  Katla is a threatening volcano in Iceland, partly covered by the
M\'yrdalsj\"okull ice cap. The volcano has a large caldera with several active
geothermal areas. A peculiar cluster of long-period seismic events started on
Katla's south flank in July 2011, during an unrest episode in the caldera that
culminated in a glacier outburst. The seismic events were tightly clustered at
shallow depth in the Gvendarfell area, 4 km south of the caldera, under a small
glacier stream on the southern margin of M\'yrdalsj\"okull. No seismic events
were known to have occurred in this area before. The most striking feature of
this seismic cluster is its temporal pattern, characterized by regular
intervals between repeating seismic events, modulated by a seasonal variation.
Remarkable is also the stability of both the time and waveform features over a
long time period, around 3.5 years. No comparable examples have been found in
the literature. Both volcanic and glacial processes can produce similar
waveforms and therefore have to be considered as potential seismic sources.
Discerning between these two causes is critical for monitoring glacier-clad
volcanoes and has been controversial at Katla. For this new seismic cluster on
the south flank we regard volcano-related processes as more likely than glacial
ones for the following reasons: 1) the seismic activity started during an
unrest episode involving sudden melting of the glacier and a j\"okulhlaup; 2)
the glacier stream is small and stagnant; 3) the seismicity remains regular and
stable for years; 4) there is no apparent correlation with short-term weather
changes, such as rain storms. We suggest that a small, shallow hydrothermal
system was activated on Katla's south flank in 2011, either by a minor magmatic
injection or by changes of permeability in a local crack system.

Geophysics