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.