Jökull - 01.12.1978, Blaðsíða 112

powerstation in August 1977 and have since
recorded tilt variations of the building. The
movement of the building is also monitored by
watertube tilt measurements and the agree-
ment with the new tiltmeter is excellent.
TEPHRAS FROM THE BÁRDARBUNGA
CORE
S. Steinthorsson,
Science Institute, University of Iceland
In the summer of 1977 a 415 drill core was
recovered from the Bárdarbunga ice sheet in
NW Vatnajökull by members of the Science
Institute and the Icelandic Glaciological
Society. The hole failed to reach the bottom of
the glacier by an unknown distance. The pur-
pose of the drilling was chiefly to procure
material for the D/H study of the glacier, with
palaeoclimatic analysis in view, but other in-
terests included a glaciological study of the
core, an investigation of the tritium profile,
and the analysis of a possible record of
atmospheric pollution over Iceland. 29 layers
of volcanic ash were found in the core; they
have been used to date the various depth levels
in the ice by the time-honoured method of
tephrochronology. The core reaches back to
the year 1650. It contains 10 known Gríms-
vötn tephras, two layers from Katla, five from
“eruptions in Vatnajökull”, and 7 layers that
could not be matched with any known erup-
tions. Five of these probably derive from a
period of fissure activity north of Vatnajökull
in the first half of the 18th century, whereas
the remaining two derive from a central vol-
cano, probably Grímsvötn.
GEOTHERMAL ACTIVITY IN THE
AXARFJÖRDUR AREA
Valgardur Stefánsson,
National Energy Authority
Several hot springs are on the Axarfjördur
gravel plain. Temperatures up to 96°C have
been measured. D. C. electrical resistivity
soundings, using both Schlumberger and
dipole-dipole methods, have been aplied in
this area. Very low resistivity (1 — 2 fi m) is
found to coincide with the main active fissure
swarm extending from Axarfjördur in the
north to Krafla and Námafjall in the south.
The Na-K-Ca geothermometer indicates a
temperature in the order of 150—200°C, and
assuming mixing of cold water the silica
geothermometer indicates a temperature in
the orderof 170—230 °C.
A DENSE NETWORK OF
SEISMOGRAPH STATIONS IN
ICELAND
Jón Sveinsson, Marteinn Sverrisson, Páll Einarsson
Sveinbjörn Björnsson,
Science Institute, University of Iceland
The number of permanent or semiper-
manent seismograph stations in Iceland has
increased from 5 to 35 during the last six years.
In addition, four portable seismographs are
available for immediate use in the field. The
new seismographs were designed and built at
the Science Institute of the University of Ice-
land. The main components of the seismo-
graph are a geophone (natural frequency
2.0—3.0 Hz), a variable amplifier with LP
and HP filters and a drum recorder. A con-
tinous radio time signal from Rugby in Great
Britain is put directly on the record. Time
corrections are thus eliminated. The seismo-
graph is most sensitive in the frequency band
3—30 Hz. Maximum sensitivity of 106 is
frequently used. All earthquakes in Iceland of
magnitude 2.0—2.5 and larger can be located
with reasonable accuracy. The seismographs
record with ink on ordinary paper. This is
particularly important in areas, where the
seismic activity is used in the evaluation of
volcano hazards.
110 JÖKULL 28. ÁR