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depth in the earth. If at some depth this increase is replaced by a decrease
over an interval of depth, again followed by an increase at some
greater depth, we have, what we cali a low-velocity layer, provided
the numerical value of the velocity decrease with depth in at least a
part of the layer surpasses the criticai value v/r (v = velocity, r = radius;
see Gutenberg, 1954 b, and Bullen, 1954, pp. 87-89). The most
marked low-velocity layer (for P waves) exists on the inner side of the
outer core. This low-velocity layer has already been recognized by ali
seismologists long ago. If a low-velocity layer exists also at the boundary
of the inner core, is not yet certain. According to Jeffreys there is one,
whereas Gutenberg does not find suffìcient observational support for it.
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