Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 80

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includes the observation system in the short line previously done
by Bott, 69/2/2C-2D/ and 69/3/3E/-3F/ (3). Fig. 2.2 shows a ray
scheme for the PM waves. It can be seen that for these waves the
observation systems provide an extremely reliable multi-overlapping
tracing within the outlines of the Iceland-Faeroe Ridge. In the west-
em portion of the line A, under Iceland, the system appear less
accurate. However, using the accurate determinations of the depths
and velocities under the Ridge it appears possible to calculate the
reliable values of the depths and velocities for main seismic botm-
daries under eastern Iceland as well.
Velocities and Travel Times of the First Arrivals
A general characteristic of the seismic wave field is obtainable
from the reduced travel time curves summarized in Fig. 3.1, the
reduction velocity being 8.0 km/sec. The latter in addition to the
NASP data contains the data from the previous studies, Pálmason’s
travel time curve (L10), for the north-western part of Iceland,
Bott’s travel time curve (3E,3F) for the Iceland-Faeroe Ridge and
Báth’s travel time graph ( WP) for westem Iceland. For comparison
the typical travel time curves for the continental cmst, Jeffreys-
Bullen (J-B) and extrapolated over great distances travel time
curves for the oceanic crust (0) (14) are shown.
In terms of inclination the experimental travel time curves fall
into two groups. Tlie first with Pk velocities essentially less than
8 km/sec is associated with the waves running in the earth’s cmst.
The second with the PM velocities approx. 8 km/sec is confined to
the waves developping in the upper mantle.
The time band of the mantle wave travel time curves over dis-
tances of 200-300 km occupies 2-3 sec. The longest travel time has
been recorded by the stations R2, R3, R4 and DU (5) lying within
the rift zone occupying the middle of Iceland. The shortest time
has been registered for line A shots by stations Rx and Mx situated
on the eastem coast of Iceland and explosion 4 fired in the north-
western coast. The offshore stations are intermediate.
The location of the cross-zones of the branches of the travel time
curves Pk and PM for various stations is characteristic. They lie at