The 2011 unrest at Katla volcano: seismicity and geological context

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.