The 2011 unrest at Katla volcano: characterization and interpretation of the tremor sources

TL;DR

This study analyzes a 23-hour tremor burst at Katla volcano in 2011, identifying different tremor phases and exploring whether they resulted from a minor eruption, hydrothermal activity, or flood-related processes, indicating increased volcanic heat release.

Contribution

It provides a detailed characterization of tremor sources at Katla, distinguishing phases linked to flood and volcanic activity, and discusses possible eruption or hydrothermal origins.

Findings

Tremor phases correlated with flood and volcanic activity.

Evidence suggests a minor sub-glacial eruption or hydrothermal processes.

Increased heat release from the volcano was observed.

Abstract

A 23 hour tremor burst was recorded on July 8-9th 2011 at the Katla subglacial volcano, one of the most active and hazardous volcanoes in Iceland. This was associated with deepening of cauldrons on the ice cap and a glacial flood that caused damage to infrastructure. Increased earthquake activity within the caldera started a few days before and lasted for months afterwards and new seismic activity started on the south flank. No visible eruption broke the ice and the question arose as to whether this episode relates to a minor subglacial eruption with the tremor being generated by volcanic processes, or by the flood. The tremor signal consisted of bursts with varying amplitude and duration. We have identified and described three different tremor phases, based on amplitude and frequency features. A tremor phase associated with the flood was recorded only at stations closest to the river that flooded, correlating in time with rising water level observed at gauging stations. Using back-projection of double cross-correlations, two other phases have been located near the active ice cauldrons and are interpreted to be caused by volcanic or hydrothermal processes. The greatly increased seismicity and evidence of rapid melting of the glacier may be explained by a minor sub-glacial eruption. It is also plausible that the tremor was generated by hydrothermal boiling and/or explosions with no magma involved. This may have been induced by pressure drop triggered by the release of water when the glacial flood started. All interpretations require an increase of heat released by the volcano.