páll Einarsson1,
Knútur Árnason2 and Hrefna Kristmannsdóttir2,3
^Science Institute, University oflceland, Haga Hofsvallagötu 53, IS-107 Reykjavík, Iceland;
bryndis@ raunvis.hi. is, palli@ raunvis.hi.is
2National Energy Authority, Grensásvegur 9, IS-108 Reykjavík, Iceland; hf@ois.is, ka@os.is
3now at the University of Akureyri, Norðurslóð, 600 Akureyri, Iceland; hk@unak.is
Abstract — Portable, digital seismographs were operated around the Svartsengi geothermalfield during May-
August 1993 in order to monitor microearthquake activity prior to and during a waste water injection test. A
total of218 thousand tons ofwater were injected into borehole H-6 from July 19, through December 1993 at
an average rate of 15 l/s, reaching a maximum of30 l/s. The borehole accepted the injectionfluid under gravity
and pumping was not required. No detectable microearthquakes occurred within the Svartsengi geothermal
field during the injection period. We conclude that the injection pressure was probably far below the level
needed to induce seismicity. Fluid pressure drawdown caused by exploitation decreases pore pressure which
again increases the rock strength. A drawdown ofmore than 20 bars in the Svartsengi field since 1976, and an
equal rate in the adjacent Eldvörp field, may have raised the fracture limit and thus reduced the microseismic
activity in these fields.
INTRODU CTION
The Reykjanes Peninsula oblique rift is a seg-
ment of the mid-Atlantic plate boundary and joins the
Reykjanes Ridge off shore with the complex series of
boundaries in Iceland (Figure 1). The presently active
plate boundary is marked by the Reykjanes seismic
zone (RSZ), a 2-5 km wide, continuous zone of seis-
mic activity which trends 80°, oblique to the en ech-
elon, 30°-trending volcanic systems of the rift zone
(Klein et al., 1977; Einarsson and Björnsson, 1979;
Einarsson, 1991). The seismic zone extends from
the westem tip of the Reykjanes Peninsula, through
the centers of three volcanic systems and towards the
Hengill triple junction, where it merges with the trans-
form South Iceland Seismic Zone.
Unlike seismicity in other parts of the active vol-
canic zones of Iceland, the seismicity of the RSZ is
not only confined to high-temperature geothermal re-
gions within individual central volcanoes but instead
is continuous along the plate boundary. Earthquakes
in the RSZ occur in swarms which follow the plate
boundary, with normal faulting being most common
in the western part of the zone and strike-slip faulting,
with fewer but larger earthquakes in the east (Einars-
son 1991). The focal mechanisms reflect a gradual
change along the RSZ from a ridge dominated seg-
ment in the west towards a transform zone in the east.
High-temperature geothermal fields on the
Reykjanes peninsula are located where the RSZ
crosses individual fissure swarms. The Svartsengi
field, along with the Eldvörp and Reykjanes high-
temperature geothermal fields, is situated within the
Reykjanes volcanic system, which is the westernmost
volcanic system (Figure 1). The Reykjanes volcanic
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