Jökull - 01.01.2014, Page 76

Ingi Þ. Bjarnason
that Bárðarbunga medium size earthquakes are highly
anomalous earthquakes due to their low stress drop
(see discussion in Appendix). There may be several
explanations to this in the case of the Bárðarbunga in-
termediate size earthquakes. The shallow depth of the
Bárðarbunga events is probably one of the principal
factors for their low stress drop. It follows from equa-
tion 1 (in Appendix), that for two earthquakes with
equal moment, the earthquake in higher shear mod-
ulus regions (i.e. usually relatively deeper source),
would tend to have higher stress drop, given same
fault geometry. At shallow depth the material fric-
tion is smaller than at greater depth, and hence the
loading force or stress needed for slip is smaller, but
loading force is proportional to stress drop (Scholz,
1990). Low stress drop events may have relatively
longer principal fault dimension than a higher stress
drop event of same magnitude, and or lower slip. The
slip of the 1973–1996 sequence events is not con-
strained, but the ∼12 km fault length of the 1996 event
is comparable to the larger Vatnafjöll event (Figure 1).
The reason for the difference in stress drop character
between the Vatnafjöll and Bárðarbunga events is per-
haps best explained by different loading mechanism.
In the former case it is plate tectonic force acting on
the entire crust, which probably holds strongest in the
brittle part of the lower crust. In the latter, however,
magma buoyancy acts upon the relatively weak up-
per crust, causing low stress drop earthquakes. This
may also explain the small aftershock activity in most
of the Bárðarbunga events; stress relaxation may be
more complete within this low stress environment.
The shallow depth of the fault slip of the Bárð-
arbunga events suggests that the loading force is also
shallow, like an increased pressure in shallow magma
chamber postulated by Nettles and Ekström (1998).
However, if the loading force originates from a greater
depth, the Bárðarbunga events may signify breaking
of shallow asperities on deeper extending well lubri-
cated faults. This has been discussed by Das and Kost-
rov (1986). Therefore, it is not certain whether the
loading force of the shallow Bárðarbunga intermedi-
ate size earthquakes originate from a shallow magma
chamber in the upper and/or middle crust, or from a
deeper magma reservoir in the lower crust.
CONCLUSION
The 1973–1996 Bárðarbunga sequence of intermedi-
ate size earthquakes is interpreted as being magmatic
induced, caused by mantle derived magma seeping
into the volcano. It led to increased pressure and lift of
the caldera block with reactivated slip on shallow ring
fault patches dipping towards its centre. On Sept. 29–
30th 1996, the pressure inside the magma reservoir
exceeded the lithostatic pressure, probably causing
lateral dyke formation resulting in large magma pres-
sure increase in the neighbouring region for the first
time in the 1973–1996 earthquake sequence. The in-
creased pressure led to volcanic eruption on the sub-
glacial volcanic ridge Gjálp, in NW Vatnajökull on
Sept. 30th. The eruption may have caused a large pres-
sure drop in Bárðarbunga and neighbouring regions,
judged by lowered seismicity in the NW Vatnajökull
area during the 8 years following the eruption.
The loading force of the shallow Bárðarbunga
main events may be due to increased pressure in a
shallow magma chamber, or it may be due to in-
creased magma pressure at greater depth.
Acknowledgements
Thanks are due to US NSF, The Icelandic Science
Foundation, the University of Iceland Research Fund,
The National Power Company and The State Road
Department for financial support. I thank The Ice-
landic Meteorological Office and Páll Einarsson for
supplying earthquake data. The expert assistance of
Bergþóra S. Þorbjarnardóttir and Bryndís Brandsdótt-
ir with graphical work is greatly appreciated, using the
Generic Mapping Tools software (Wessel and Smith,
1998). The work benefited from discussion with
Bergþóra S. Þorbjarnardóttir, Selwyn Sacks, Hauk-
ur Jóhannesson, Guðmundur Ó. Friðleifsson, Ágúst
Guðmundsson, Helgi Torfason, Hjalti Franzson, Ing-
var B. Friðleifsson, Steffi Burchardt, Martin Hensch,
Helgi Björnsson and Karl Grönvold and from com-
ments on the manuscript made by Ólafur Grímur
Björnsson, Sveinbjörn Björnsson and Leó Kristjáns-
son. The manuscript was critically reviewed by Páll
Einarsson and an anonymous reviewer. Special thanks
to Kristján Sæmundsson for most constructive discus-
sions on the subject and comments on the manuscript.
76 JÖKULL No. 64, 2014