At 08:57 AM 1999/09/02 +0100, Stuart Crampin wrote:
>What and where is shear stress? Seismology suggests true triaxial
>stress is present throughout much of the crust regardless of tectonics
>or the presumed stress state.
The question seems easy - perhaps I misapprehend. If the triaxial stress field
is such that the maximum principal (compressive) stress is horizontal
north-south and the minimum principal stress is horizontal east-west, then
left- and right-handed shear stress would be maximal on vertical planes
oriented respectively NE-SW and NW-SE.
>...
>I suggest the implication is that the dynamic microstructure of
>fluid-saturateed microcracks in situ rocks is controlled (certainly in
>the uppermost 15km, and probably throughout the crust) by purely
>triaxial stress. Shear-stress is only apparent when there is
>discontinuity or fracture and its effects can only be seen in
>retrospect.
But purely triaxial stress implies shear stress. You can't have one without
the other. Rocks may yield by extensional microcracks oriented normal to
the minimum compressive stress, but this does not imply that shear stress
was nonexistent. Similarly, in regimes where rocks fail by
shear-fracturing, this does not imply the absence of a "purely triaxial
stress". Or am I somehow missing Stuart's point?
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