Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 87

CRUSTAL STRUCTURE OF ICELAND
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P2a and S2a- The amplitudes are shown in Figs. C7 and C8, and in
the following table (average values).
P2a(dB) P2b(dB) S2a(dB) S2b(dB)
50 69 55 66
60 80 59 83
Profile
64a
64b
The P2b-amplitude is about 20 dB above the P2a-amplitude and
the S2b-amplitude is about 10—20 dB ahove the S2a-amplitude. The
difference is about 10 dB larger than is commonly found between
head waves from different layers in Iceland. A possible explanation
may be the relatively small velocity difference between the layers
on this profile, and the relatively large difference in head wave coef-
ficients for waves P2a and P2b (and S2a and S2b). These head wave
coefficients depend on the velocity and density ratios of the media
on both sides of the refracting boundary, and on the Poisson’s ratios.
Mýrar-Snaefellsnes (Ll). On this profile first arrival P-wave
amplitudes were measured at distances beyond about 15 km. The
reduced amplitudes increase at first but beyond about 50 km they
remain essentially constant, with a relatively small random varia-
tion (Fig. C9). The apparent velocity of the P4-wave is low, and it
is not possible from the travel time diagram to distinguish between
the P3- and the P4-wave on the basis of apparent velocities. The
amplitude graph does not either give any change in amplitude that
could be related to a change in first arrival from the P3-wave to the
P4-wave. The existence of the P4-wave along the distant part of this
profile is thus somewhat in doubt. Since on the basis of results from
other profiles it appears to be present at the eastem end of this pro-
file, the most likely interpretation is that it is also present at the
westem end, but that the upper boundary of layer 4 is dipping down
to the west in the area south of Snaefellsjökull.
Mýrar-Gardskagi (L2). The P2-, P3- and P4-amplitudes were
measured. The P3-amplitude varies somewhat, but a relationship
with delay time variations is not immediately visible. Attenuation
seems to be small. The P4-amplitude varies 10-20 dB but seems on
the average to be slightly stronger than the P3-amplitude (Fig. C10).
Graenavatn-Eyjafjöll (L3). The P3-wave has a somewhat irre-
gular amplitude with no clear attenuation visible. The P4-wave is
the first arrival beyond about 70 km. It is about 5-10 dB stronger