A Creep Instability in High SFE Materials at Intermediate Homologous Temperatures with Application to Slow Earthquakes and Slow Slip

Abstract:

Recent high­pressure torsion (HPT) experiments have revealed a mechanism that produces unstable creep at intermediate homologous temperatures in the range of those at the base of the seismogenic zone. In these experiments, a thin disc or ring is first subjected to a normal stress of 1­6 GPa and then to a simple shear deformation by rotating one of the loading pistons. The high normal stress suppresses fracture allowing ductile flow at intermediate values of T/Tm. A decrease in the flow stress was observed to be associated with dynamic recrystallization and the growth of larger grains with lower dislocation densities. This new mechanism is especially promising for slow earthquakes because, unlike runaway thermal weakening, the strain weakening associated with recrystallization is not catastrophic. The observed recrystallization extends over a large strain, producing a long effective slip­weakening displacement with potential to lead to slow earthquakes. Although this weakening mechanism was observed in Al, Mg, Zn and wet NaCl, it was not observed in Cu. Two possible explanations are: 1) Al, Mg, Zn, and NaCl are deformed at a significantly higher homologous temperature and 2) the stacking fault energies (SFE) of Al, Mg, Zn, and NaCl are significantly higher than Cu. A high SFE facilitates the dynamic recrystallization process through easy cross-slip of screw dislocations. Although there are no robust observations of stacking faults in olivine, it is expected to be very high because the separation between the partial dislocations is very small (l < 40 Å). Shear weakening via dynamic recrystallization is likely in silicates near the brittle­ductile transition. We present here new HPT experimental results for an alloy (Al­6061) that show strain weakening indicating that the phenomenon is not limited to high­purity materials. An analytic slip pulse model was used to show that very slow propagation is possible for reasonable values of shear zone width and slip pulse dimensions. A 2D viscoelastic model was used to simulate the nucleation and propagation of slow events in a layer having strain weakening corresponding to the experimental results.