Cohabitation of slow and moderate-sized earthquakes in blueschist-facies linked to fault microstructure and rheology

We explore how slow earthquakes and
moderate-sized intraplate earthquakes cohabit in blueschist-facies at
intermediate-depth. We measure friction on blueschist gouges at
elevated temperatures (100-500℃), effective normal stresses
(60-220 MPa) and fluid pressures (30-75
MPa) to represent conditions along the descending slab
and to link observed rheology to microtextural evolution.
We identify a transition into potentially
unstable rheologies at elevated temperatures
>200℃ - meeting the conditions necessary for generation of
earthquake instabilities at blueschist-facies depths. This
transition is controlled by a mechanism of thermally-activated
dissolution creep indicated by the observed presence of dissolved
glaucophane and suggestive of dilation-suppressing
fluid-assisted mass transfer processes. Reducing effective normal stress
results in increasingly unstable behavior as evident
in dynamic stick-slip. This is driven by an increase in effective
fault stiffness k'c as a result of enhanced shear
localization in the gouge fabric. However, this rupture
growth, promoted by lower effective
stress, may be limited by the
increase in fluid pressure which independently promotes
stable deformation with slip-dependent strengthening. From these multiple competing feedbacks, we infer that slip instabilities nucleated within blueschist grade slabs should favor earthquakes of limited-size. This is consistent with observations that both slow and moderate-sized dynamic earthquakes cohabit the cold subducting slab at blueschist-facies depths.