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the problem of the upper mantle structure by studying the strain relea.se
variation with depth. If the method and data used in this paper are adequate,
we may be allowed to say that although there is no strain release
evidence for the depth of the upper boundary of the asthenospliere zone on
account of lack of adequate accuracy in the determination of focal depths,
nevertheless there is ample indication of a discontinuity at about 125 km
depth. The abrupt change in the rate of decrease in the strain release
with depth near this level clearly indicates that a sudden decrease in the
yield strength of the material in the earth should occur at about this depth.
I t might even be possible to think that the melting point of some kind of
crystal grains or rocks in the earth is attained at that depth. However,
this does not involve a completely molten state. This state should rather
occur at depths where there is a complete lack of strain release. Regionally
this state is attained at different depths, but in some regions the partially
molten state, i. e. the heterogeneity of the mantle, probably recurs or increases
due to the pressure increase or some other reason and reaches a minor
maximum beyond which it might be possible to speculate that the heterogeneity
of the mantle falls off rapidly and a continuous layer of material
in molten state covers the whole earth. If data from other sources will
confirm this structure, there will be good reasons to think of redefining
the upper boundaries of surface and intermediate shocks at depths of 125
and 425 km or thereabouts, respectively.
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