Jökull - 01.01.2014, Blaðsíða 82

Ingi Þ. Bjarnason
Figure A1. Waveform of the Bárðarbunga main
event on Sept. 29th, 1996, recorded on vertical
component at ICEMELT broadband seismic sta-
tion SKOT at epicenter distances 77 km. A 9 sec-
ond long window is shown. – Bylgjugögn meg-
inskjálfta í Bárðarbungu 29. sept. 1996, skráð á
lóðréttan þátt stöðvar í Svartárkoti í ICEMELT
breiðbandsskjálftanetinu í 77 km fjarlægð frá
skjálftaupptökum. Tímagluggi er 9 sekúndur. Sjá
staðsetningar skjálftamæla á 1. mynd.
−2
0
2
SKOT
Z
−2
0
2
KAF
Z
−2
0
2
LJOP
Z
−2
0
2 HOFF Z
−2
0
2 BLOL Z
−1
0
1
ASBS
Z
0.0
0.5
1.0
1.5
0 10 20 30 40 50 60 70 80 90 100 110 120
seconds
REYV
Z
Figure A2. Waveforms of Bárðarbunga main
event on Sept. 29th 1996, recorded at ICEMELT
broadband seismic stations (see Figure 1 for
station locations). Two minutes long, verti-
cal component (Z) records are shown. Epicen-
tral distances increase from the top record and
down. – Bylgjugögn meginskjálfta í Bárðarbungu
29. sept. 1996, skráð á lóðrétta þætti stöðva í
breiðbandsskjálftanetinu ICEMELT. Athugið, að
skali á útslagi (y-ás) er breytilegur milli stöðva,
og fjarlægð þeirra frá upptökum eykst frá efsta
línuriti og niður.
tio Vr/β. Equivalent form exists for S-wave, but with dif-
ferent scaling constant. Combining the relation of moment
and stress drop for a circular crack (e.g. Madariaga, 1980),
Mo =
16
7
∆σdynR
3 (eq. 3), with eq. 2. gives ∆σdyn =
7
16
(2π/Cpα)
3Mo〈fp〉3 (eq. 4). It is informative to evaluate
what effect uncertainty in the centroid depth (or the Vr/β
ratio) of Bárðarbunga events, or what affect the scale factor
Cpα in eq. 4, might have on stress drop calculations in the
Bárðarbunga region. The evaluation shows that variations
in Cpα for plausible values of Vr/β in the range 0.7–0.9 do
not (<2%) affect the stress drop significantly, therefore Vr/β
is constrained at 0.9.
The dynamic stress drop (eq. 4) is calculated for a pair
of tectonic earthquakes in Iceland and a pair of Bárðarbunga
earthquakes, for which the moment magnitude is available
(Table 2). The velocity models used for these calculations
are from Bjarnason and Schmeling (2009) for the Bárðar-
bunga events, and for the other events the standard earth-
quake location model for Iceland (SIL model), that was con-
structed from the work of Bjarnason et al. (1993). The stress
drops of both Bárðarbunga earthquakes analysed here are
exceptionally low, 5±3 bars in 1994 and 4±2 bars in 1996.
At the time of the Vatnafjöll earthquake, there was only
one broadband seismic station operating in Iceland; hence
the corner frequency is estimated only from one direction,
which increases the uncertainty of the estimate. However,
based on the previous work of Bjarnason and Einarsson
(1991) and calculation of 120 bars (-60/+90 bars) stress it
is concluded that the Vatnafjöll earthquake was a high stress
drop event. The event off-shore Skagafjörður in 1994 had
15±2 bars stress drop, which is below global average stress
drop but not unusual (Allmann et al., 2009).
82 JÖKULL No. 64, 2014