Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 78

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GUÐMUNDUR PÁLMASON
thesis is that the low apparent P4-wave velocity on L1 is caused by
a combination of all the above mentioned possibihties.
A search was made for reflected waves from the 3—4 interface on
the seismograms of profiles L1 and L2. Amplitudes as well as travel
times were observed and compared with the structure deduced above.
The results are further discussed in a later section (cf. sect. 8).
The profile Kjalarnes-Faxaflói, which runs from a land station on
Kjalarnes westwards to a distance of 57.7 km, was discussed in sec-
tion 6.4. A depth of about 3 km to layer 3 is indicated along the
westem half of the profile.
Off the south coast the profile L6 will be discussed first. It nms
from a land station near the mouth of river Thjórsá in a westward
direction, terminating near Eldey at a distance of 104.2 km. At the
shot point layer 1 is found at a relatively shallow depth with an ap-
parent velocity of 4.1 km/sec. Due to the relatively large spacing be-
tween stations, which was used on the offshore profiles, layers 2 and
3 are not well defined by segments on the travel time curve. Layer 2
is probably not present, as indicated by the neighbouring profiles on
the Reykjanes peninsula. The apparent P3-wave velocity is low,
ahout 6.1 km/sec. Beyond approximately 40 km the P4-wave is ob-
served as a first arrival with an apparent velocity of 7.13 km/sec.
A delay time of the P3-wave of about 0.80 sec is indicated near the
shot point of profile L6. With only layer 1 present with a probable
true P-wave velocity of 4.4 km/sec this gives a depth of 4.8 km to
layer 3. The delay time is increasing to about 0.95 see at a distance
of about 35 km which, if caused by increased thickness of layer 1,
gives a depth of about 5.5 km to layer 3. It is, however, also possible
and more probable that a lower-velocity surface layer is at least partly
responsible for the increased delay time, and that the actual depth
to layer 3 is about 5.0 km on this part of the profile.
At distances heyond about 40 km the P4-wave is the first arrival.
The apparent velocity of 7.13 km/sec, which is close to the most
probable value of the true P4-wave velocity, indicates that no major
structural changes are found under the distant part of the profile.
The total delay time of the P4-wave is about 2.20 sec. Dividing this
equally between the shot and detector ends and using the thickness
of layer 1 deduced above, a depth of 8.4 km is obtained. This is
slightly less than the depth of 8.8 km obtained in Faxaflói on profile
L2, and the 8.9 km obtained on the Kerid—Hvítárvatn profile (L7).