Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 65

CRUSTAL STRUCTURE OF ICELAND
65
which therefore introduces an uncertainty in the gravity model.
The structure deduced from the seismic and gravity data is shown
in Fig. 23. It is based on the assumption that the P3-wave velocity
is 6.5 km/sec. The three shot points Si, S2 and S3 for the shorter
profiles are shown, hut the shot point of profile 2 is on the north
coast at the mouth of Vatnsdalsá.
The seismic structure has been deduced using the same methods
as on Snaefellsnes (cf. sect. 6.5). The true velocity of the P3-wave
between Sx and the shot point of profile 2 is close to 6.55 km/sec.
There are on the other hand some indications that the true velocity
in layer 3 under the anomalous structure may be somewhat lower,
possibly 6.3-6.4 lon/sec. If such a horizontal change in velocity oc-
curs the interpretation becomes more uncertain and the structure
section would be somewhat modified. The main features would,
however, remain.
The residual gravity anomaly is also shown in Fig. 23. Good
agreement between observed and calculated gravity variation is
found by using a density difference of 0.19 g/cm3 between layers
3 and 2. This is in good agreement with the Hrútafjördur value of
0.18 g/cm3.
In the Skagafjördur valley four profiles have been measured, two
on each side of the valley, including profile E of Tryggvason and Báth
(1961). Large structural variations are found to occur in the crust
under the valley, both in an east-west direction and in a north-south
direction.
The total delay times for the P3-wave are given in Fig. 20. On
the Tindastóll profile (3) the delay time is fairly constant giving a
value of about 0.35 sec near the shot point and increasing to 0.39 sec
near the southern end of the profile just south of Saudárkrókur. This
corresponds to a depth of about 2.2 km to layer 3 north of Saudár-
krókur. On the opposite side the Höfdaströnd profile (4) gives a
fairly constant total delay time of about 0.92 sec, corresponding to a
station delay time of 0.46 sec. The depth to layer 3 indicated here is
about 3.2 km, which is 1 km greater than on the Tindastóll profile.
Both these profiles have apparent P3-wave velocities of between 6.4
and 6.5 km/sec indicating that the upper boundary of layer 3 is
largely horizontal.
Farther to the south conditions change. The Blönduhlíd profile
(5) on the east side indicates a delay time of 0.50 sec at the shot point
which is close to the end of the Höfdaströnd profile. Near Flugumýri
5