台灣及鄰近地區地殼變形之數值研究模式(III)：應力軸轉換之三維數值模式分析

Using 3-D distinct-element modeling, we
explore a variety of simulations to characterize
the stress permutations observed in brittle
tectonics. Stress inversions of fault slip data or
earthquake focal mechanisms often reveal such
permutations. The main aim of our study is to
produce mechanically consistent 3-D models that
account in a simple way for switches between
principal stress axes σ1-σ2 or σ2-σ3. Other
phenomena, such as those related to elastic
rebound, are beyond the scope of this work.
It appears that the stress changes induced
by variations in rheology are large enough to
modify the local tectonic behavior and produce
permutations of principal stress axes, despite the
simplicity of far-field boundary conditions.
Rather than simple directional changes, which
exist but are limited, the relative variations in
principal stress values are the major cause of
permutations σ1-σ2 and σ2-σ3. This is in good
agreement with observations in nature, where
despite permutations the orientations of axes
often remain tightly clustered. Note that the most
demonstrative experiments were done with a
ratio Φ of 0.5, implying that σ2 is the arithmetic
mean between σ1 and σ3 (low Φ ratios favor
σ2/σ3 permutations, whereas high Φ ratios favor
σ1/σ2 ones).
In terms of geological significance, we
conclude that the major causes of stress
permutations are the heterogeneity of the brittle
deformation (e.g., intact rock massifs between
heavily faulted grabens of deformation zones)and the anisotropy of the mechanical properties
that results from the fracturing and faulting (that
is, a rock more resistant in the direction parallel
to faults than in other directions). Our modeling
effectively revealed that anisotropy in rock
properties favor stress permutations. Of major
importance seems to be the existence of
relatively resistant zones at the tips of deformed
ones, acting as channels where stress
concentrates and switches occur. Because in
nature such zones move in time and space, it is
not surprising that stress permutations are so
pervasive.
In our modeling experiments, we explored a
variety of compressional, extensional and
strike-slip tectonic situations involving stress
permutations.