Jökull - 01.01.2014, Page 70

Ingi Þ. Bjarnason
two do not need to be decoupled in general. A plate-
tectonic origin could be in the form of ridge-push
driven tectonics, acting on the bend in the Central Ice-
land Rift at Bárðarbunga (i.e. the bend from SW-NE
strike to a SSW-NNE strike) (Figure 1). There are
several problems with this hypothesis. A ridge-push
that generates thrust faulting would probably be larger
outside the rift than within it, and evidence of regional
compression has not been observed in Iceland (Kho-
dayar and Einarsson, 2004). Rifting is also generally
perceived as a passive process, a passive opening due
to plate tectonics, that does not cause major thrust
earthquakes as occur in Bárðarbunga.
The evidence of a magmatic origin of the Bárð-
arbunga medium size earthquakes comes from local
seismic recordings. They tend to show relatively
emergent P and S waves with low corner frequency
and large surface waves that characterise volcanic
earthquakes (e.g. Lahr et al., 1994) (Table 2, Figure 4
and Appendix Figures A1 and A2). The tight cluster-
ing of the seismicity within the Bárðarbunga volcano
also suggests magmatic origin (Einarsson, 1991).
Kinematics of the Bárðarbunga 1973–1996 earth-
quake sequence
It is argued in the present communication that dur-
ing the largest earthquakes of the sequence (MW =5.4–
5.7), faults or fault patches of the size of ∼1/3 of Bárð-
arbunga caldera circumference are being moved. The
total circumference of the ice covered Bárðarbunga
caldera is estimated to be ∼30 km (Björnsson, 1988).
These fault lengths can be inferred from the size of the
earthquakes, assuming an average fault-slip-to-fault-
length ratio to be 10−4 to 10−5 (Scholz et al., 1986).
The length of the 1996 fault can also be inferred from
the distribution of aftershocks as located by Stefáns-
son et al. (1996). The whole aftershock sequence of
the 1996 event is complicated, and it is likely to be
generated by a number of processes, e.g. afterslip on
the main fault, slip on neighbouring faults, and possi-
bly lateral magma migration away from Bárðarbunga.
However, in the first 12 hours after the main event of
Sept. 29th 1996, many of the aftershocks were lo-
cated in a relatively narrow arch shaped region that
approximately follows the western half of the caldera
rim, indicating that a ∼12 km long fault segment rup-
tured on Sept. 29th 1996. The locations of the IMO
(Stefánsson et al., 1996) are not accurate enough to
determine if the caldera fault moved, or a concentric
fault to the west of it.
The Earth does often select pre-existing planes of
weakness for stress release, even when they are def-
initely outside the plane of maximum shear stress.
Cone sheets (inclined sheets) are common within
eroded central volcanoes in Iceland (e.g. Annells,
1968; Sigurdsson, 1970; Fridleifsson, 1973; Jóhann-
esson, 1975; Torfason, 1979; Fridleifsson, 1983; Siler
and Karson, 2009; Burchardt et al., 2011, Gudmunds-
son et al., 2014), and they may form arch shaped
planes of weakness. However, the size of the Bárð-
arbunga earthquakes makes the caldera ring fault per-
haps a more likely candidate. The third possibility is
shear fracturing during the formation of a cone sheet.
Anderson (1936) proposed that cone sheet formation
is pure tension fracturing, but Phillips (1974) con-
cluded that cone sheets occupy shear fractures. Gud-
mundsson (2002) studied thousands of cone sheets in
Iceland. He concluded that they are primarily ten-
sion fractures. However, Torfason (1979) has doc-
umented in South-East Iceland several instances of
shear movement across cone sheets usually with re-
verse sense of motion. Reactivated faulting on a cone
sheet, or cone sheet formation, as a possible source
of the Bárðarbunga earthquakes should therefore not
be disregarded (see an excellent discussion by Shuler
et al., 2013). However, if cone sheets are pure tension
fractures, then the moment tensor solutions of Bárðar-
bunga events (Nettles and Ekström, 1998; Konstanti-
nou et al., 2003; Tkalc̆ić et al., 2009) would probably
rule out cone sheets as their main source.
A fault slip of an area 10 km by 3 km, with the
shape of a steeply dipping cylindrical wall with cen-
troid depths of 1.5 km and 5.0 km depth, respectively,
allows us to calculate 23 to 66 cm and 13 to 36 cm
slip for earthquakes of magnitude 5.4–5.7 (MW ), re-
spectively. The assumed rigidity structure of Bárðar-
bunga for these calculations is derived from the VSV
velocity model of Bjarnason and Schmeling (2009)
for Central Iceland (10 GPa and 32.5 GPa at 1.5 km
and 5.0 km depths, respectively). Higher slip val-
ues would be obtained for cone shaped fault segments
70 JÖKULL No. 64, 2014