Jökull - 01.12.1963, Side 52

cropping. A gabbro outcrop is found at river
Vididalsá some 15 km west of the profile (Jóns-
son, 1964).
3— 5. Profiles 3 to 5 are located in the Skaga-
fjördur valley in northern lceland. Profile 3
is on the western side of the valley, but profiles
4 and 5 are on the eastern side. South of pro-
file 3 on the western side is profile E of Tryggva-
son & Báth (1961). Profile 3 gives a depth of
2.10 km to layer 3, whereas profile 4 gives a
depth of 3.26 km. The structure at profile 4
is somewhat uncertain, as P2 arrivals are weak,
and apparently missing at distances of 10—15
km. The S2 phase is very distinct at 9.98 km
distance. Despite the uncertainty in structure
the depth to layer 3 should not be appreciably
in error in profile 4. There is thus a change in
depth to layer 3 of more than one kilometer
across Skagafjördur between profiles 3 and 4.
On profile 5 recording conditions were rather
poor and the number of useful stations near the
shot point small. Below a thin low-velocity sur-
face layer one apparently has a velocity of 4.79
km/sec, which would mean that layer 1 is not
present here. Due to the scarcity of stations,
however, the structure must be considered un-
certain here, and both layer 1 and 2 may well
be present. The depth to layer 3 is probably
not very much in error.
Profile E of Tryggvason & Báth (1961) gives
a depth of 3.04 km to layer 3, which is similar
to profiles 4 and 5 in the present investigation.
6.-8. These three profiles are located in the
central highlands south of the Tertiary basalt
area of northern Iceland. They are at an
altitude of 700—1,000 meters. Being in the Neo-
volcanic zone the surface rocks are Quaternary
volcanic products. This surface layer is here
about 200 meters thick and has a P velocity of
about 3.0 krn/sec. Below the surface layer the
Tertiary basalt structure is obtained. Layer 3 is
found at depths from 3.2 to 4.1 km below sea
level. The greatest depth is found on profile 6,
which is nearest to profile F of Tryggvason &:
Báth (1961), where a depth of 4.7 km to layer 3
was found.
9. This profile runs along the western side
of the valley Fnjóskadalur in the Tertiary basalt
area of northern Iceland. It is about 15 km
east of profile F of Tryggvason Sc Báth ment-
ioned above. Layer 1 is found ltere under a
thin alluvial surface layer. A layer with P velo-
city 5.6 km/sec as found by Tryggvason & Báth
in Eyjafjördur was not found in Fnjóskadalur.
In the Eyjafjördur area no profiles were mea-
sured to study the deeper structure of the lava,
as this had already been done before (Tryggva-
son &: Báth, 1961). On the other hand a series
of shorter unreversed profiles were measured
here to study the uppermost layer. These mea-
surements will not be described here in detail
but the rnain results will be given. The profiles
were measured in the area between lake Hóla-
vatn in the south and the village Hjalteyri in the
north. The rnean P velocities of layers 1 and 2
and their standard deviations, assuming the lay-
ers to be everywhere the sarne, are as follows
Layer P velocity km/sec
1 4.19 ± 0.04
2 5.00 ± 0.04
Both rnean values are computed from a set
of 8 profiles. The thickness of layer 1 ranges
from about 0.3 km to about 0.6 km.
There is a discrepancy between the present
results in Eyjafjördur ancl those of profile F of
Tryggvason ancl Báth (1961), which was founcl
to be due to an insufficient nurnber of geophone
stations near the shot point of profile F as well
as to a low-velocity surface layer of limitecl ex-
tent near the shot point of this profile.
10,—12. These profiles are all near the west-
ern boundary of the Neovolcanic zone in north-
eastern Iceland. The Bárdardalur fault line
which is a prominent topographical feature in
this area runs in a north-south direction near
the western boundary of the Neovolcanic zone.
Gravity profiles have been measured across this
line but have failed to reveal any major anomaly
associated with it (Schleusener (1943), Hospers
(1952), Einarsson (1954)). It was therefore of con-
siderable interest to make a seismic study of the
area for comparison.
Profiles 10 and lOa run in opposite directions
along the pass Ljósavatnsskard. Profile 10 is
rather short (13.5 km) and runs from lake Ljósa-
vatn west to Fnjóskadalur. Profile lOa runs
from Fnjóskadalur to the east across the Bárdar-
dalur line to Reykjadalur. Profile II runs from
JÖKULL 1963
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