Jökull - 01.12.2003, Síða 29

The 1783–1785 Laki-Grímsvötn eruptions
activity that was followed by a longer-lasting phase of
quiet lava effusion (i.e., Helland, 1886; Thoroddsen,
1879, 1894, 1925; and Thorarinsson, 1967, 1969).
However, studies of the proximal tephra strati-
graphy have shown this view to be oversimplistic
and that the Laki eruption featured at least 10 dis-
tinct eruption episodes (Thordarson and Self, 1993).
Furthermore, the Grímsvötn volcano erupted intermit-
tently from June 1783 through May 1785, adding at
least four eruption episodes to the Laki-Grímsvötn
event (Table 2). These results are substantiated by the
contemporary descriptions (Thordarson, 1990, 1991),
which provide strong evidence for the episodic nature
of the Laki eruption as indicated by repeated occur-
rences of earthquake swarms, explosive activity and
lava surges. The nature and association of these phe-
nomena, as revealed in the contemporary accounts,
will now be considered in more detail.
SEISMIC ACTIVITY
Three facets of the seismic activity associated with the
Laki eruption have important implications. Firstly, the
contemporary accounts only document earthquakes
that were strong enough to be felt by the inhabi-
tants at a distance of 30–80km from the Laki fissure.
Secondly, they indicate that earthquakes occurred in
swarms that often lasted for days. Thirdly, these earth-
quake swarms commonly preceded or coincided with
the onset of explosive activity and increased lava pro-
duction at the vents (Table 2).
The evidence indicates that the earthquakes asso-
ciated with the Laki eruption were readily detected at
distance of >30 km from source vents, implying that
their magnitude was ≥4 on the Richter scale (Brands-
dóttir, 1992). Consequently, rifting of brittle crust
rather than volcanic tremor (sensu stricto) is a more
likely source for these earthquakes.
Also, despite a significant difference in peri-
odicity, the episodic occurrence of intense earth-
quake swarms in association with increased activity
at the Laki fissures shows a strong resemblance to
the pattern observed in recent volcano-tectonic rift-
ing episodes in Northern Iceland (Bjornsson et al.,
1977; Brandsdóttir and Einarsson, 1979, Brandsdóttir,
1992). Consequently, it is logical to conclude that the
3–4 weeks of gradually increasing earthquake activ-
ity that preceded the Laki eruption mark the onset of
rifting within the Grímsvötn volcanic system and the
initial emplacement of the Laki feeder dyke. By the
same token, the subsequent earthquake swarms rep-
resent ongoing rifting that resulted in gradual north-
east extension of the erupting fissures (Thordarson
and Self, 1993).
The strong earthquakes felt in the Fire districts
in February 1784 are worthy of further note. These
earthquakes occurred towards the end or after ces-
sation of activity at the Laki fissures. Furthermore,
Steingrímsson describes these earthquakes as being
different from those clearly associated with the erup-
tion and it is very likely that these earthquakes were
not directly related to the activity at the Laki fissures.
It is interesting that strong earthquakes were also felt
on the Reykjanes peninsula in Southwest Iceland on
24 and 25 February 1784 (Lievog, 1784). It is possible
that these earthquakes resulted from larger-scale dis-
locations on the South Iceland Seismic Zone (SISZ)
and may have been the precursors to strong earth-
quakes that occurred on the SISZ in August that year,
causing significant damage to structures and buildings
in the region. Furthermore, it is difficult to believe
that this sequence of events (i.e., the Laki eruption and
subsequent earthquakes on the SISZ) is a random oc-
currence because not only do they show close associ-
ation in terms of time, but occur on structures that are
closely linked geologically. The SISZ is a complex
zone of faulting linking the overlapping sectors of
the Western and the Eastern Volcanic Zones and thus
it is conceivable that the earthquake activity on the
SISZ was triggered by the Laki-Grímsvötn volcano-
tectonic episode (Stefánsson and Halldórsson, 1988;
Gudmundsson, 2000).
EXPLOSIVE ACTIVITY AND TEPHRA
DISPERSAL
Although the tephra fall deposits only amount to 2.7%
of the erupted magma volume (Thordarson and Self,
1993), the explosive activity at the Laki fissures was
significant and should not be overlooked. The mag-
nitude of the explosive activity at Laki gains a new
perspective when it is acknowledged that the 0.4 km3
JÖKULL No. 53, 2003 27