Jökull - 01.01.2019, Side 60

The 2011 unrest at Katla volcano
Figure 4. Magnitude-time evolution of the seismicity at Goðabunga (left) and inside the caldera (right) during
1998–2015, from IMO catalogue (events with magnitude ML>1). – Stærð teiknuð sem fall af tíma fyrir skjálfta
í Goðabungu-þyrpingunni (vinstri) og fyrir skjálfta innan öskjunnar (hægri) á tímabilinu 1998–2015. Skjálftar
með ML > 1. Gögn frá Veðurstofu Íslands.
ter described by Soosalu et al. (2006) and Jónsdóttir et
al. (2009) still occurs, but has gradually become less
clear. In addition, this peak seems to have shifted to
an earlier time of the year, probably reflecting climate
and weather changes.
– The seasonal correlation of the caldera seismicity
is less pronounced than at Goðabunga (as was al-
ready observed in previous studies), but, opposite to
Goðabunga, it has become clearer in the past few
years, especially in 2012, 2013 and 2014.
– A sharp increase of the seismicity rate inside the
caldera occurred in July 2011, coinciding with the un-
rest episode and jökulhlaup, and continued until the
beginning of 2012 with 660 events with ML>1 and
20 with ML>2.5.
– The Gvendarfell seismic cluster on the southern
flank became active in July 2011. It includes nearly
all events located on the southern flank of Katla (Fig-
ure 3), with the spread of the hypocenters due to loca-
tion uncertainty. The size of the cluster is in fact very
JÖKULL No. 69, 2019 59