Quantifying the Role of 3D Fault Geometry Complexities on Slow and Fast Earthquakes

Abstract Traditional models of slow slip events (SSEs) oversimplify fault geometry, although imaging shows subduction faults are segmented and complex. We examine how fault interactions control slip behavior using 3D quasi‐dynamic simulations of two parallel faults with uniform rate‐weakening friction accelerated by hierarchical matrices. Four regimes emerge—periodic earthquakes, coexisting SSEs and earthquakes, only SSEs, and complex sequences—whereas a single planar fault with the same friction produces only earthquakes. We quantify interaction using the maximum Coulomb stress induced by a unit stress drop on a neighboring fault. This interaction metric depends on geometry, allowing extension to arbitrary fault systems. SSEs occur at intermediate interaction strengths. Low interaction strengths produce periodic earthquakes, whereas high interaction strengths generate complex earthquake sequences with irregular recurrence and variable magnitudes. Simulations reproduce moment–duration scaling and show sensitivity to detection thresholds. These results demonstrate that geometric complexity alone generates both slow and fast earthquakes through evolving traction heterogeneity.