The Variable Depth of Earthquake Foci

Abstract

PROF. H. H. TURNER has given reasons (Mon. Not. R.A.S., Geophys. Suppt. No. 1) for believing that the depths of the foci of earthquakes differ among themselves by quantities up to about 300 km. It may be pointed out that this is precisely what may be expected from the theory of a cooling earth. The available information concerning the thermal state of the earth indicates that the rocks in the asthenosphere, at depths of 400 km. and more, must be very much weaker than those at the surface ; this is amply confirmed by the geodetic evidence collected by Barrell, which also suggests that the rocks at depths comparable with 100 km. are considerably stronger than those at the surface. Accordingly, whatever may be the cause of crustal deformation in the earth, yield will occur in the asthenosphere for smaller stresses than are necessary to produce it in the upper parts of the crust. Thus the earthquakes arising from fractures below 400 km. would be more numerous but much less violent than those occurring at higher levels, and the greatest earthquakes should have their foci at the depth of greatest strength. We should therefore expect that the depths of earthquake foci may range from zero to 200 or 300 km.