Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 79

CRUSTAL STRUCTURE OF ICELAND
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Considering the relatively large thickness of layer 1 on profile L6
and the possibility that this thickness may be overestimated, which
in tum would underestimate the depth to layer 4, it is concluded that
no significant difference in depth to layer 4 exists between the pro-
file L6 on the one hand and the profiles L2 and L7 on the other. A
depth between 8.5 and 9.0 km is indicated on all three profiles.
A pair of profiles, L4 and L5, run from a common end station on
land near Thorlákshöfn. L4 mns due south to a distance of 98.51 km
and L5 runs to the southeast and passes just south of the Vestmann
Islands to a distance of 96.45 km. A low-velocity surface layer is indi-
cated near the shot point. Its velocity was not measured but will he as-
sumed to he about 3.0 km/sec, which gives it a thickness of 0.60 km.
The underlying apparent velocity is 4.5 km/sec on both profiles.
This may he a tme velocity, but it is also possible that due to the
relatively large spacing between stations the corresponding segment
on the travel time curve may actually consist of two segments with
velocities corresponding to layers 1 and 2. For the calculation of
depths to layers 3 and 4 a velocity of 4.5 km/sec will be used for this
layer. The delay time of the P3-wave indicated near the shot point is
about 0.71 sec, which gives a depth to layer 3 of 3.9 km. At a distance
of about 20 km the P3-wave delay time increases on both profiles, by
about 0.10 sec on L5 and about 0.06 sec on L4. If this increase is
caused by a deepening of the 2—3 boundary, this would amount to
0.6 km and 0.4 km respectively. If on the other hand it is caused by
a thickening of the surface layer, this would amount to 0.35 km and
0.20 km respectively. Somewhere between these two limits is the in-
crease in depth to layer 3 in this area.
The P^-wave appears as a first arrival on both profiles beyond
about 45 km. Between 45 and approximately 70 km the apparent
velocity of the P4-wave is about 7.1—7.2 km/sec. If the true P4-velo-
city is 7.2 km/sec as assumed here, this means that the 3-4 boundary
is largely horizontal. Beyond about 70 km the apparent velocity of
the P4-wave decreases to well below 7 km/sec, probably due to a
deepening of the 3-4 interface, as is indicated by the two profiles
L8 and L9 in the eastem part of southern Iceland. The average ap-
parent P4-wave velocity on profiles L5 and L4 are 6.76 and 6.83
km/sec, respectively.
The total delay time of the P4-wave at distances less than 70 km
is about 2.20 sec. Dividing this equally between the shot and receiver