Dear Rob,
You make a good point that fault-slip data speaks directly about
incremental strain axes and not stress. On the other hand, with
fault-slip inversion of data from fault systems in the brittle upper
crust the validity of its assumptions is scale dependent.
Interferometric images allow us to measure coseismic displacements of
crust around earthquakes in geologically complex parts of the world.
Despite geological complexity these displacements have been matched
to a very accurate degree by simple models where the crust behaves as
an isotropic elastic medium. Therefore, on the scale of a fault
system in the brittle upper crust, it seems to be quite valid to
assume a mechanically isotropic, rheologically linear material. From
your work this would indicate that the principle axes from inversion
are stress axes.
Many of the applications I have seen of fault-slip inversion do not
go beyond constructing a deformation history for an area. In these
cases it is totally unnecessary to attempt to talk about stress axes.
It would be foolish however to be dualistic in our interpretations of
fault systems and the rock record i.e. thinking of fault systems only
in terms of either strain or stress. Strain will not tell us anything
about where there were greater accumulations of aftershock events
around fault systems over time; a question which has important
implications for fluid flow associated with those fault systems (Cox
& Ruming JSG v26 p1109+, Micklethwaite & Cox Geology v32 p813+). If
we want to research fault mechanics from field observations then
stress is more useful. Likewise understanding fault systems in
mineralised terranes has been held up for many years because
geologists have tended to approach them with simple concepts of
strain.
Cheers,
Steve
--
Steven Micklethwaite
Postdoctoral Fellow
Rock Physics,
Research School of Earth Sciences,
Mills Road, ANU
Canberra, ACT 0200
T: +61 2 61255169
F: +61 2 61258253
http://rses.anu.edu.au/petrophysics/Staff/StevenMHome.html
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