Rit (Vísindafélag Íslendinga) - 01.06.1971, Síða 81

CRUSTAL STRUCTURE OF ICELAND
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wave is fairly constant at 3.6—3.7 sec at distances of over 50 km where
the P4-wave is a first arrival. If the above depth to layer 3 is assumed
to be correct, the thickness of layer 3 will be 4.9 km and the depth to
layer 4 14.8 km. If on the other hand layer 3 is assumed to be missing
under this profile, the thickness of layer 2 would be 8.6 km and the
depth to layer 4 would be 12.3 km. The first possibility seems to be
more likely since layer 3 is present on the profile Vík—Lómagnúpur
which runs parallel to the profile L9 at a distance of some 20-40 km.
Furthermore layer 3 is known to exist in general imder the ocean
deeps (the “oceanic” layer). It will therefore be assumed that the
first altemative is the correct one and that the depth to layer 4 under
this profile is about 14.8 km.
A comparison of profile L9 with the Vík-Lómagnúpur profile
(L8) is of interest (cf. sect. 6.2). The structure inferred is similar on
both. A thin surface layer is present on L8 with a P-velocity of about
3.0 km/sec and a thickness of about 0.5 km. Layers 1 and 2 have
thicknesses of 2.9 and 5.0 km, respectively. Layer 3 is 6.1 km thick,
giving a total depth of 14.5 km to layer 4. This result is similar to the
one obtained on profile L9 and shows that no major structural changes
are taking place between these two profiles. The relatively large
structural change between these two profiles and those which are
farther west, for instance Graenavatn-Eyjafjöll (L3) and Kerid-
Hvítárvatn (L7), takes place over a relatively narrow belt, perhaps
40-50 km wide. The offshore profiles L4 and L5 indicate that this
belt coincides with the presently active volcanic zone in southern
Iceland between Hekla in the west and Katla-Eldgjá in the east.
A particular study was made of the Breidamerkurdjúp submarine
valley near the eastern end of the profile L9. A series of eight shots
were fired on a line crossing the central part of the valley. The
average interval between the shots was about 3 km. The location of
the shots is shown in Fig. A45.
The amplitudes of the first arrival P4-waves were corrected for
geometrical spreading and variations in charge size (9.8 to 22.6 kgs).
Instrumental settings were kept unchanged. The reduced amph-
tudes in decibel are plotted in Fig. C18 for all P4-arrivals between
40 and 140 km. Along the main part of profile L9 the amplitude is
slightly decreasing, about 0.13 dB/km, which is probably an effect
of attenuation in layer 4. The short profile across Breidamerkurdjúp
is shown separately in Fig. C18. Three stations west of the valley
have normal amplitudes and arrival times. On the remaining sta-
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