Endo-exo classification of episodic rock creep in underground mines: Implications for forecasting violent rockbursts

Rock masses in deep underground environments under high in-situ stress often exhibit episodic creep behavior, driven by complex interactions between external perturbation and internal reorganization. The causes of these creep episodes and their link to potential catastrophic failure remain poorly understood. Here, we present a novel “endo-exo” framework for analyzing episodic rock creep in underground mines, capturing the interplay between exogenous triggers (e.g., blasting and excavation) and endogenous processes (e.g., damage and healing within rock masses). The underlying physical mechanism involves cascades of locally triggered rock block movements due to fracturing and sliding. We identify four fundamental types of episodic dynamics, classified by the origin of disturbance (endogenous or exogenous) and the level of criticality (subcritical or critical). All four types exhibit power law relaxations with distinct exponents: 1+ θ (exogenous-subcritical), 1- θ (exogenous-critical), 1‒2 θ (endogenous-critical), and 0 (endogenous-subcritical), all governed by a single parameter 0 < θ < 1. Our theoretical predictions are examined using the comprehensive dataset of a platinum mine in South Africa, where stopes display episodic closure behavior during successive mining operations. All creep episodes recorded can be accounted for in our classification with θ ≈ 0.35 ± 0.1, providing strong validation of our theory. This θ value is interpreted in terms of a first-passage process driven by anomalous stress diffusion, represented by fractional Brownian motion or Lévy-type processes. Finally, we offer new insights into endo-exo interactions and the system's transition from episodic creep to catastrophic failure, with important implications for forecasting violent rockbursts.