Jökull - 01.01.2019, Side 56

The 2011 unrest at Katla volcano
Figure 2. Recent unrest at Katla. Black
cross: 1918 eruption site (Björnsson et
al., 2000). Black, grey and red dashed
arrows: routes of 1955, 1999 and 2011
jökulhlaups, respectively. Circles with
same outline or fill colors mark the
cauldrons newly formed or deepened
during the same unrest episode. Red
star: new seismic source in the Gvend-
arfell area. GR: Gvendarfell ridge.
Light grey lines: water divides of main
outlet glaciers (Björnsson et al., 2000).
Topography information from the Na-
tional Land Survey of Iceland. – Yfir-
litskort af nýlegum atburðum við Kötlu.
Svartur kross: Gosstöðvar 1918. Svört,
grá og rauð ör sýnir leiðir jökulhlaup-
anna 1955, 1999 og 2011. Hringir í
sömu litum sýna sigkatlana sem mynd-
uðust í jöklinum við upptök þessara
hlaupa. Rauð stjarna: Upptakasvæði
skjálftarununnar við Gvendarfell. GR:
Gvendarfell. Ljósgráar línur: Vatna-
skil helstu skriðjökla. Landslagsgögn
eru frá Landmælingum Íslands.
(2009), but no detailed description of their features is
reported. We thus decided to undertake a geological
survey in the area around the Gvendarfell ridge, where
the new seismic cluster is located, aimed at identify-
ing the main geological and tectonic features possibly
connected with the seismic sources.
In this article, we place the 2011 unrest episode
at Katla in the broader context of its geology and
eruptive history. Moreover, we analyze the general
changes of the historical seismicity at Katla by look-
ing at the seismic catalogue from 1998 to 2015. We
use this information to shed light on the 2011 unrest
and suggest a possible interpretation.
THE KATLA VOLCANIC SYSTEM
Geological overview
Katla is located just south of the intersection between
the active rifting zone of the Eastern Volcanic Zone
(EVZ) and the transform boundary of the South Ice-
land Seismic Zone and may be classified as an in-
traplate volcano (Figure 1). An occasional connec-
tion with rifting in the EVZ is exemplified by the AD
934 Eldgjá eruption (Sturkell et al., 2008). Katla and
Eyjafjallajökull appear to be tectonically connected,
as the Eyjafjallajökull E-W fissure swarm merges with
the Katla radial fissure system (Einarsson and Brands-
dóttir, 2000; Einarsson and Hjartardóttir, 2015).
The Katla volcanic system consists of a large cen-
tral volcano mostly covered by the Mýrdalsjökull ice
cap, connected to the Eldgjá fissure system extend-
ing 75 km to the northeast (Larsen, 2000; Thordarson
et al., 2001). The central volcano hosts a 10×14 km
wide and 650–750 m deep, ice-filled caldera (Björns-
son et al., 2000). Three main glaciers descend from
the ice cap through deep gaps in the southeast, south-
west and northwest caldera walls (Figure 1b), cor-
responding to the three main possible paths for jök-
ulhlaups via Kötlujökull, Sólheimajökull and Entu-
jökull, respectively. Several ice cauldrons (at least
16) located within the caldera and at its rim are the
JÖKULL No. 69, 2019 55