Brittle-ductile transition and associated seismicity: Experimental and numerical studies and relationship with the b-value

The acoustic emission (AE) and the mechanical behavior of granite samples during triaxial compression tests have been analyzed. The size of AE events displays power-law distributions, conforming to the Gutenberg-Richter law observed for earthquakes which is characterized by the b-value. As the con ning pressure increases, the macroscopic behavior becomes more ductile. For all di erent stages of the rock mechanical behavior (linear, non-linear pre-peak, non linear post-peak, shearing), there is a systematic decrease of the b-value with increasing con ning pressure. A numerical model based on progressive elastic damage and the nite element method allows simulations of the main experimental observations on AE and of a wide range of macroscopic behaviors from brittleness to ductility. The model reproduces a decrease in the b-value that appears to be related to the type of macroscopic behavior (brittle-ductile) rather than to the con ning pressure. Both experimental and numerical results suggest a relationship between the b-value and the brittle-ductile transition. Moreover these results are consistent with recent earthquake observations and give new insight into the behavior of the Earth's crust.