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.

Crustal movements due to Iceland’s shrinking ice caps mimic magma inflow signal at Katla volcano

Many volcanic systems around the world are located beneath, or in close proximity to, ice caps. Mass change of these ice caps causes surface movements, which are typically neglected when interpreting surface deformation measurements around these volcanoes. These movements can however be significant and may closely resemble movements due to magma accumulation. Here we show such an example, from Katla volcano, Iceland. Horizontal movements observed by GPS on the flank of Katla have led to the inference of significant inflow of magma into a chamber beneath the caldera, starting in 2000 and continuing over several years. We use satellite radar interferometry and GPS data to show that between 2001 and 2010, the horizontal movements seen on the flank can be explained by the response to the long term shrinking of ice caps and that erratic movements seen at stations within the caldera are also not likely to signify magma inflow. It is important that interpretations of geodetic measurements at volcanoes in glaciated areas consider the effect of ice mass change and previous studies should be carefully reevaluated.