Summary Fault systems have geometrically complex structures in nature, such as stepovers, bends, branches, and roughness. Many geological and geophysical studies have shown that the geometrical complexity of fault systems in nature decisively influences the initiation, arrest, and recurrence of seismic and aseismic events. However, a vast majority of models of slip dynamics are conducted on planar faults due to algorithmic limitations. We develop a 3D quasi-dynamic slip dynamics model to overcome this restriction. The calculation of the elastic response due to slip is a matrix-vector multiplication in boundary element method, which can be accelerated by using Hierarchical Matrices. The computational complexity is reduced from O(N2) to O(Nlog N), where N is the number of degrees of freedom used. We validate our code with a static crack analytical solution and the SEAS benchmark/validation exercise from Southern California Earthquake Center. We further employ this method on a realistic fault system with complex geometry that was reactivated during the 2023 Kahramanmaraş–Türkiye doublet earthquakes, generating slip sequences that closely match real observations.