Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 75

CRUSTAL STRUCTURE OF ICELAND
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structural interpretation. Possibly the poor arrivals on this profile
may be related to the structural change occurring between the westem
and the eastern part of southem Iceland.
The offshore profiles were all arranged in such a way that the re-
cording station was kept in a fixed position on the shore, while charges
were exploded from a ship at various distances along the line. This
arrangement proved fairly efficient and much smaller explosive
charges were needed than on the land profiles (cf. Fig. 4).
The two profiles L1 and L2 in Faxaflói mn from a common end
point on the shore close to Akrar. L1 mns west parallel to the Snae-
fellsnes peninsula to a distance of 100.54 km, and L2 runs to the SSW
terminating just west of the Reykjanes peninsula at a distance of
90.31 km. As it was difficult to get close to the shore on Mýrar be-
cause of shallow water depth, no particular emphasis was placed on
obtaining data at short distances, less than about 6-7 km. It appears,
however, that layer 1 is very thin here, and also that the delay time
and depth to layer 3 are relatively small. The delay times for the
P3-waves on these and the other offshore profiles are shown in Fig. 27.
The delay time of the P3-wave near the land station of profiles
L1 and L2 appears to be about 0.34 sec. With a probable vertical
velocity distribution this gives a depth of about 2.3 km to layer 3.
The P3-wave delay time increases by about 0.10 sec on both profiles
farther out, on profile L1 at a distance of 25—30 km and on profile
L2 at a distance of 18—20 km. This presumably indicates the termina-
tion of the structure causing the relatively low delay time at the re-
cording station of these profiles. Both profiles then show a slight in-
crease in delay time at distances of 40-50 km but decreasing again
farther out. The major P3-wave delay time anomalies on these pro-
files appear to be in the vicinity of the end station on land.
Beyond a distance of about 55 km on profile L2 the P4-wave is
observed as a first arrival. The total delay time of this wave is shown
in Fig. 28, and also of the first arrival on L1 beyond a distance of
about 50 km. The delay time of the P4-wave along the first half of
these profiles appears to be about 0.82 sec on the average. Using the
existing information on layers 1 and 2 and the assumption of true
P-velocity in layer 4 of 7.2 km/sec, this gives a depth of 8.8 km to
layer 4. On profile L2 the P4-wave delay time is slightly increasing
at larger distances which may mean increased depth to layer 4 by
1-2 km. The increase could also be due to a change in the velocity