Rit (Vísindafélag Íslendinga) - 01.06.1971, Blaðsíða 80

80
GUÐMUNDUR PÁLMASON
ends and assuming the structure above layer 4 as given above, this
gives a depth of 8.8 km to layer 4. This is practically the same as for
profiles L2, L7 and L6 described earlier.
Beyond 70 km the delay time of the P4-wave increases on both
profiles, more strongly on L5. At the ends of the profiles it is 1.40 sec
on L4 and 1.60 sec on L5. If this is caused entirely by a depression
of the 3-4 boundary, the corresponding depths are 13.3 km and 16.3
km respectively. It is, however, just as likely that a part of the delay
time increase is caused by increased thisknesses of layers 1 and 2 as
appears to be the case on profiles L8 and L9. If the thicknesses of
layers 0, 1 and 2 on the profile L8 are assumed to be valid at the
distant ends of profiles L4 and L5, the corresponding depths to layer
4 will be 9.4 and 12.6 km. The most likely depth to layer 4 at the
ends of profiles L4 and L5 will accordingly be put at 11.3 and 14.4
km respectively, with an uncertainty probably not in excess of ±
2 km.
A search was made for reflected waves from the 3-4 interface on
profiles L4 and L5. The results are described in section 8.
The profile L9 runs from a recording station on the shore at Al-
vidruhamrar, about 35 km east of Vík, in an eastnortheast direction
to a distance of 140 km. It passes over the northem part of the sub-
marine valley Breidamerkurdjúp. A line of shot points was mn across
this valley somewhat farther to the south in order to detect possible
travel time anomalies which might give an indication about the
shallower structure under the valley (cf. Fig. A45).
As in the other offshore profiles no particular emphasis was placed
on obtaining dense travel time data near the land station. A thin low-
velocity surface layer is present with an assumed velocity of 3.0
km/sec. Its thickness is about 0.3 km. Layer 1 has an apparent velo-
city of 4.5 km/sec, which is rather high, but appears to be common in
southern Iceland. Its computed thickness on the basis of horizontal
interfaces is 3.4 km. Layer 2 is fairly well defined by first arrivals
between 30 and 48 km. Its apparent velocity is 5.19 km/sec. The P3-
wave on the other hand is not well defined by first arrivals, but a
minimum delay time for this wave can be estimated from the first
arrivals at 48-53 km which is the range where the P4-wave overtakes
the P2-wave as a first arrival. The minimum total delay time of the
P3-wave thus found is about 2.84 sec. If this is divided equally be-
tween shot and detector stations it gives a thickness of layer 2 of 6.2
km and a depth to layer 3 of 9.9 km. The total delay time of the P4-