Jökull - 01.01.2014, Blaðsíða 74

Ingi Þ. Bjarnason
shallow magma chamber. Tkalc̆ić et al., (2009) pro-
pose two models for the 1996 event: a) a complex
magma chamber, where volume decreases at the bot-
tom of the chamber, but increases at the top of it,
or b) volume loss in a magma chamber by opening
of a dyke above it. Both models of Tkalc̆ić et al.,
(2009) are constrained with no net volume change so-
lution of the moment tensor under Bárðarbunga, with
a 2/3 part of the moment as non-double compensated-
linear-vector-dipole.
The renewed activity in Bárðarbunga in the sec-
ond half of the year 2014 does give a hint of the driv-
ing force of the earthquake sequence in 1973–1996. In
the 2014 episode, GPS measurements of Bárðarbunga
volcano show high rate vertical subsidence of the
caldera floor, ∼1.0 m/day during the first few weeks
of the episode (Sigmundsson et al., 2015). All mo-
ment tensor solutions of a series of intermediate size
earthquakes within the Bárðarbunga 2014 episode,
calculated from data recorded on international seismic
networks, show predominantly normal faulting, with a
large non-double-couple component (Global Moment
Tensor Program by Ekström et al., 2012; GEOSCOPE
by Vallée et al., 2011; GEOFON Program, 2014).
These observations suggest a correlation between the
non-double-couple normal faulting and caldera floor
subsidence. The waveform characteristics of the cur-
rent intermediate earthquakes are the same as pre-
viously described for the 1973–1996 earthquake se-
quence, except presumably for the direction of motion
as determined by the international moment tensor so-
lutions (e.g. the seismic station BORG IRIS/IDA in
West Iceland, of the Global Seismic Network). There-
fore, it is concluded that similar fault patches are mov-
ing in the 2014 episode as in the 1973–1996 sequence,
but with opposite sense of motion. This supports the
hypothesis that the 1973–1996 sequence was due to
uplift of the caldera block, possibly piecemeal uplift
of different parts of the block, due to increased pres-
sure inside the volcano.
Einarsson (1991) proposed magma deflation in
Bárðarbunga to be the cause of the thrust earthquake
sequence. He observed a correlation between the
Bárðarbunga main events and magma activity dur-
ing the 1975–1984 volcanic episodes of the Krafla
central volcano, located 110 km north of Bárðar-
bunga. He proposed a pressure connection between
the two volcanoes, along a hypothesised partially
molten layer under Iceland. Magma flow into the
Krafla magma chamber would thus cause pressure de-
crease and eventual collapse of the caldera floor in
Bárðarbunga. The volcanic inflation of Krafla ceased
in 1984, but the Bárðarbunga events continued until
1996, undermining the proposed mechanism. An al-
ternative deflation model can be suggested in which
the magma reservoir of Bárðarbunga is filled with
magma from the mantle and partially emptied with
subsurface lateral magma ejection, occurring periodi-
cally for 22 years, until Sept. 1996, when it reached
the surface through weak zones of the region. An ar-
gument against this hypothesis is lack of observation
of clear intrusion tremors, or other seismic activity
that can been associated with magma injection into
neighbouring regions before or after each of the Bárð-
arbunga main events. Such major seismic activity was
only observed after the 1996 main event.
The driving force of an inflation model is ascend-
ing magma from the mantle that gradually saturates
the storage capacity of the magma reservoir under
the volcano. However, instead of a dominant lateral
magma ejection when critical pressure is reached in-
side the magma chamber, the pressure lifts the caldera
block. The main earthquakes occur when cylindri-
cal faults (e.g. the caldera fault), that dip to the cen-
tre of the volcano, fail (Bjarnason and Þorbjarnar-
dóttir, 1996). Immediately following the earthquake
the pressure is decreased due to the increased volume
of the volcano. The magma does therefore probably
not flow out of the volcano in large quantities unless
a dyke intrusion opens up volume outside the vol-
cano, to the surface or subsurface, or if there is a rel-
atively quick pressure increase after the earthquake.
Such an increase in pressure can result from gas bub-
bles, rising up through the magma, causing a sudden
pressure increase in the magma chamber (Linde et al.,
1994). Increased pressure of this kind could explain
the hypothesised flow of magma out of magma satu-
rated Bárðarbunga volcano, following the main earth-
quake of 1996. Neither the inflation nor deflation
models do, however, explain the contrast in after seis-
74 JÖKULL No. 64, 2014