Rit (Vísindafélag Íslendinga) - 01.06.1971, Síða 97

CRUSTAL STRUCTURE OF ICELAND
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amplitude beyond the critical distance on all four profiles, which
strongly supports the interpretation of these waves as reflections from
the 3-4 boundary. Since the arrival times agree fairly well with what
would be expected, this lends support to the correctness of the refrac-
tion interpretation.
9. SHEAR WAVES AND MULTIPLE EVENTS
A noteworthy characteristic of seismograms of most of the refrac-
tion profiles in Iceland is a strong arrival or a group of arrivals oc-
curring at a time which would be expected for S-waves. These waves
are generally of lower frequency than the P-waves. They were identi-
fied by Tryggvason and Báth (1961) as S-waves.
It was observed by Steinhart and Mayer (1961) that stronger S-
waves were generated by explosions on a water bottom than by
suspended charges. Theoretical calculations by Pursey (1956) have
shown that for a surface source a large proportion of the energy is
radiated as shear waves. In the Icelandic refraction studies the explo-
sions have usually been made in lakes at a depth of less than 5 meters.
The charges have been fired at the bottom which often consisted of
solid basalt lava. It thus appears that conditions for the generation
of S-waves may have been favourable and that the interpretation in
terms of S-waves, generated close to the shot point, is correct.
In the Icelandic measurements the S-waves were usually more
prominent in the Tertiary basalt areas than in the Neovolcanic zone.
Two explanations can be offered for this. Firstly, as the S-waves are
later arrivals, they may have been masked by noise caused by scat-
tering in the relatively inhomogeneous low-velocity surface layer in
the Neovolcanic zone. This is supported by the relatively noisy ap-
pearance of seismograms from these areas. Secondly, shot point condi-
tions may have been different in the Neovolcanic zone. The lake
bottoms there usually consisted of looser material than in the Tertiary
areas, the more solid lava occurring at greater depth. This would rela-
tively weaken the S-waves and make them less conspicuous on the
seismograms.
For the profiles in northem and eastem Iceland the S-wave velo-
cities were determined and used to compute the Poisson’s ratio for the
various seismic layers. In other parts of the country a few determina-
tions of S-wave velocities have also been made, particularly in westem
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