Jökull - 01.01.2014, Side 71

Earthquake Sequence 1973–1996 in Bárðarbunga volcano
with the same area, located at similar depths, because
their centroid depth would tend to be shallower (i.e.
higher proportion of the fault lies at a shallower depth
with lower average rigidity). Assuming that displace-
ments of the Bárðarbunga main earthquakes are accu-
mulative, and by connecting their mb and MW magni-
tudes with a linear relationship, the accumulated mo-
ment can be calculated and related to the dimensions
of the Bárðarbunga volcano. If for such an exercise,
the total slip is distributed along the entire caldera rim
fault (i.e. 30 km long and 1 km wide fault with cen-
troid depth of 1.5 km), a ∼9.5 meters total displace-
ment is calculated with corresponding volume change
of ∼0.7 km3 of the caldera. If, however, the centroid
depth is at the lower crustal boundary of 4–5 km depth
(Darbyshire et al., 1998; Bjarnason and Schmeling,
2009), the accumulated slip on the same fault geome-
try would be ∼3.0–3.5 meters with a volume change
of 0.2–0.25 km3. These numbers are likely to be min-
imum estimates, because earthquakes of lower size
than 4.5 do not enter the calculation, and part of the
deformation is most likely aseismic.
The above calculated volume changes can be com-
pared with results from Árnadóttir et al. (2009) who
carried out country wide GPS measurements in Ice-
land over the time period 1993 to 2004. These re-
searchers observed a significant uplift (∼8–18 mm/yr)
of a broad area of Central and Southeast Iceland,
which they modelled with glacial isostatic adjust-
ments due to recent thinning of the largest glaciers
in the country. In spite of relatively coarse GPS
measurements around the Vatnajökull glacier, they
do model a net ∼0.1 km3 volume contraction under
Bárðarbunga during the interval of observations, car-
ried out in 1993 and 2004, respectively. Due to lack of
temporal resolution in the GPS data, their study can-
not resolve a possible variation in volume change be-
fore the 1996 Bárðarbunga main event and the Gjálp
eruption, and a post eruption volume change, making
comparison somewhat limited.
The moment tensors of the Bárðarbunga earth-
quakes that have been determined have a large non-
double-couple component that could be consistent
with earthquakes on circular faults or a collection of
fault surfaces that form a circular assemblage (Ek-
ström, 1994; Nettles and Ekström, 1998; Konstan-
tinou et al., 2003; Tkalc̆ić et al., 2009). Nettles
and Ekström (1998) and Tkalc̆ić et al. (2009) inter-
pret moment tensors to show that the main motion
is subsidence on outward-dipping fault (with respect
to the volcano), while Bjarnason and Þorbjarnardóttir
(1996) interpreted the main motion to be an upward
movement on inward dipping fault (Figure 5). Full
moment tensor solution of Konstantinou et al. (2003)
for the 1996 event resolved implosive isotropic com-
ponent, with normal faulting, in contrast to the thrust
faulting determined by Nettles and Ekström (1998),
Tkalc̆ić et al. (2009) and by Einarsson (1991) for
previous Bárðarbunga events. However, Tkalc̆ić et
al. (2009) show with synthetic waveforms how data
noise and slight error in velocity structure can lead to
false isotropic component. Assuming that the main
motion of the Bárðarbunga intermediate earthquakes
is due to thrust faulting, then this motion can be in-
terpreted as pure volcano deflation (Einarsson, 1991),
or inflation (Bjarnason and Þorbjarnardóttir, 1996), or
a combination of both (Nettles and Ekström, 1998;
Tkalc̆ić et al., 2009). Without further information, i.e.
on the dip of the faults, or detailed geodetic measure-
ments of the volcano, which were not carried out at
the time, a clear cause cannot be fully constrained. It
is conceivable that the dip of the active faults can be
determined, e.g., with relative locations of the 1996
aftershocks. However, the number of recorded after-
shocks may not be high enough to allow for such an
analysis. There is also uncertainty in identifying the
true aftershocks. Other geological events may have
occurred after the main event, e.g. formation of a ring
dyke or cone sheets in the caldera area, inducing seis-
micity outside the main fault.
Structures of Icelandic calderas
Of paramount value is to gain information on the
dip of the faults that have moved during the Bárðar-
bunga events, in order to understand the nature of their
sources. Cone sheets generally dip towards the centre
of volcanoes. There is a consensus that the drop of
a piston type caldera (i.e. drop with relatively intact
caldera floor) is accommodated on a steeply dipping
near vertical ring fault. However, there is a lack of
consensus on the general direction of the dip of the
JÖKULL No. 64, 2014 71