Rit (Vísindafélag Íslendinga) - 01.06.1967, Qupperneq 91

87
seismograph stations, so only the largest are recorded at so
many stations that a reliable mechanism solution is possible.
A few reliable mechanism solutions of earthquakes from the
Mid-Atlantic Ridge by Stauder and Bollinger (1964, 1966)
and by Stefánsson (1966) all clearly indicate strike-slip
movement on a nearly vertical plane. This is very striking.
The current idea is that the Mid-Atlantic rift system has de-
veloped by the action of horizontal tensional stresses. These
would be released by a dip-slip movement on a plane dipping
about 63° (see, e.g., Heiskanen and Vening Meinesz, 1958).
The mechanism solutions cited above, would seem to contra-
dict the current idea, but this is not necessarily so. It is gene-
rally accepted that the breaking strength of rocks is much
less under tensional than under compressional forces. It is
quite possible that the general tensional stress system of the
Mid-Atlantic Ridge cannot build up a potential stress field
of such a magnitude as would be necessary to cause large
earthquakes, but the stress would be released more gradually
by smaller earthquakes or by plastic deformation. However,
distinct systems of horizontal shearing stresses could develop
in certain localities, e.g., at bends on the ridge. This is in
accordance with the distribution of large earthquakes on the
ridge, and the locations of the large earthquakes mentioned
here.
The method applied by Scheidegger (1966) for tectonic
interpretation of fault plane solutions from the Mid-Atlantic
Ridge indicates very clearly east-west tension in the area be-
tween the equator and Iceland (fault plane solutions from
14 earthquakes were applied). In the Greenland arctic seis-
mic belt southeast-northwest tension was indicated (7 earth-
quakes). In the Russian arctic belt on the other hand there
was a compressional tectonics, with nearly east-west com-
pression (6 earthquakes). Although the number of earth-
quakes is small the results are of great significance.
In Iceland large earthquakes seem to be confined to east-
west running belts in south and north Iceland (see Fig. 4).