Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 81

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the longest distances (more than 200 km) for stations R2, R3 and
R4 and the shortest (130-150 km) for R4 and M4 stations. The
location of the branches of the travel time curves Pk and PM is well
emphasized by the dynamic of the first waves (see fig. 3.2). The
amplitude curves for the station Rj change in inclination over a
shorter distance than for the stations R2 and R3.
Pálmason’s and Bott’s short travel time curves fit well into the
summary as well as a longer one of M. Báth. Comparison of our
travel time curves with theirs shows that the first PM waves with
high velocities, at the distances obtained by the above mentioned
authors, have failed to be distinguished as the first arrivals.
In Fig. 3.1 it is seen that at a distance of more than 350 km from
the station R2 a short hranch of the travel time curve with high
velocity (more than 10 km/sec) is obtained. It corresponds to the
weak but distinct waves of the first arrivals prior to more pro-
nounced mantle waves. Such waves have also been recorded at other
stations. For these waves the observation systems appear inefficient
and so far the question of their nature has not been solved.
The studies of the travel time curves to analyse the features and
the existence of the low velocity layers in the earth crust have not
yet yielded the final results and this problem needs further investi-
gation.
The analysis of all travel time curves obtained at the line A as
well as at other NASP lines made it possible to subdivide these
curves by the location of the Pk and PM waves into the groups
corresponding to the various geographic zones and differently con-
structed block of the earth’s crust. They are the Iceland block
(I-I, Fig. 4), the Iceland-Faeroe block (II-II) occupying the ridge
in the middle, the Faeroe block (III-III), the block of the Faeroe-
Shetland channel (IV-IV) and the Shetland block (V-V).
In velocities of the Pk waves these blocks fall into three groups.
Velocities of the Pk waves of approx. 6 km/sec are found for the
Faeroe and Shetland-Scotland shelf zones (blocks); the velocities
exceeding 6 km/sec (approx. 6.5-7.0 km/sec) — by the Iceland
and Iceland-Faeroe blocks and the velocities less than 6 km/sec —
by the Faeroe-Shetland channel block.
The different blocks have different curves for the dependence of
the average velocities on depth. The graphs on Fig. 5 have been con-