Jökull - 01.01.2014, Síða 68

Ingi Þ. Bjarnason
In the routine location of earthquakes in the Vatna-
jökull region the IMO uses velocity models with ra-
tio VP /VS=1.78. Combining the velocity models of
Darbyshire et al. (1997) and Bjarnason and Schmel-
ing (2009) for Central Iceland, there is indication
that a ratio VP /VS ∼1.85 may be more appropri-
ate for the closer stations in the Central Iceland re-
gion. Such higher VP /VS ratio would, however, tend
to make the located depth of the earthquakes shal-
lower. If the depths of these deeper Bárðarbunga
earthquakes can be constrained, even at shallower
depth than currently located (i.e. >15 km), it will be
postulated that deeper earthquakes under Northwest
Vatnajökull are caused by fracturing of rocks around
the crust-mantle boundary and lower crust by ascend-
ing magma from the mantle, as occurred during the
Westman Islands eruption in 1973. There, however, a
temporary dense seismic net was installed during the
eruption and recorded well constrained earthquakes at
15–25 km depth under the eruption site, which are ex-
plained by magma induced strain release (Björnsson
and Einarsson, 1981; Einarsson 1991). It is unknown
if similar deep earthquakes may have occurred in the
1973–1996 earthquake sequence. However, the Sci-
ence Institute of University of Iceland operated ana-
logue seismic stations in Central Iceland during good
part of the years 1973–1996. One of these stations
was located within the 1.4 focal depth distance of po-
tential deep earthquakes in the Bárðarbunga area, as
well as one station in the ICEMELT network in the
years 1995–1996. It is conceivable, that the question
regarding deep earthquakes under Bárðarbunga can be
answered by analysing these old data. Monitoring
deep earthquakes under volcanic systems in Iceland
could become an important tool for volcanic hazard
prediction within intermediate time frame (years to
decades).
Focal mechanisms have been constructed for a
number of the Bárðarbunga earthquakes. The mech-
anisms indicate thrust faulting with a strike-slip com-
ponent, with vertical or sub-vertical T-axis (Einars-
son, 1991). Moment tensor solutions show also thrust
faulting with a significant non-double-couple com-
ponent (Ekström, 1994; Nettles and Ekström, 1998;
Tkalc̆ić et al., 2009). Rifting and transform are the
predominant tectonic motions in Iceland (Sæmunds-
son, 1979), and thrust faulting as indicated by small
and medium size earthquakes is not often observed in
the surface tectonics of the country (Gudmundsson et
al., 2008).
In 1994–1996 the ICEMELT digital broadband
seismic network (Bjarnason et al., 1996a; 1996b)
recorded the last three main events of the Bárðarbunga
sequence (Figure 4). Waveforms were similar in all
of them and characterised by emergent P waves and
large amplitude surface waves. The 1996 event was
clearly the largest of the three. They have in common
low corner frequency compared to a number of earth-
quakes in Iceland of similar size that have been exam-
ined (Table 2). The 1996 event has the lowest P wave
corner frequency (0.17±0.03 Hz) of the three events
with very low frequency P waves (≈0.2 Hz) arriving
approximately 0–3 s after the first motion (Appendix,
Figures A1 and A2). These low frequency P waves
are likely to be produced at or near to the source, as
they clearly arrive with the first motion on many of
the ICEMELT stations (Figure A2).
At the time of the 1996 main event in Bárðar-
bunga, the closest seismic station in the IMO- net-
work to the epicenter was at ∼100 km distance
(Jakobsdóttir, 2008). Hence the hypocenter depth of
the 1996 earthquake and most previous Bárðarbunga
main events are currently in general unconstrained in
the seismic bulletins, but some improvements may
be possible. All moment tensor inversions of the
1996 main event find the best fitting centroid depth at
3.5 km (Nettles and Ekström, 1998; Konstantinou et
al., 2003; Tkalc̆ić et al., 2009). In the following sec-
tion it will be argued that Bárðarbunga main events
are unusually shallow (<5 km) for earthquakes of in-
termediate size. Such a shallow depth for intermediate
earthquakes is unusual in a global perspective, and
can account for the unusual source properties of the
sequence.
DISCUSSION
The Bárðarbunga sequence is reasonably well docu-
mented, but the forces behind it are not well under-
stood. One can speculate that it is either of plate tec-
tonic or of localised magmatic origin, although the
68 JÖKULL No. 64, 2014