Post-peak properties of intact hard rocks (UCS > 250 MPa) at highly confined compression corresponding to seismogenic depths and shear rupture mechanism operating at these conditions are still experimentally unexplored due to spontaneous failure of extreme dynamics and violence. Recently conducted comprehensive analysis of different side effects accompanying the spontaneous failure resulted in the identification of a unique shear rupture mechanism according to which shear rupture propagation is governed by the fan-shaped fracture head characterized by very low shear resistance (less than frictional strength). Such low shear resistance of the fan-head determines the transient strength of intact rocks during the failure process. The paper discusses briefly main principles of the fan-mechanism and proposes an improved model of strength profiles for hard rocks involving the fan transient strength profile.

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