Jökull - 01.12.1978, Page 100

outwards to less than 1 mm/day outside the
caldera. Seven subsidence events have occur-
red over the last 2 years. Each time the total
subsidence has been from 3 cm and up to 2 m.
Gravity measurements show that the ele-
vation changes are largely caused by mass
movements beneath the area and not by
compression and expansion of the crust alone.
Model calculation indicates that the in-
flation is caused by the inflow of magma into a
magma chamber at a depth of 3 km beneath
the Krafla caldera. The mean mass flow is
about 5 km3/sec. Subsidence events are inter-
preted as being caused by an east-west rifting
of the crust and by a horizontal flow of magma
along fissures into the fissure swarm to the
north and south of the caldera. This flow of
magma can be calculated by considering it
being a flow of viscous fluid between two
parallel plates.
A RADIO-ECHO EQUIPMENT FOR
SURVEYING OF TEMPERATE ICE
SHEETS
Helgi Björnsson, Marteinn Sverrisson, Ævar
Jóhannesson,
Science Institute, University of Iceland
A radio-echo equipment has been designed
and built for depth sounding in temperate ice
sheets. Monopulses of 7kW peak power and
300 nanoseconds duration are transmitted at a
rate of 1000 pulses per second from a
thyristor-circuit into a 30 m long resistively
loaded broad-band dipole antenna. The
reflected signal from the ice is picked up by the
same type of antenna, fed into a
videoamplifier and photographed from an
oscilloscope. The receiver has a 2—5 MHz
band pass filter. Navigation on the ice sheet is
done by Loran-C and satellite navigation.
RADIO-ECHO SOUNDINGS ON
MÝRDALSJÖKULL AND
VATNAJÖKULL
Helgi Björnsson, Marteinn Sverrisson, Ævar
Jóhannesson,
Science Institute, University of Iceland
The bedrock topography of southern-Mýr-
dalsjökull has been surveyed by radio-echo
soundings. A plateau at 900—1000 m above
sea level is surrounded by mountains and cut
into by glacier outlets. Further, close south of
Austmannsbunga, a valley enters from the
northeast, cuts southwards into the plateau
leaving a depression down to 700 m beneath
the centre of the ice cap. The remains of the
plateau form two ridges. The ridge across the
glacier from Háabunga to Merkurjökull defi-
nes the main subglacial water divide. In 1918
Katla erupted on the other ridge which runs
SW—NE from Háabunga between the
depression and Höfdabrekkujökull.
A cross-section in Vatnajökull from Gríms-
fjall to Tungnárjökull shows a maximum
depth of 750 m. An extensive layer at 250 m
depth in Mýrdalsjökull is considered to be the
tephra from Katla in 1918. Radio-echo
sounding of volcanic ash layers may open new
aspects to studies of the mass balance and
dynamics of glaciers in Iceland, and the his-
tory of volcanic eruptions.
EARTHQUAKES AND TECTONIC
STRESS IN SOUTHERN ICELAND
Sveinbjörn Björnsson,
Science Institute, University of Iceland
The epicentra of large earthquakes in
Southern Iceland line up along an E—W
zone, from Selsund in the East across Hestfjall
towards Thurá in Ölfus in the West. The
epicentral zone is 70 km long but only about
10 km wide and is interpreted to be underlain
98 JÖKULL 28. ÁR