Rit (Vísindafélag Íslendinga) - 01.06.1971, Qupperneq 61

CRUSTAL STRUCTURE OF ICELAND
61
depth to layer 3 southwards. The total delay times of the P3-wave on
profiles in N-Iceland are given in Fig. 20.
In Hrútafjördur a reversed north-south profile was measxured (65a
and b) which shows a marked change in depth to layer 3 along its
length. Layer 1 is at the surface with a thickness of 0.6 km at the
northern end and of 1.5 km at the southern end. It is underlain by
layer 2. Delay times for the P3-wave have been computed using eqn.
(4.1.4) for the central part of the line where the P3-wave is the first
arrival on both profile directions. To obtain the true P-wave velocity
in layer 3 these delay times are suhtracted from the observed travel
times on hoth profile directions and straight lines drawn through the
resulting points. The velocities obtained from the slopes of the lines
are as follows:
Profile 65a V3 = 6.57
„ 65b V3 = 6.45
Average V3 = 6.51 km/sec
When the true V3-velocity has been found it is also possible to
calculate delay times for the P3-wave near both ends of the profile
including the shot points. The following delay times are found at the
shot points. Sx denotes the northern and S2 the southern shot points:
51 : 0.30 sec
52 : 0.63 sec
The difference, 0.33 sec, agrees very well with the value 0.34 sec,
which is f ound from the arrival times midway between the shot points
(bTSl s2 = Tax Ta2 ).
The delay times for the P3-wave have been converted to depths
which are plotted in Fig. 21. At the northern end of the profile the
depth to layer 3 is about 2.0 km while at the southern end it is about
4.5 km.
It is instructive to compare the gravity field along this profile
with the seismic structure. The Bouguer anomaly is shown in Fig. 21.
From the general gravity picture of Iceland (Einarsson, 1954) it is
not clear whether a regional N-S trend due to deeper density varia-
tions exists in the Hrútafjördur area. It appears just as likely that
such a trend does not exist. It will therefore be assumed that the
gravity variation is entirely due to the variation in depth to layer 3.
A theoretical gravity variation is calculated using the method of
Talwani et al. (1959a) for a two-dimensional structure. A good fit
with the observed gravity is obtained by using a density difference