Jökull - 01.01.2019, Blaðsíða 74

Variation in b-value of caldera earthquakes of Bárðarbunga Volcano
caldera collapse 2014 to 2015 (Gudmundsson et al.,
2016), and re-inflation of the volcano, 2015 to present
(Sigmundsson et al., 2018). In this paper we test the
proposition that this last change is in some way re-
flected in a changing b-value.
RECENT ACTIVITY OF THE
BÁRÐARBUNGA VOLCANO
Bárðarbunga is located in Central Iceland, near the
center of the Iceland Hotspot (Wolfe et al., 1997),
at a triple junction where three branches of the Ice-
landic plate boundary meet (e.g. Einarsson, 2008).
Due to its remoteness Bárðarbunga was not discov-
ered as an active volcano until 1973 when the first
satellite images of Iceland became available (Thorar-
insson et al., 1973) and a caldera structure was evi-
dent beneath the Vatnajökull glacier. The caldera was
found to be the central element in an extensive vol-
canic system, which fissure swarms extend 100 km to
the SW and at least 60 km to the NE (e.g., Sæmunds-
son, 1978; Einarsson and Sæmundsson, 1987; Hjart-
ardóttir et al., 2016a). The caldera is about 700 m
deep and filled to the rim by glacier ice (Björnsson,
1988). The caldera and the NE fissure swarm were
found to be seismically quite active (Björnsson and
Einarsson, 1990; Einarsson, 1986, 1991a). Studies of
tephra deposits revealed large historic and prehistoric
fissure eruptions in the fissure swarms, the SW swarm
in particular (Larsen, 1984). The largest known is
the Thjórsá Lava of South Iceland, a 20 km3 vol-
ume lava flow emplaced around 8000 a (e.g., Hjart-
arson, 1994). Lava flows from the Bárðarbunga vol-
canic system have reached the south shore of Iceland
and almost to the north shore as well (Hjartarson and
Sæmundsson, 2014; Svavarsdóttir et al., 2017). No
other Icelandic volcano has spread lavas as widely as
this. Large recent eruptions include the Vatnaöldur
eruption of ∼870, Veiðivötn eruption ∼1480, and
Tröllahraun eruption of 1864–1866 (Larsen, 1984),
all within the SW fissure swarm, a part of the East-
ern Volcanic Zone in South Iceland.
Earthquake monitoring of the Bárðarbunga area
goes as far back as 1975, when the first stations in a
country-wide seismic network were installed in North
Iceland (Einarsson and Björnsson, 1987) and epicen-
tral locations became sufficiently accurate to separate
the activity of different volcanoes in the area. The ac-
curacy increased significantly in 1985 when teleme-
tered stations were installed in Central Iceland. A
new digital network replaced these analog stations
in 1990–1994, again increasing the detectability of
events in Central Iceland. All this time it was clear
that the caldera region of Bárðarbunga was seismi-
cally active (e.g., Björnsson and Einarsson, 1990;
Einarsson, 1991a; Jakobsdóttir, 2008) and was go-
ing through different phases of activity. The period
1974–1996 was particularly active. About 15 earth-
quakes of magnitude 5 and larger occurred at regular
intervals during this time but had been unknown be-
fore that. Their focal mechanisms had a large com-
ponent of reverse faulting, interpreted by Einarsson
(1986) to be the result of slow deflation of the vol-
cano. Other authors considered this to be due to infla-
tion of the volcano (Zobin,1999; Nettles and Ekström,
1998; Bjarnason, 2014). This activity ended abruptly
in 1996 when the Gjálp eruption occurred, a fissure
eruption about 15 km south of Bárðarbunga (Einars-
son et al., 1997; Gudmundsson et al., 1997). The ac-
tivity remained relatively low for several years but be-
gan to increase slowly in 2002 (Jakobsdóttir, 2008).
A new phase began in August 2014 when a dike
started propagating away from the volcano and the
caldera floor subsided. The dike propagated 48 km
laterally for two weeks, accompanied by earthquake
activity and graben formation until a lava eruption be-
gan from its distal end (Sigmundsson et al., 2015;
Hjartardóttir et al., 2016b; Ágústsdóttir et al., 2019).
The eruption continued for six months and produced
1.4 km3 of lava. This was accompanied by a slow
collapse of the Bárðarbunga caldera, a total of 65 m,
and a series of earthquakes there, at least 70 of which
were of magnitude 5 and larger (Guðmundsson et
al., 2016). Following the end of the eruption the
earthquake activity became relatively low for several
months. But in the fall of 2015 the earthquake activity
in the caldera increased again and continued at a con-
stant rate for about two years, then slowly decreasing.
The largest events of this last period exceeded magni-
tude 4, but were smaller than 5.
JÖKULL No. 69, 2019 73