Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 69

CRUSTAL STRUCTURE OF ICELAND
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profile. On the basis of the travel time diagram it is also possible to
assume that under the surface layer with velocity 3.6 km/sec, layer
2 with P-wave velocity of about 5 km/sec is found. This in turn would
be underlain by layer 3 with a slight dip to the north. This latter
structure is more likely to be true by comparison with other neigh-
bouring profiles, and this will be assumed here. The depths to layer
3 will then be about 3.9 km near the shot point in lake Hvítárvatn,
and about 4.7 km near the northern end of the profile. Since the shot
point is at an elevation of 420 meters, these depths will be 3.5 and 4.3
km below sea level. This is in reasonably good agreement with other
profiles to the south and north, which give a greater depth to layer 3
north of Kjölur than to the south.
The three profiles on the highland south of Eyjafjördur all show
a similar delay time for the P3-wave. A thin low-velocity surface
layer is present near all the shot points. It has a velocity of about 3.0
km/sec and a thickness of about 0.2 km. Beneath the surface layer a
typical Tertiary basalt structure is obtained with layer 1, about 1
km thick, underlain by layer 2. Delay times for the P3-wave are in
the range 0.67-0.73 sec, which gives depths to layer 3 of 4.3-5.0 km.
The shot point elevations are close to 0.8 km, which gives a depth
range of 3.5-4.2 km below sea level for layer 3.
6.8. NE-Iceland.
This section comprises the area from Bárdardalur in the west to
Vopnafjördur in the east. The southemmost profile is near Askja.
All the profiles have been discussed earlier (Pálmason, 1963), but
will here be discussed from a different point of view, based on the
delay time concept and the assumption of a constant velocity in layer
3. The total delay times thus obtained for the P3-wave are given in
Fig. 25.
In the previous section a discussion was given of the profile in
Ljósavatnsskard, which crosses the hypothetical fault along Bárdar-
dalur and shows no delay time anomaly on crossing this topographic
feature. To the east of this profile, in Reykjadalur, two profiles (12
and 12a) run in opposite directions from shot points which are about
6 km apart. Along the northem profile (12) which mns along the
main road in Adaldalur towards Húsavík, the delay time is more or
less constant, about 0.53 sec, which gives a depth to layer 3 of about
3.3 km. Along the southern profile (12a) the P3-wave delay time