Exploring the role of hydromechanics in back-propagating rupture dynamics

Back-propagating rupture (BPR), a phenomenon where seismic rupture migrates rapidly backward away from its advancing front, has been observed across various geological settings. Previous studies have proposed that the occurrence of BPR is attributed to fault zone complexity and heterogeneity, including the presence of fluids, but how fluid flow controls BPR behavior remains poorly understood. In this study, we use the Hydro-Mechanical Earthquake Cycles (H-MECs) model to explore the interplay between fluid flow and BPR in a fault with a poro-visco-elasto-plastic medium, governed by rate- and state-dependent friction. Our simulations show that back-propagating rupture can occur either on a homogeneous fault or fault zone with heterogeneous hydro-mechanical structure. By increasing the background pre-stress on a homogeneous fault, we observe different rupture modes, from steady pulse rupture with BPR, steady-growing pulse rupture to crack-like rupture, with rupture speed increasing linearly. Further simulations accounting for fault zone heterogeneity demonstrate that regions with low pore-fluid pressure are more likely to undergo seismic rupture, while regions with high pore-fluid pressure remain stable and creeping. BPR occurs when rupture transitions from a low to a high pore-fluid pressure region.  This pore-fluid pressure transition induces oscillations in slip rate and shear stress, triggering a shift from pulse-like to crack-like rupture behavior, generating a secondary rupture front that propagates backward and causing re-rupture along the fault. Our findings indicate that the length of the high pore-fluid pressure region and background pre-stress significantly influence the occurrence and propagation of BPR — smaller background pre-stress and larger high pore-fluid pressure regions promote BPR, driven by a self-healing front behind the forward rupture. These results emphasize the critical role of pore-fluid pressure heterogeneity and stress conditions in fault dynamics, providing a plausible mechanism for back-propagating rupture consistent with observations.