Jökull - 01.01.2013, Side 27

Recent fault movements in the Tungnafellsjökull fissure swarm
Structures indicating recent movements
Figure 7 shows three different types of structures that
provide evidence of recent movements in the fissure
swarm of Tungnafellsjökull. Type 1 (Figure 7a) shows
a step in a glacially eroded area. The step in the glacial
ground moraine that normally has no obvious fea-
tures indicates movements on faults in the Holocene.
Type 2 (Figure 7b and c) shows sinkholes in glacial
moraine. Sinkholes form when there is movement
on faults or fractures and loose surface material is
washed into the underlying fracture. This is clear indi-
cation of movements in the Holocene since the Pleis-
tocene ice sheet can be assumed to have left fissures
packed with debris. Type 3 (Figures 7d, e and f) show
sinkholes and fractures that bear obvious sign of very
recent movements. The fresh wounds in the rim of
the sinkholes (Figures 7d and e) and in the edge of the
fracture (Figure 7f) indicate movements as recent as
the spring of 2010 since they cannot be expected to
survive the spring thaw. These features are not com-
monly observed in the rift zones except in the areas
were very recent movements have taken place. A re-
connaissance in 2010 of fractures and sinkholes pro-
duced during the Krafla rifting episode in 1975–1984,
e.g. did not reveal structures as fresh-looking as these.
Earthquake activity
Seismic activity at Tungnafellsjökull is relatively low
(Figure 8) compared to many other volcanic systems
(Einarsson, 1991; Jakobsdóttir, 2008). The earth-
quake epicenters form a diffuse pattern, which is not
explained by uncertainty of epicentral determination.
A plot of cumulative seismic moment for the years
1995–2011 (Figure 9) shows 3 earthquake swarms
that may have been associated with surface move-
ments in faults and fractures. The first swarm oc-
curred during the Gjálp eruption in October 1996, the
second in August 2008 and the third in November
2009. The August 2008 swarm was the smallest event
of the three. InSAR detected surface movements,
which coincided with the 1996 earthquake locations
(Pagli et al., 2007) most likely mark the time of the
movements observed in the area as InSAR data from
April 2004 to September 2010 do not provide coher-
ent results (Amandine Auriac, pers. comm. 2011).
None of the earthquakes is large. If the total seis-
mic moment was released in one earthquake its mag-
nitude would be only 3.4. The length of the three
fractures identified by the InSAR study of Pagli et
al. (2007) was in the range 3–4 km and the displace-
ments of the order of one fringe, i.e. about 3 cm. If
converted to seismic moment this corresponds to one
earthquake of magnitude 5.0. Additional displace-
ments since the study of Pagli et al. (2007) would add
to the size of that event. There is clearly a discrepancy
between the observed seismic moment release and the
surface faulting. This discrepancy suggests that the
fault movements are of magmatic origin rather than
purely tectonic (see e.g. Pedersen et al., 2007).
DISCUSSION
Research on the fissure swarms of the volcanic sys-
tems in the Central Iceland Volcanic zone has been
more limited than in other volcanic zones. Some of
the rift zone branches have been studied more ex-
tensively, e.g. the Northern Volcanic Zone (Hjartar-
dóttir et al., 2009; Hjartardóttir and Einarsson, 2012,
Hjartardóttir et al., 2012), and the Reykjanes oblique
rift (e.g. Clifton and Kattenhorn, 2006). The North-
ern Volcanic Zone, on the divergent boundary, is
characterized by fissure swarms, approximately 5–
20 km wide and 40–120 km in length, each extend-
ing through a specific central volcano. The fissure
swarms consist of volcanic fissures, faults and frac-
tures. Volcanic fissures are usually dominant near the
central volcano, but non-eruptive fractures become
more frequent further away from the central volcano.
The activity of the fissure swarms seems to be high-
est in the middle and their altitude is higher there
than elsewhere (Hjartardóttir et al., 2009; Hjartardótt-
ir and Einarsson, 2012). These characteristics can also
be found elsewhere in the world for example in the
Northern Main Ethiopian Rift (NMER) in East Africa.
The NMER contains 4 tectono-magmatic segments.
Each segment consists of a volcanic center and rift
tips characterized by brittle deformation. The area
between the central volcano and the tips is similar
to the fissure swarms in the Northern Volcanic Zone
in Iceland. Nearest to the central volcano the defor-
mation is mostly magmatically induced but moving
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