Jökull - 01.01.2013, Side 30

Þ. Björnsdóttir and P. Einarsson
Figure 9. Cumulative seismic
moment around Tungnafells-
jökull in the period 1996–
2011. Most of the mo-
ment release occurred in Oc-
tober 1996, August 2008 and
November 2009. – Uppsafn-
að skjálftavægi við Tungna-
fellsjökul 1996–2011. Mest
skjálftavægi losnaði í hrinum
í október 1996, ágúst 2008 og
nóvember 2009.
from the center to the tips the deformation changes
to brittle. In the fissure swarm one can find open fis-
sures, aligned basaltic cones and faults (Kurtz et al.,
2007). The fissure swarm of Tungnafellsjökull bears
some of these characteristics, but nevertheless appears
to be a little different. Although the volcanic system
of Tungnafellsjökull is situated at the plate bound-
ary, activity in the system has been scarce for the last
ca. 10,000 years. The fissure swarm is unusually wide
and short. This is one of the reasons we prefer to clas-
sify the Tungnafellsjökull system with the Central Ice-
land Volcanic Zone rather than the much more active
Eastern Volcanic Zone.
CONCLUSIONS
1. Ground checks of fissures and faults in the summers
of 2009 and 2010 revealed evidence of recent move-
ments in the Tungnafellsjökull fissure swarm, consis-
tent with InSAR studies of Pagli et al. (2007). Some
of the evidence strongly suggest that movements oc-
curred as recently as the spring of 2010.
2. Three types of fault structures related to movements
could be differentiated. Type 1 is a step in a glacial
ground moraine that normally doesn’t have any ob-
vious features. The step thus indicates Postglacial
movements. Type 2 and 3 are sinkholes or fractures
on the ground. Both types indicate Postglacial move-
ments, but type 3 has open and fresh wounds in the
rim of the sinkholes and the edge of the fractures indi-
cating movements during the last year, even as recent
as the last thaw.
3. Study of the seismic activity in the period 1996 to
2011 reveals 2–3 events or episodes during which the
movements may have taken place. The first episode
was in 1996, during the Gjálp eruption in Vatnajökull,
which was located about 35 km away from the survey
area. This event was detected in SAR interferograms
by Pagli et al. (2007) and probably had the largest in-
fluence on the faults in the fissure swarm. The other
events are smaller, one in August 2008 and the other
in November 2009.
4. The cumulative seismic moment of all these seis-
mic events amounts to that of a single earthquakes of
magnitude 3.4. The total geometric moment of the ob-
served displacements, on the other hand, is equivalent
to a magnitude of at least 5.0 if released in one earth-
quake. This discrepancy suggests that the fault move-
ments were not purely of tectonic origin, but rather
associated with magma movements at depth.
5. The fissure swarm of the Tungnafellsjökull volcanic
system is relatively short (40 km) and wide (20 km),
30 JÖKULL No. 63, 2013