Jökull - 01.01.2015, Page 51

Reviewed research article
Shallow geothermal and deep seismicity beneath Þeistareykir,
NE-Iceland
Juerg Schuler1,2, Robert S. White2,3, Bryndís Brandsdóttir3 and Jon Tarasewicz2,4
1Seismological Laboratory, California Institute of Technology, Pasadena, USA
2Dept. of Earth Sciences, University of Cambridge, Cambridge, UK
3Institute of Earth Sciences, Science Institute, University of Iceland, Reykjavík, Iceland
4now at BP, Sunbury on Thames, UK
Corresponding author email: schuler@cantab.net
Abstract – The seismicity in the central Þeistareykir volcanic system, NE Iceland, between 2009–2012 consists
of spatially clustered earthquakes at 2–5 km depth (b.s.l.) southwest of the main geothermal fields. Deep
earthquakes are located in a scattered pattern southeast of Þeistareykir at 8–20 km depth (b.s.l.). Although not
associated with detectable surface uplift they may be caused by high strain rates within the plastic regime of
the crust in the vicinity of the Húsavík-Flatey transform fault system or due to reduced normal friction caused
by melt movements at depth.
INTRODUCTION
The Þeistareykir volcanic system marks the north-
western boundary of the Northern Volcanic Zone
(NVZ) in Iceland. The volcanic system consists of
a diffuse central volcano transected by a 7–9 km wide
and 70–80 km long fissure swarm, extending from
Lake Mývatn to Öxarfjörður at an average azimuth
of N27◦E (Einarsson and Sæmundsson, 1987; Magn-
úsdóttir and Brandsdóttir, 2011; Hjartardóttir et al.,
2015). The Þeistareykir fissure swarm is character-
ized by large normal faults with maximum displace-
ments of 200–300 meters along its western rim and
numerous rift fissures further east. The volcanic cen-
ter is dominated by Holocene lava shields (Sæmunds-
son et al., 2012) and primitive lavas (Maclennan et
al., 2003). Surface alterations from geothermal ac-
tivity are found on the northern slope of the hyalo-
clastite mountain Bæjarfjall covering an area of about
11 km2 (Gíslason et al., 1985), although recent re-
sults of transient electromagnetic (TEM) soundings
suggest an area of geothermally altered crust of up to
45 km2 at a depth of 800–1000 m below the surface
(Karlsdóttir et al., 2012). The study area exhibits rel-
atively low relief (300–500 m elevation) with Bæjar-
fjall reaching 560 m above sea level. We subsequently
refer to depth as depth below the surface whereas we
mean depth below sea level when it is followed by the
acronym b.s.l.
Geodetic surface deformation measurements and
models (e.g., Metzger and Jónsson, 2014) suggest
a transient inflation of the Þeistareykir volcano be-
tween 1992–2009. A localized uplift at Þeistareykir
observed during 2007–2008 was most likely caused
by magma accumulation at 8.5 km depth (Metzger
et al., 2011; Spaans et al., 2012). The location of
the Mogi source used to model the surface deforma-
tion is shown by the red cross in Figure 1a. A broad
uplift zone within the NVZ has been interpreted as
magma accumulating at the crust-mantle boundary at
about 21 km depth (Zeeuw-van Dalfsen et al., 2004;
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