Jökull - 01.12.1990, Síða 155

of 10 m and larger can thus be resolved, e.g. hyalo-
clastite ridges and major normal faults. Fissure zones
with minor vertical displacements cannot, however,
be delineated.
Fig. 3 shows the surface forms on western and
northern Vatnajökull. The map includes the out-
lets Tungnaárjökull, Sylgjujökull, Köldukvíslarjökull,
Dyngjujökull and Brúarjökull and the interior of the
ice cap. The highest parts of the glacier cover
the mountains Hamarinn, Bárðarbunga, Kverkfjöll,
Grímsfjall and Háabunga. The map illustrates ice sur-
face depressions that are created by subglacial geother-
mal activity and are underlain by water-filled vaults
from which water is drained in jökulhlaups (Bjöms-
son, 1974, 1975, 1977, 1988). The largest one is in
the Grímsvötn area and contains one of the most (if
not the most) powerful geothermal systems in Ice-
land. The subglacial lake Grímsvötn is the source
°f frequent jökulhlaups that drain to the sandur plain
Skeiðarársandur. Three ice cauldrons are located on
a row striking E-W from Hamarinn towards Gríms-
vötn. The water-filled vaults beneath the two eastem-
niost ice cauldrons (Skaftárkatlar) drain to the river
Skaftá (see e.g. Björnsson, 1988). The westemmost
ice cauldron drains to the proglacial lake Hamarslón
whichfeeds Kaldakvísl (Bjömsson, 1983,1988). One
ice cauldron is located about 1 km E of Pálsfjall and
another in the Kverkfjöll area.
Fig. 4 shows the bedrock topography of the same
area. In its northwestern part, the landscape is dom-
inated by the large mountains Bárðarbunga, Hamar-
inn, Háabunga, Grímsfjall, Esjufjöll and Kverkfjöll
and mountain ridges stretching out from them. To
the east of the neovolcanic zone, on the other hand,
the subglacial landscape is strikingly different. It is
characterized by glacially eroded features, the most
prominent of which is a broad valley bordered by the
mountains at the glacier edge to the east and Esjufjöll
to the south.
SEISMIC ACTIVITY
The western part of Vatnajökull has been recog-
nized as a seismically active area since instrumen-
tal locations of earthquake epicentres became avail-
able in Iceland (Tryggvason et al., 1958). The activ-
ity increased markedly in 1954 (Tryggvason, 1973)
and again in 1974 (Einarsson and Bjömsson, 1980;
Brandsdóttir, 1984). Improved seismograph coverage
of the country in the seventies (Einarsson and Bjöms-
son, 1987) led to better epicentral locations. The most
significant improvements occurred in 1975 when seis-
mographs were installed in NE-Iceland, and in 1977
when instruments were added in E- and SE-Iceland.
Epicentres of the period 1975-1985 are shown in
Fig. 5. Formal errors of the locations are not greater
than 2 km horizontally, but most locations have consid-
erably smaller errors. Depths are poorly constrained,
but the data are all consistent with a crustal (< 10 km)
origin of the events. The locations are determined
with the location program HYPOINVERSE (Klein,
1978). Velocity structure is derived from Gebrande
et al. (1980), and station corrections are found from
calibration explosions. It is found that formal errors
of the locations represent well the true uncertainties.
Generally, epicentres in the eastern neovolcanic
zone of Iceland do not seem to delineate faults or
plate boundaries (Einarsson, 1991). The earthquakes
are clustered and activity within each cluster is gen-
erally persistent through time. The most prominent
clusters coincide with the mountains shown in Fig. 4;
Bárðarbunga, Hamarinn, Grímsfjall and Kverkfjöll.
The clustered nature of the activity, evident in Fig. 5,
suggests that the earthquakes are caused by concen-
trated sources of stress in the crust. A possible ex-
planation of the clustering may lie in a concentration
of regional stress around crustal inhomogeneities, but
inflating and deflating magma chambers also produce
local stress and clustered seismicity. Earthquakes as-
sociated with deflation and inflation are known from
the Krafla Volcano in NE-Iceland (Einarsson, 1978,
1991), and earthquakes of Grímsvötn and Bárðar-
bunga have been interpreted in a similar way (Einars-
son and Brandsdóttir, 1984; Einarsson, 1986, 1991).
VOLCANIC SYSTEMS OF VATNA-
JÖKULL
Fig. 1 shows the principal central volcanic com-
plexes of the neovolcanic zones in Central Eastern
Iceland, as shown by Sæmundsson (1982) and Einars-
JÖKULL, No. 40, 1990 151