From atomic defects to microcracks: Tracing damage with EPR

The accumulation of microdamage fundamentally controls the rheological behavior and dynamic permeability of the lithosphere. However, quantifying the internal subcritical damage that precedes macroscopic failure remains a challenge even during controlled laboratory experiments. We present a proof-of-concept study using Electron Paramagnetic Resonance (EPR) spectroscopy to bridge the gap between atomic-scale defects and rock-scale failure. 

EPR provides a direct, chemically specific measure of unpaired electron spins associated with broken atomic bonds, effectively serving as a proxy of lattice-scale damage. To test this, we subjected quartz-rich sandstone to intermittent mechanical loading (brazilian splitting and uniaxial compression) and thermal treatment (300°C) to track the evolution of paramagnetic defects. 

Preliminary results indicate that the EPR signal is highly sensitive to damage accumulation. Heat-treated specimens show a substantial signal increase, likely reflecting microfracturing driven by thermal expansion of mineral grains. Furthermore, under mechanical loading, the spectral intensity scales with the imposed load level. This suggests that the concentration of paramagnetic defects can track the progression of strain accumulation. We conclude that solid-state spectroscopy offers a promising non-destructive method to probe the fundamental microprocesses accompanying rock deformation, shedding light on the atomic-to-granular mechanisms that ultimately govern rock strength and permeability.