Rit (Vísindafélag Íslendinga) - 01.06.1971, Síða 99

CRUSTAL STRUCTURE OF ICELAND
99
layer in the Tertiary districts in westem, northem and eastem Ice-
land. In southeastern Iceland it is missing and layer 2 is exposed at
the surface. In Eyjafjördur in northern Iceland where the thickness
of layer 1 is 0.3 to 0.6 km (Pálmason, 1963), a hole has been drilled
to a depth of close to 1000 meters without indicating any change of
rock type with depth (Saemundsson, 1969). Layer 2 appears to he
missing on the Reykjanes peninsula, as it does not correspond to any
first arrival waves on the travel time diagrams. It is possible however,
that it is relatively thin here, and that the P2-wave therefore does
not appear as a first arrival.
The P-wave velocities in layers 1 and 2 are no doubt somewhat
variable from one area to another. This is apparent where layer 1 is
a surface layer, and the measured velocities probahly are tme velo-
cities. Relatively high values are found at Rorgarnes (profiles 53 and
54), Skagaströnd (1) and Vopnafjördur (22). Low values occur in
southem Iceland, where the measured values appear to have a syste-
matic geographical distribution with a minimum near Apavatn,
where layer 1 is a surface layer (cf. Fig. 9).
Layer 3 is found on every profile of sufficient length, and its
existence has been demonstrated everywhere under Iceland, with the
possible exception of the central NW-peninsula. It may, however,
quite well exist there also at a greater depth than reached by the re-
fraction lines in that area (i.e. over 6 km). A particular emphasis has
been placed on mapping in detail the upper boundary of this layer
by following variations in the delay time of the P3-wave. The avail-
able data on depth to this layer are summarized in Fig. 37.
Several areas of relatively shallow depth to layer 3 are seen in
Fig. 37, the most conspicuous ones being the western Iceland area
from Hengill to Mýrar, the Snaefellsnes area and the Húnavatns-
sýsla area in N-Iceland. Other shallow-depth areas of interest are in
eastem and southeastern Iceland. Relatively great depths to layer 3
are found under the eastern part of southem Iceland, central northern
Iceland and a part of the NW-peninsula. A slight depression in the
2-3 boundary is also found under the volcanic zone in NE-Iceland.
The P-wave velocity in layer 3 is probably more uniform than in
the upper layers. The value which has heen chosen for delay time
calculations is 6.50 km/sec. The reasons for choosing this value have
been discussed in section 5.5. The true value is helieved to deviate
from this value by less than 0.1 km/sec in most cases, although larger
deviations are known to exist (cf. sect. 6.5).