Fluid-induced earthquake nucleation controlled by shear-induced compaction and dilation

The conventional understanding of tectonic faults primarily categorizes them based on frictional behavior: stable due to velocity-strengthening (VS) behavior, or unstable owing to velocity-weakening (VW) that lead to seismic ruptures. This classification has traditionally led to the assumption that VS faults are unlikely candidates for earthquake nucleation. However, emerging evidence from recent laboratory experiments and field studies is challenging this simplistic view, pointing towards a more complex mechanism. In this study, we utilize a hydro-mechanically coupled fault model, which integrates VS friction governed by rate-and-state friction laws with dynamic weakening influenced by poroelastic effects. A key aspect of our findings is the impact of fluid injection on the mechanical state of the fault. This process decreases the effective normal stress and frictional resistance, initially paving the way for the propagation of an aseismic, slow-slip event. The transition from aseismic to seismic slip on VS faults hinges on the balance between shear-induced dilation and compaction. These opposing mechanisms respectively lead to a decrease and an increase in pore-fluid pressure, dictating the balance between fault stability or instability. Our results show that when the effect of compaction-induced pressurization surpasses the initial dilatancy phase, it enables the propagation of dynamic rupture as a solitary pore-pressure wave. Conversely, when dilation predominates over compaction, an aseismic slow-slip event propagates through the fault, maintaining stability and preventing rapid seismic activity. These findings advance our understanding of seismic risk associated with VS faults. They are especially relevant in the context of fluid injection practices in geothermal energy production and CO2 storage, demonstrating how such activities might activate faults that are considered nominally stable. Additionally, our results underscore the critical need for more experimental and theoretical investigations into shear-induced compaction as an efficient mechanism for fault self-pressurization, which plays a key role in leading to seismic instabilities.