Effects of Dilatant Hardening on Fault Stabilization and Structural Development

Dilatant hardening is one proposed mechanism that causes slow earthquakes along faults. Previous experiments and models show that dilatant hardening can stabilize fault rupture and slip in several lithologies. However, few studies have systematically measured the mechanical behavior across the transition from dynamic to slow rupture or considered how the associated damage varies. To constrain the processes and scales of dilatant hardening, we conducted triaxial compression experiments on cores of Crab Orchard sandstone and structural analyses using micro‐computed tomography imaging and petrographic analysis. Experiments were conducted at an effective confining pressure of ∼10 MPa, while varying confining pressure (10–130 MPa) and pore fluid pressure (1–120 MPa). Above 15 MPa pore fluid pressure, dilatant hardening slows the rate of fault rupture and slip and deformation becomes more distributed amongst multiple faults as microfracturing increases. The resulting increase in fracture energy has the potential to control fault slip behavior. Plain Language Summary When rocks are breaking, the pore spaces and developing fractures dilate, resulting in a decrease in pore fluid pressures. This decrease can strengthen the rock from ongoing deformation in a process known as dilatant hardening. We conducted experiments to better understand how this strengthening effect works, in particular looking at the ratio of pore fluid pressure to the external confining pressure (simulating rocks buried at depth), and also analyzed how the fractures that develop can vary from dilatant hardening. We found a threshold pressure at which the strengthening peaked, and increasing pore fluid pressure did not change how strong the rocks got from continuing deformation. We also observed a drastic increase in how damage was distributed due to this hardening effect at both a large (visible to the naked eye) and small scale (only visible in a high‐magnification microscope). These results indicate that dilatant hardening can increase how much energy must be expended to break the rock and to cause faults to slip when pore fluid pressures are high enough, and likely plays a role in stabilizing fault slip, causing earthquakes to slow down and be less dynamic. Key Points We measured the transition between dynamic and stable rupture as a result of dilatant hardening We observed differences in microstructural development tied to the shift in rupture style We developed a model of fracture nucleatio