Jökull - 01.01.2005, Page 113

Seismic activity in Iceland during 2004
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Grímsvötn, M > 0
Grímsvötn, M > 1
Bárðabunga, M > 1
Figure 5. a) A graph showing cumula-
tive number of earthquakes in Gríms-
vötn from January 1997 to December
2004 for Mlw ≥ 1 and a magnitude
vs. time plot below. The inserted fig-
ure shows the cumulative number of
earthquakes for Mlw ≥ 0 for a shorter
period, i.e. from August until mid
November 2004. The three vertical
lines mark the detection of the water
leakage from Grímsvötn at 28 Octo-
ber, the beginning of the jökulhaup at
29 October and the beginning of the
eruption at 1 November. b) A graph
showing cumulative number of earth-
quakes in Bárðarbunga from January
1997 to December 2004 for M lw ≥ 1,
and a magnitude vs. time plot be-
low.– a) Uppsafnaður fjöldi skjálfta í
Grímsvötnum frá 1997 til ársloka 2004
fyrir Mlw ≥ 1, ásamt stærð skjálfta
sem fall af tíma fyrir sama gagnasafn.
Á innfeldu myndinni má sjá uppsafn-
aðan fjölda skjálfta fyrir Mlw ≥ 0 frá
ágúst fram í miðjan nóvember 2004.
Lóðréttu línurnar þrjár sýna hvenær
vísbendingar komu fram um rennsli
úr Grímsvötnum 28. október, upphaf
flóðsins þann 29. og upphaf gossins
1. nóvember. b) Uppsafnaður fjöldi
skjálfta í Bárðarbunga frá 1997 til
ársloka 2004 fyrir Mlw ≥ 1, ásamt
stærð skjálfta sem fall af tíma.
verified by P-wave particle motion from a few earth-
quakes recorded at the grf seismic station.
After the beginning of the eruption, which lasted
for six days, only 12 earthquakes were located at
Grímsvötn through the remainder of 2004. Two days
before the eruption, water began to drain from Gríms-
vötn, resulting in a jökulhlaup from Skeiðarárjökull
(Harðardóttir et al., 2004). However, analysis of
tremor signals from 28 October suggests that a leak
was already established (Figure 5a). It has been
suggested (Sigmundsson and Guðmundsson, 2004)
that the pressure decrease over the magma chamber
caused by the water leaking from Grímsvötn trig-
gered the eruption. During the jökulhlaup, twenty-
nine locatable icequakes were detected in Skeiðarár-
jökull (Vogfjörd et al., 2005b) and the glacier moved
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