Jökull - 01.01.2014, Page 75

Earthquake Sequence 1973–1996 in Bárðarbunga volcano
Figure 5. Two possible fault movements that are consistent with the observed focal mechanisms and moment tensors of the
Bárðarbunga main events. To the left is an inflation model (resurgent caldera) with inward dipping caldera fault, and to the
right is a deflation model with outward dipping caldera fault. Identification of the fault(s) that slip within the volcano in the
main events is uncertain. Highly simplified tectonic picture of a central volcano with caldera is depicted, e.g. field observa-
tions in Iceland find caldera faults with near vertical dip. – Mögulegar hreyfistefnur á aðalmisgengi Bárðarbunguöskjunnar,
sem báðar geta skýrt brotlausnir og vægisþinur meginskjálfta í Bárðarbungu. Líklegast eiga meginskjálftar Bárðarbungu
upptök á hringlaga öskjumisgengi. Tvær ólíkar túlkanir á orsökum skjálftanna koma til greina. Ef misgenginu hallar inn á
við, er öskjuris líkleg orsök þeirra, en ef misgenginu hallar út, er líkleg orsök öskjusig. Athugið, að hér er dregin upp mjög
einfölduð mynd af tektóník megineldstöðvar með öskju. Jarðfræðilegar athuganir á Íslandi sýna t.d. að halli öskjumisgengja
er nærri lóðréttur.
mic activity of the 1996 event in the caldera region
and lack of such activity in previous main events. It
can only be speculated that in 1996 the loading force
had reached a maximum, and that the stress release
was less complete in the 1996 main event compared to
previous events, or that the ring fault had become sig-
nificantly weakened compared to before, possibly due
to magma lubrication with formation of a ring dyke
or cone sheets entering the fault zone. In case of such
complex geological events, the seismicity following
the 1996 main event consists only partly of true after-
shocks in the caldera region.
It is useful to compare source parameters of
the Bárðarbunga events, recorded by the ICEMELT
broadband seismic network in 1994–1996, with a cou-
ple of other events in the Iceland region recorded
by the same network, as well as the larger 1987
Vatnafjöll event (MW =5.9) (Bjarnason and Einarsson,
1991; Figure 1), recorded on a broadband seismo-
graph installed by the Carnegie Institution of Wash-
ington, USA, in Akureyri North Iceland (Evans and
Sacks, 1980).
One source parameter to be considered is the cor-
ner frequency of the seismic wave spectra, as the
Bárðarbunga events have anomalously low frequency
content or low corner frequency. In the Brune (1970)
earthquake model, corner frequency is proportional
to rupture velocity, which in turn is proportional to
the S-wave velocity of the ruptured material (Scholz,
1990). Corner frequency is also frequently related to
the concept dynamic stress drop of earthquakes, with
low dynamic stress drop correlating with low corner
frequency and slow rupture velocity. A vast literature
exists on that subject of earthquake stress drop. Sev-
eral authors have warned that there is a non-unique
relation between static stress drop (equation 1 in Ap-
pendix) and dynamic stress drop, derived from the
corner frequency of the earthquake spectra (Scholz,
1990; Atkinson and Beresnev, 1997). Stress drop es-
timate for an earthquake can differ by a factor of 4
to 5, especially if dynamic stress drop estimates are
mixed with static stress drop estimates.
From the observations presented on long source
duration and low corner frequency, it is concluded
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