  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.
