Jökull - 01.01.2014, Síða 61

Reviewed research article
Earthquake Sequence 1973–1996 in Bárðarbunga Volcano:
Seismic Activity Leading up to Eruptions
in the NW-Vatnajökull Area
Ingi Þorleifur Bjarnason
Institute of Earth Sciences – Science Institute, University of Iceland, Sturlugata 7, 101 Reykjavík, ingib@hi.is
Abstract — A day and a half after the earthquake (mb=5.3, MS=5.6, MW =5.6) in the Bárðarbunga central
volcano on Sept. 29th 1996, a volcanic eruption broke out under the Vatnajökull glacier. The eruption was
located approximately 20 km SSE of the earthquake epicenter, midway between the Bárðarbunga and Gríms-
vötn central volcanoes. Course of events suggests a connection between earthquake and eruption and therefore
a connection with a sequence of earthquakes of the same characteristics in Bárðarbunga during the years
1973–1996. The earthquakes in question are of an unusually low frequency character (corner frequency),
explained by exceptionally low dynamic stress drop (< 10 bars) at shallow depth (≤5.0 km). The sequence
which lasted for 22 years is characterised by ∼annual main events of magnitudes in the range of 4.5–5.7 (mb).
It intensified in the 1990s, with some of the largest earthquakes of the whole episode occurring at that time.
Moment tensor solutions of teleseismic signals and locally recorded waveforms reveal that the main events
are thrust faulting earthquakes with a significant non-double couple component. Arguments are presented that
the faulting occurred on a steeply inward dipping caldera fault, with reactivated motion on a weak fault. As
a consequence of this hypothesis magma inflation in Bárðarbunga is the most probable cause of the 1973–
1996 events. However, the loading force (the magma) may or may not have resided at a similar shallow depth
as the earthquakes. Cast in the frame of the inflation model, the Bárðarbunga 1973–1996 sequence implies a
resurgent caldera of at least 0.2–0.7 km3 for approximately a quarter of a century, exceeding its magma storage
capacity in 1996. However, these calculations are model dependent. Bárðarbunga and neighbouring area were
relatively calm during the period mid-1997 to 2004. There was a renewed activity of small earthquakes during
the years 2005–2009. From the beginning of continuous seismic recording in Iceland in 1925, all eruptions in
Vatnajökull on record have been accompanied with earthquake(s) of magnitude ≥4.0, within two months of the
initial eruption.
INTRODUCTION
In 1973 an earthquake sequence started in Bárðar-
bunga, a central volcano under the North-Western part
of Vatnajökull glacier (Figures 1 and 2). The sequence
comprises a series of 20 main events of mb magni-
tudes in the range of 4.5–5.7; shocks occurring once
a year on average (Table 1). The last main event
of this sequence occurred at 10:48:17.09 (GMT) on
Sept. 29th 1996. A day and a half later an eruption
broke out under Vatnajökull, ∼20 km SSE of the main
event, midway between the Bárðarbunga and Gríms-
vötn central volcanoes. The eruption was named the
Gjálp-eruption (Einarsson et al., 1997; Guðmundsson
et al., 1997). The Bárðarbunga and Grímsvötn central
volcanoes and their associated fissures are among the
most active volcanic systems in Iceland. They have
a history of causing volcanic disasters in the form
of widespread poisonous gases, as well as produc-
ing the largest lava flow in historic time on earth, the
1783–1784 Laki eruption (the Skaftár fires), and large
JÖKULL No. 64, 2014 61