Jökull - 01.12.1963, Blaðsíða 54

rections north and south. The third is located
in the area east o£ the tablemountain Herdu-
breid and the volcano Askja, 'which had an
eruption in 1961. The surface low-velocity layer
is of a rather varying thickness in the area of
these profiles. Layer 2 is thicker than in most
other places and consequently the depth. to
layer 3 also greater, about 4.5 km below sea
level. Apparently layer 2 on profile 21 consists
of two layers with velocities 4.61 and 5.39 knt/
sec. This division into two layers may not be
real, as the last 10 km of the profile are located
about 8—10 km west of the main profile line,
which runs in a north-south direction, and
lateral structural changes in an east-west direc-
tion could affect the arrival times at these last
10 km of the profile.
On profile 19 at a distance of 12-47 km the
P2 wave arrives about 0.1 sec too early compared
to the neighbouring stations. This discrepancy
has not been explained yet, but may be due to
an anomalous structure under this station.
22.-23. These two profiles are in the Vopna-
fjördur area in the Tertiary basalts of eastern
Iceland. Profile 22 is on the lowland near the
village Vopnafjördur. It is only 19.7 km long
and does not give depth to layer 3. The velo-
cities of layers 1 and 2 are here somewhat higher
than usually is found. Profile 23 is on the
highland southwest of Vopnafjördur. It extends
partly into the Neovolcanic zone, that is into
an area with a Quaternary volcanic surface
layer. The rather lów velocity found for layer
3 is probably due to a thickening of the surface
low-velocity layer towards the end of the pro-
file.
24. This is a short profile near lake Saenauta-
vatn in the eastern part of the Neovolcanic
zone. The profile is too short to give depth to
layer 3, but gives thickness of layers 0 and 1.
25.-27. These three profiles are all in the
Tertiary basalt area of eastern Icelancl. Profile
26—26a is a reversed profile. Although the
distance between profiles 25 and 26 is only
from 5—15 km the structure is apparently clif-
ferent. Profile 25 gives a thickness of about 1.5
km of layer 1, whereas profile 26 gives only
about 0.5 km. And on profile 27 in Breiddalur
on the east coast it is either missing completely
at sea level or thinner than 100—200 meters. On
the other hand layer 2 is rnuch thicker in Breid-
dalur than in the area of profiles 25 ancl 26.
The structure is thus far from uniform in the
area of profiles 25—27 although they are all
in a geologically similar part of the country.
28.—30. The area in southeastern Iceland
where profiles 28 to 30 are located is character-
ized by a number o£ large gabbroic and acid
intrusions, which are not common in other
parts of Iceland, as well as by flood basalts.
It was therefore of considerable interest to
compare the seismic structure of this area with
other parts of the country. The profiles were
arranged so that the shot points were close
to the largest gabbroic intrusions, i. e. Eystra-
Horn, Vestra-Horn and Vidbordsfjall. It turn-
ecl out that the surface rocks on all three
profiles had velocities corresponding to layer
2, which meant that layer 1 was absent. Layer
3 was found at a relatively small depth, 1.2
to 1.8 km below sea level. On profile 30,
which runs in a southwest direction from Vid-
bordsfjall, the velocities are somewhat lower
than on the other two profiles. This may be
due to a possible existence of layer 1 at the
southwestern encl of the profile, where flood
basalts are the predominant surface rocks.
50
JÖKULL 1963