Jökull - 01.01.2014, Side 64

Ingi Þ. Bjarnason
SEISMIC ACTIVITY AND VOLCANIC
ERUPTIONS IN THE VATNAJÖKULL
REGION
The early seismic net, sensitivity analysis
In Iceland continuous recording of seismic activity
began (Reykjavík) in 1925 (Tryggvason, 1978a). In
1951–1954 a new and a more sensitive seismometer
(Sprengnether) was placed in Reykjavík and the old
one (Mainka) was transferred to Akureyri in North
Iceland (Tryggvason, 1973). In the early days based
on the Reykjavík recordings, epicenter locations were
rather inaccurate for the distant Vatnajökull area.
With the instrument upgrading of the 1950s, detection
of earthquakes of magnitude 3.0 and greater improved
for the Vatnajökull region, but location determinations
were still inaccurate. Locations of small earthquakes
in Vatnajökull were not well constrained until mid-
1970s after the instalment of seismographs in North-
east and later in East and Southeast Iceland (Einars-
son, 1991). From the beginning of recording, the lo-
cal magnitude scale (ML) of Icelandic earthquakes is
thought to have remained rather uniform (Tryggvason,
1973), except possibly after the installment of a digi-
tal national network in the 1990s (see Table 1).
Since 1954 the earthquake bulletins show increase
in earthquakes of magnitude ≥3.0 in the Vatnajökull
region (Tryggvason, 1979). Previously, authors came
to the conclusion that the detection threshold for the
Vatnajökull region in the years 1925–1953 was as
good as ML ≥3.5–4.0 (Tryggvason, 1973; Brands-
dóttir, 1984). In good weather conditions the detec-
tion threshold will certainly improve significantly, but
in bad conditions it deteriorates. Therefore, to in-
terpret the natural changes in seismic activity in the
Vatnajökull region in the 20th century, it is necessary
to evaluate the magnitude of completeness (Mc) of the
seismic bulletins preserved. Mc is a fixed value of
an earthquake magnitude which subdivides a seismic
bulletin into two parts. 95% of earthquakes of mag-
nitude >Mc will have a record in the bulletin. Many
earthquakes that occurred in the region with magni-
tude ≤Mc will, however, not have a record. Assess-
ment of Mc for early recorded earthquakes in Iceland
seems not have been carried out before.
The magnitude frequency relation of Gutenberg
and Richter can be used to make a preliminary es-
timate of the Mc value for the Vatnajökull region.
Tryggvason (1973) reported relatively high b-values,
1.2–1.3, in the Vatnajökull and Dyngjufjöll area (i.e.
where Askja is located in Figure 1) compared to other
parts of Iceland. In the early years, 9 earthquakes can
be interpreted to be within the Vatnajökull region in
the range ML = 41/4–51/4. Based on these observa-
tions the Mc is estimated to be in the range 4.4–4.5
(ML), assuming the Tryggvason (1973) b-values, and
using the Gutenberg and Richter magnitude frequency
relationship. If the global average b-value of unity
would be assumed for Vatnajökull region, a lower Mc
of ∼41/4 would be expected.
In the early decades of seismic recording, earth-
quakes in Iceland were often detected at seismic sta-
tions outside the country, i.e. in Greenland or in conti-
nental Europe, even sometimes as low as 4.0 in mag-
nitude (Tryggvason, 1978a, 1978b). Hence, with the
local Mc ∼4.4, and relatively good detection outside
Iceland, it is unlikely that earthquakes of magnitude
∼5.0, like those of the 1973–1996 Bárðarbunga earth-
quake sequence, would have escaped detection in the
early years of recordings.
Tryggvason (1973) concluded that the observed
increased rate of earthquakes since 1955 in the Vatna-
jökull area to be real, and Björnsson and Einarsson
(1990) pointed out a possible correlation between this
increase in seismicity and increased geothermal activ-
ity in the Loki Ridge glacier cauldrons at the same
time (Figure 1). Assuming Mc∼4.4 (ML) in the early
period, there is uncertainty if there is in fact a real
increase in the rate of earthquakes with ML > 4.4
in Vatnajökull region since 1955. However, the small
number of earthquakes in the Vatnajökull area re-
ported in the early period, limits what can be con-
cluded.
From the mid-1970s the ability to locate epicen-
ters became accurate enough in the Vatnajökull re-
gion, making it possible to map distinctly the seismi-
cally active areas in the region (Björnsson and Einars-
son, 1990; Einarsson, 1991). The density of the per-
manent seismic network was not high enough to con-
strain sufficiently earthquake depth. However, the net-
work did indicate that the majority of the seismic-
64 JÖKULL No. 64, 2014