Page "lore" Paragraph 617
from
Brown Corpus
Earthquakes originate as far as 400 miles below the surface, but they do not occur at greater depths.
Two unsolved mysteries are based on these facts.
( 1 ) As far down as 400 miles below the surface the material should be hot enough to be plastic and adjust itself to twisting forces by sluggish flow rather than by breaking, as rigid surface rocks do.
( 2 ) If earthquakes do occur at such depths, why not deeper??
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