Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 43

CRUSTAL STRUCTURE OF ICELAND
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jumps which possihly could be interpreted as faults. The travel
times have been reduced to a cominon height above sea level so that
elevation differences should not have any effect. Apart from these
]umps there are no major irregularities in the delay time, that could
be related to the volcanic zone.
The amplitude of the P3-wave was also read from the seismo-
grams and reduced to common conditions as described in an earlier
section. The reduced amplitude is also plotted in Fig. 11. It has a
niaximum close to an azimuth of N65°E. This maximum is probably
a real one, judging from the relatively smooth variation of the ampli-
tudes on both sides, although it is known from other profiles that a
relatively large scatter may occur in reduced amplitudes at different
stations along the same profile. It may be of some significance that
the direction of minimnm attenuation coincides closely with the main
direction of the volcanic zone on the Reykjanes peninsula, as shown
on the geological map of SW-Iceland (Kjartansson, 1960).
A 30 km long unreversed profile (42) runs from a shot point in
lake Thingvallavatn south across Hengill terminating just north of
Thorlákshöfn. The surface layer in Grafningur just south of the shot
point has a P-wave velocity of 3.05 km/sec and is 0.4 km thick. The
most probable value of the shot point delay time for the P3-wave is,
judging from the total delay time, about 0.44 sec. Layer 1 has an
apparent velocity of 3.78 km/sec and layer 2 appears to be absent.
The depth to layer 3 between Thingvallavatn and Hengill is then
found to be 1.9 km. The delay time of the P3-wave increases to the
south along the profile and reaches about 0.55 sec at the southem
end of the profile. This is in fairly good agreement with delay times
along the Graenavatn-Eyjafjöll profile.
A short (about 10 km) profile runs from the same shot point in
Thingvallavatn to the east along the road in Grafningur. It continues
south on the west side of river Sog towards Ingólfsfjall. The travel
times along this profile are generally about 0.10 sec longer than on
the previous profile. This is probably mainly due to a larger thick-
ness of the surface layer, which here may be up to 0.8 km thick. This
is similar to its thickness in the Thingvellir graben.
Profile A of Tryggvason and Báth (1961), which is reversed
(AI and AII), runs in a westward direction from lake Thingvalla-
vatn. The delay times of the P3-wave have been calculated for these
profiles and are plotted with other profiles in Fig. 10. The delay
times are close to those obtained on profile 42.