Transformation shear instabili13 ' and the seismogenic zone for deep earthquakes


Abstract. We use a numerical model for olivine-spinel trans- In contrast, for shallow earthquakes, a quantitative theory for formation to study deep earthquake nucleation and to delineate nucleation and instability has been developed using laboratorythe seismogenic region within a subducting slab. The model in- derived friction laws [e.g., Roy and Marone, 1996]. In these cludes laboratory-derived flow laws, latent heat release, and studies, simplified constitutive laws are combined with elastic phase transformation kinetics. We calculate deformation, trans- coupling to determine the conditions for instability and parameter formation state, grain growth, and rheology for several paths ranges consistent with earthquake nucleation. A similar approach within a subducting slab. Strain rate perturbations are imposed to may be useful in evaluating the mechanism of deep earthquakes. define the necessary conditions for instability. Strain rate pertur- In this paper, we focus on the instability condition during nubations decay for • < a critical value •,, and thus the coldest, in- cleation of deep earthquakes. We study only the onset of instaterior portion of the metastable wedge deformstably. For • •c, bility and do not consider the more general problem of rupture strain rate perturbations grow, shear strength decreases with propagation and arrest. We use a model for coupled olivinestrain, and the system is potentially unstable. The instability con- spinel transformation and deformation described by Liu [1997] dition is mapped to delineate the seismogenic zone within a sub- who presentedetails of the model and showed how nucleation of ducting slab. The model seismogenic zone is bounded by •c, and, deep earthquakes could be understood in terms of stability criteat larger percentransformations, by coarsening of spinel grains ria. Here, we apply the model to a subducting slab and delineat

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