Rit (Vísindafélag Íslendinga) - 01.06.1967, Side 86

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from about 6.7 km/s to 7.4 km/s. Tryggvason (1959) found
layers with such velocities from an investigation of the ap-
parent velocity of P waves of earthquakes in Iceland. Ewing
and Ewing (1959) found the 7.4 velocity at a considerably
shallower depth south-west of Iceland. The existence of the
7.4 layer has been verified many times since then, and it
certainly exists in many localities, especially under the mid-
ocean ridges. Tryggvason (1961) tried to find the lower
boundary of the 7.4-layer, but this has up to now proved
to be difficult. He used P-onsets from near and distant seismo-
graph stations of some earthquakes in the region between
Norway and Greenland. The method is to some extent simi-
lar to the seismic refraction method where explosions are
used instead of earthquakes. Tryggvason concluded that a
depth of 100—140 km of the lower boundary of the 7.4 layer
could explain the velocity of propagation. It was estimated
that the width of this layer was of the order of 1000 km.
A study of delays of P waves from a large earthquake in
Iceland (Stefánsson, 1966) throws some doubt on the unique-
ness of Tryggvason’s result. It is possible that the velocities
of propagation which Tryggvason found can be explained by
a much shallower 7.4-layer. Below such a shallow 7.4-layer
we should have the velocity 8—8.2 km/s which is usually
found just below the crust, and then a low velocity layer
at a depth of 100—200 km, with P velocity 7.8—8 km/s, but
such layers have been found in other parts in the world.
Such low velocity layers are often problematic for seismo-
logists, especially as they cause shadows at particular di-
stances, and this can cause errors in reading arrival times.
It may be that the first arrival is hidden, but a later arrival
is taken for the first. A thorough study of the existence and
depth of such low velocity layers is of great significance.
The best way to study this is to observe amplitudes of the
seismic waves and shadow zones. This is a study which can
hardly be cari’ied out at any place on the Mid-Atlantic Ridge
except Iceland.