Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 88

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the head waves shows them to coincide practically in the ridge
center in the areas of the horizontal strike of the boundary, and
to differ essentially in eastem Iceland where the boundary plunges,
which is due to an error in the seismic setoff determinations by the
time term method discarding inclination angles.
The agreement of the two variations of the M discontinuity on
A-line construction can be improved by means of the subsequent
iterations of the time term method. It appears more difficult to
compare the section in the velocity isolines with that of the M
discontinuity constructed by a time term method in lines Bx and
B2 because of a thin observation grid in the upper portion of the
cmst.
Thus the cross-section on fig. 7.2 on line — Iceland — Faeroes
— Shetland Islands illustrates the distribution of the velocities in
different cmst blocks and a position of a crust-to-mantle transition
zone. The M discontinuity and its velocity characteristics have
been determined most reliably in the region of the Iceland-Faeroe
Ridge and beneath the eastem Iceland. In the east of the studied
region the velocity parameters of the M discontinuity are less ac-
curate. The extreme west of the section beneath Iceland has been
extrapolated with account for all principal features of the wave
propagation in the transition from Iceland to the Iceland-Faeroe
Ridge.
In selecting the section on fig. 7.2 for Iceland a variant of a low
velocity layer (LVL) was also analysed. For this a travel time
curve obtained from shot point 4 (fig. 1) was used. The calcula-
tions have shown such layers to be kinematically possible at depths
not less than 15 km.
For the western portion of the cross-section (the first variant in
the velocity isolines) gravity effect has been also determined. The
calculations illustrated on fig. 7.1 show an experimental Bouguer
anomaly and a curve calculated for the cross-section in velocity
isolines. It has been assumed that the velocity and density are di-
rectly proportional, the density gradient diminishing with depth.
At the top of the cmst section the velocity difference of 0.5 km/sec
corresponds to the density difference of 0.1 g/cm3, at the bottom
— 0.05 g/cm3.