Rit (Vísindafélag Íslendinga) - 01.06.1971, Síða 27

CRUSTAL STRUCTURE OF ICELAND
27
The “average” thicknesses of layers 1 and 2 are given below in
Tahle 5.6. The standard deviation s for a single determination is
also shown.
Table 5.6.
Layer Thickness km s km n
1 1.04 0.41 62
2 2.15 0.73 52
Layer 0 is found only in the Neovolcanic zone where its thickness
varies from 0 to 1.0 km.
Some data on the thickness of layer 3 have been obtained from
the longer profiles L1 to L10, using the delay time method. In most
cases it appears to be in the range 4-7 km, hut a larger thickness is
mdicated in northem Iceland, perhaps 12-13 km.
5.5. P-wave velocities in layers 3 and 4.
For detailed depth determinations by the delay time method it is
necessary to know as accurately as possible the P-wave velocity in the
layer whose depth is sought. There is considerable evidence that layer
3 is more homogeneous with respect to velocity than the overlying
layers, and it is therefore well suited to depth determination by this
method. The average P3-wave velocity as found in sect. 5.2 was 6.35
km/sec, but there are fairly strong arguments for choosing a some-
what higher value, about 6.5 km/sec, as being more representative
of this layer. These arguments are given below.
a) When the P3-wave velocity data are looked at more carefully,
it is seen that the lower velocities occur mainly on the shorter
profiles, while on longer profiles the velocity approaches a value
of about 6.5 km/sec. This is seen clearly in Fig. 6, where the ap-
parent velocity is plotted versus length of profile. By length of
profile is then meant either the actual length, if the P3-wave
is recorded as a first arrival to the end of the profile, or the
maximum distance at which the P3-wave is recorded as a first
arrival, if the profile is still longer. The apparently lower velo-
cities at the shorter profile lengths may perhaps be accounted
for by the following two possibilities.
(i) The velocity may increase more or less continuously with
depth from layer 2 to layer 3. This possibility may be true