Jökull - 01.01.2019, Page 54

The 2011 unrest at Katla volcano:
seismicity and geological context
Giulia Sgattoni1,2,3?, Federico Lucchi1, Páll Einarsson2, Ólafur Gudmundsson3,
Gianfilippo De Astis4, and Claudio Antonio Tranne1
1Department of Biological, Geological and Environmental Sciences, University of Bologna, Bologna, Italy
2Institute of Earth Sciences, Science Institute, University of Iceland, Reykjavik, Iceland
3Department of Earth Sciences, Uppsala University, Uppsala, Sweden
4National Institute of Geophysics and Volcanology (INGV), Rome, Italy
?Corresponding author: giulia.sgattoni@unibo.it https://doi.org/10.33799/jokull2019.69.053
Abstract — Katla is one of the most active volcanoes in Iceland and is characterised by persistent seismicity.
It is partly covered by the Mýrdalsjökull glacier and its historic activity is dominated by phreatomagmatic
eruptions within the caldera associated with catastrophic glacial floods. In July 2011 a sudden jökulhlaup was
released from the glacier, associated with tremor, elevated seismicity inside the caldera and a new cluster of
seismicity on the south flank. This was likely caused by a hydrothermal or magmatic event, possibly a small
subglacial eruption. Similar unrests occurred in 1955 and 1999. We have identified changes of the seismicity
pattern coinciding with the 2011 unrest, suggesting a modification in the volcanic system. It may be speculated
that if the persistent seismicity at Katla is an indication of a pressurized magma system ready to erupt, small
events like those of 1955, 1999 and 2011 may trigger larger eruptions in the future. We have also conducted
a pilot study of the geology of the southern flank, where the new seismicity is recorded, and identified sources
for flank eruptions in the recent eruptive history of Katla. These include rhyolitic domes and surtseyan craters.
Therefore, a wide range of volcanic processes have to be taken into account as possible source for the new
seismicity and volcanic hazard.
Keywords: Katla volcano, volcano seismicity, long-period earthquakes, silicic domes, flank activity
INTRODUCTION
The study of subglacial volcanoes is crucial, as
magma-ice interaction can produce highly explosive
eruptions and jökulhlaups (glacial floods; Major and
Newhall, 1989, Gudmundsson et al., 2008), but prob-
lematic, because the ice cover prevents direct observa-
tions and complicates the understanding of geophysi-
cal signals. The Katla volcanic system, in south Ice-
land, is a prime example. Katla hosts a large caldera
covered by the Mýrdalsjökull ice cap (Figure 1) and
is a peculiar volcano for its persistent seismicity also
during periods of volcanic quiescence (Einarsson,
1991). It is one of the most active volcanoes in Iceland
and its volcanic activity is dominated by explosive,
phreatomagmatic eruptions (Óladóttir et al., 2008).
The last eruption to break the ice surface occurred in
1918 and the current repose time is the longest known
in history (Larsen, 2000).
After the eruption of Eyjafjallajökull in 2010, sci-
entists’ attention was pointed to Katla, as several pre-
vious eruptions of Eyjafjallajökull were swiftly fol-
lowed by eruptions at neighbouring Katla (Einarsson
and Hjartardóttir, 2015). However, no visible eruption
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