The Rheology of Faults Triggered by the Olivine Spinel Transformation in Mg2geo4 and Its Implications for the Mechanism of Deep-Focus Earthquakes

The relevance of the anticrack faulting mechanism to the origin of deep-focus earthquakes depends critically on the physical processes operating during fault generation and movement. We show here that the resistance to sliding on fault zones produced by this mechanism in Mg2GeO4 depends only weakly on confining pressure, but strongly on sliding rate. Both of these characteristics are inconsistent with friction. The microstructures of the fault zones and the crystal-plastic rheology of the constituent phases indicate that sliding must occur primarily by grain-boundary sliding. Previously, we reported that anticrack faulting is accompanied by elastic radiation of energy (ar-oustic emissions), and that it can operate during the alpha --> beta transformation in (Mg,Fe)2SiO4 at the pressures and temperatures at which deep earthquakes occur. The present results and the recent demonstration that metastable olivine is present in the subducting slab beneath Japan provide additional support for the anticrack theory of deep-focus earthquakes.