Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 86

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small areas in the eastern part of the section show a greater
deviation accounted for by the inefficient observation system for
the upper portion of the section.
Taking into account abruptly varying inclinations of the travel
time curves in transition from Pk to MM waves (Fig. 3), in a com-
puter there has been selected a version of the cross-section in the
velocity isolines for a weaker gradient at the crust bottom starting
with a 7 km/sec isoline and with a velocity jump on the M dis-
continuity from 7.3 to 8.0 km/sec. For the M discontinuity this
version is similar to that of the head waves.
In the second assumption the cross-section was constructed by
a time field method and only for the west of the A line. Here the
system of the experimental travel time curves enabled two boun-
daries to be constructed, one in the upper portion of the section
and another in the bottom of the earth crust.
The upper boundary is characterized by a velocity of 6.5-6.8
km/sec. It agrees well with the boundaries constructed previously
by Bott for the Iceland-Faeroe Ridge and by Pálmason for Iceland.
On fig. 7.2 it practically coincides with a 6.5 km/sec isoline.
The M discontinuity shown on Fig. 7.2 has been constructed
by a time field method for various combinations of the first arrival
system.
For the travel time curve beneath Iceland the reverse times
from st. R2 — R4 above the bottom station BSS and the system
of the overlapping travel curves for the Faeroe stations (fig. 2.1)
have been used. The average velocity has been taken as constant
and equal to 6.5 km/sec. The boundary velocity along the M dis-
continuity varies. In the ridge centre it is 8.0 km/sec, approach-
ing the shelf and beneath eastern Iceland where the data are less
reliable, it ranges from 7.8 up to 8.2 km/sec.
For the head waves the M discontinuity has been constructed
by a method of time term as well. The results of the calculation
of t and depth of the M discontinuity were taken to be as much as
the velocity determined in the scheme of the difference travel
time curves and ranged from 7.7 to 8 km/sec. In the calculations
of x and depth of the M discontinuity it was taken as much as
7.8 km/sec, the average velocity in the crust being 6.5 km/sec.
The comparison of the two versions of the M discontinuity for