From seismic to aseismic slip in the lower crust: Results from hydrothermal ring-shear experiments

Understanding why earthquakes nucleate unusually deep in the crust is essential for improving seismic hazard assessments. These events occur under pressures and temperatures where rocks are expected to deform ductilely, challenging standard models of rock strength and faulting. Constraining the conditions that allow frictional instabilities to persist at depth therefore has important scientific and societal implications. To investigate the transition from potentially seismic to aseismic slip in lower-crustal environments, we conducted hydrothermal friction experiments on simulated gouges derived from epidote-rich (65%) and amphibole-rich (58%) basement gneiss from the western branch of the East African Rift System. We characterized their frictional strength and stability across temperatures of 350-600 °C, 150 MPa effective normal stress, and 100 MPa pore fluid pressure. We tested three slip-velocity protocols spanning slow (0.01-1 μm/s), intermediate (0.1-10 μm/s), and fast (1-100 μm/s) rates. Both samples show frictional strengths that vary with temperature and slip velocity. Rate-and-state friction parameters (a-b) indicate that the epidote-rich gouge exhibits velocity-weakening behaviour between 350-500 °C and at 0.3-100 μm/s, whereas the amphibole-rich gouge remains velocity-weakening across the full temperature range and at 1-100 μm/s. Microstructural observations indicate that deformation is primarily accommodated within a broad slip zone, where frictional granular flow and cataclasis dominate under both high- and low-temperature conditions. At the highest temperatures (600 °C) and slow slip rates, however, additional evidence for dissolution-precipitation creep was found, indicating the operation of viscous deformation. Our results suggest that epidote and amphibole-rich gouges can host seismic slip under lower-crustal temperature conditions at elevated slip rates. Under natural lower-crustal conditions, these elevated slip rates, sufficient to trigger frictional instability, could be facilitated temporarily by stress transfer, strain localization, or transient fluid-pressure variations.