Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 72

72
GUÐMUNDUR PÁLMASON
Two profiles were measured in opposite directions along river
Jökulsá from a common shot point in Langavatn near Grímsstadir.
These are the Grímsstadir-Axarfjördur (19) and Grímsstadir—
Herduhreid (20) profiles. Thcy both give a high P3-wave delay
time relative to other profiles in NE-Iceland. At the shot point the
delay time is about 0.76 sec increasing to 0.84 sec along the northem
profile and decreasing slightly to about 0.72 sec to the south. Using
the velocity information near the shot point this gives a depth of
about 5.0 km to layer 3 at the shot point, increasing to 5.2 km north-
wards and decreasing to about 4.8 km southwards. Since the shot
point is at an elevation of about 0.4 km these depths are 4.6, 4.8 and
4.4 km below to sea level.
Farther to the south an unreversed profile (21) was measured be-
tween Herdubreid and Askja. The total delay time is lower here than
on the previous two profiles. The low-velocity surface layer is also
thinner so that the depth to layer 3 is similar. Dividing the delay
time of the P3-wave equally between shot point and seismometer
stations gives a delay time of 0.68 sec. This gives a depth of 4.9 km
to layer 3, or 4.4 km below sea level.
In Vopnafjördur two profiles were measured but only one of them,
23, was long enough to record the P3-wave as a first arrival. The P3-
wave delay time increases towards the southwestem end of the pro-
file, probably due to a thickening of the surface low-velocity layer as
the profile enters the zone with relatively young volcanic rocks at the
surface. At the shot point end of the profile the Tertiary hasalts (layer
1) are at the surface, and here the delay time of the P3-wave is about
0.59 sec. This gives a depth to layer 3 of 4.0 km or 3.6 km below
sea level.
6.9. E-Iceland.
Six profiles from eastern and southeastern Iceland will be dis-
cussed in this section. One of them is reversed but the other five are
not. They have all been described earlier (Pálmason, 1963) hut will
here he treated from the delay time point of view. The total P3-wave
delay times are shown in Fig. 26.
In Fljótsdalur (25) the total P3-wave delay time is about 0.80 sec,
increasing slightly to the northeast. The inferred depth to layer 3
along most of the profile is about 3.0 km. The apparent P-velocities
in layers 1 and 2 are rather high on this profile.
In Skriddalur a reversed profile (26 and 26a) runs from Egils-