Jökull - 01.01.2019, Page 55

Sgattoni et al.
Figure 1. Map of Iceland showing volcanic systems in orange (Einarsson and Sæmundsson, 1987). In the en-
larged inset, main seismic and geological features of Katla. Red dots: epicentres before July 2011, located
inside the caldera and at Goðabunga. Black star: new cluster near Gvendarfell Ridge (GR) started in July 2011.
It is represented with a star because of its small size, derived from relative location (Sgattoni et al., 2016a,b).
Dashed lines: Katla and Eyjafjallajökull caldera rims. Open circles: ice cauldrons (Guðmundsson et al., 2007).
White areas: glaciers. To the NE, the location of Eldgjá fissure. Topography information from the National
Land Survey of Iceland. – Kort af eldstöðvakerfum Íslands. Stækkaða kortið til hægri sýnir skjálftavirkni á
Kötlusvæðinu og staði sem koma við sögu í greininni. Rauðir punktar sýna upptakastaði skjálfta fyrir 2011,
að mestu innan öskjunnar og í þyrpingu vestan Goðabungu. Svört stjarna sýnir þéttu skjálftaþyrpinguna við
Gvendarfell sem hófst í júlí 2011. Umfang hennar er minna en stjarnan. Slitnu línurnar sýna öskjur Kötlu og
Eyjafjallajökuls, litlir hringir sigkatla í yfirborði jökulsins. Jöklar eru sýndir með hvítum lit. Eldgjá, sem er
hluti eldstöðvakerfis Kötlu, er sýnd norðaustan Mýrdalsjökuls. Landslagsgögn eru frá Landmælingum Íslands.
occurred. But in July 2011, increased earthquake ac-
tivity occurred at Katla, together with a tremor burst
and a jökulhlaup, the latter causing considerable dam-
age to infrastructure. This unrest episode was sim-
ilar to two others that occurred in 1955 and 1999,
which some authors have interpreted as minor sub-
glacial eruptions (Thorarinsson, 1975; Guðmundsson
et al., 2007). However, the interpretation is controver-
sial, as no eruptive products were identified. The same
general controversy applies to the 2011 unrest. This
episode may have been related to either a geothermal
event with no magma involved or a small subglacial
eruption (Sgattoni et al., 2017).
The seismic activity at Katla in recent decades has
been concentrated in two main source areas, inside the
caldera and on the west flank, at Goðabunga (Fig.1b),
with the latter being much more active, at least un-
til the end of 2004 (Sturkell et al., 2006). Seasonal
patterns have also been reported with different fea-
tures for the two clusters (Einarsson and Brandsdóttir,
2000; Jónsdóttir et al., 2009). Another cluster has
been recently identified on the eastern flank of the vol-
cano by Jeddi et al. (2016). Clear changes in the seis-
micity at Katla occurred in July 2011. The 2011 un-
rest was associated not only with increased seismicity
inside the caldera, but also with the onset of new seis-
micity on the southern flank of the volcano, west of
the Gvendarfell ridge (GR in Figure 2), at the glacier
margin (Sgattoni et al., 2016b).
Attention has therefore been recently directed to-
wards the southern flank of Katla, which to date
has not been considered a source of serious volcanic
hazard, and thus its geology has been little studied.
Basaltic to intermediate hyaloclastites and lavas, to-
gether with rhyolitic domes were identified by La-
casse et al. (2007) and Jóhannesson and Saemundsson
54 JÖKULL No. 69, 2019