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The 2011 unrest at Katla volcano
cic eruptions are not reported since then, possibly in-
dicating a substantial change in the plumbing system
caused by the Eldgjá eruption (Óladóttir et al., 2008).
In historical times, Katla has been the most pro-
ductive volcanic system in Iceland in terms of magma
volume erupted (Thordarson and Larsen, 2006). It has
generated observable subaerial eruptions fairly regu-
larly at a rate of 1–3 eruptions per century, with re-
pose periods ranging between 13 and 98 years, and
averaging 47 years since AD 1500 (Larsen, 2000).
Recent activity
The last eruption to break the ice surface was an ex-
plosive basaltic eruption in 1918 that produced a ∼14
km high eruptive plume and tephra fallout (0.7 km3),
accompanied by a massive jökulhlaup that deposited a
large volume of juvenile eruptive materials. The erup-
tion site was located near the south rim of the caldera
beneath ∼400 m of ice (Eggertsson, 1919; Sveinsson,
1919; Larsen, 2000).
Periods of elevated seismicity, not associated with
eruptive activity, occurred at Katla in 1967 and 1976–
77 inside the caldera and at Goðabunga (Einarsson,
1991). Two minor subglacial eruptions, but with no
subaerial tephra emission, may have occurred in June
1955 and July 1999. The 1955 event took place near
the eastern rim of the caldera (Figure 2) where two
shallow ice cauldrons formed and a small jökulhlaup
drained from south-east Mýrdalsjökull (Thorarinsson,
1975). In 1999, a new ice cauldron formed on the
glacier (Guðmundsson et al., 2007) and a jökulhlaup
was released from Sólheimajökull (Sigurðsson et al.,
2000; Roberts et al., 2003), associated with earth-
quakes and bursts of tremor.
From 1999 to 2004, GPS measurements on
nunataks exposed along the caldera rim revealed
steady uplift of the volcano, interpreted to result from
0.01 km3 of magma accumulation (Sturkell et al.,
2006, 2008). Consequently, Guðmundsson et al.
(2007) showed that increased geothermal heat output
occurred in 2001–2003 based on the evolution of ice
cauldrons, together with the increased seismicity and
ground uplift. However, a recent study by Spaans et
al. (2015) reported that the uplift may be due to glacial
isostatic adjustment as a consequence of mass loss of
Iceland’s ice caps.
JULY 2011 UNREST
A significant, general increase in seismicity started at
Katla in July 2011 and lasted until winter 2011. This
followed a period of uplift of most ice cauldrons on
Mýrdalsjökull (11–12 m at cauldron 16) between Au-
gust 2010 and July 2011, resulting from water accu-
mulation under the glacier (Guðmundsson and Sólnes,
2013).
The seismicity intensified especially in the south-
ern sector of the caldera and culminated with a 23-
hour tremor burst on July 8–9th (Sgattoni et al.,
2016b, 2017). No signs of eruption breaking the ice
were observed, but a ∼18 million m3 jökulhlaup
drained from Kötlujökull and some ice cauldrons
deepened in the southern and eastern parts of Mýr-
dalsjökull (Figure 2). The jökulhlaup swept away the
bridge over Múlakvísl river early in the morning of
July 9th. This coincided with a clear flood-related
tremor phase lasting around 5 hours. Two main tremor
sources were identified in the southern and eastern
caldera, corresponding to the active cauldrons, and
were interpreted to be associated with either geother-
mal or magmatic processes (Sgattoni et al., 2017).
At the same time, a new earthquake source be-
came active on the southern flank near the Gvendarfell
ridge, at the edge of the glacier (Figure 2). This seis-
micity consists of long-period events with an emer-
gent P wave and an unclear S wave, and has a peculiar
temporal pattern characterized by regular inter-event
times modulated by a seasonal correlation. Due to
their temporal pattern and the depth distribution of the
hypocentres (Sgattoni et al., 2016a), they are inferred
to relate to volcanic rather than glacial processes. The
events locate in the shallow subsurface between 0.5
and 0.9 km depth, but the depth is marginally re-
solved to differ from zero. The size of the cluster in-
ferred from relative location is on the order of 100 me-
ters (Sgattoni et al., 2016a). Both magmatic and hy-
drothermal processes are considered possible (Sgat-
toni et al., 2016a,c).
EARTHQUAKE ACTIVITY 1998–2015
Catalogue data from the Icelandic Meteorological Of-
fice (IMO) give an overview of the seismic activity at
Katla during the period 1998–2015, as a framework
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