A software developed for slip modelling on a 3D heterogeneous fault with rate-and-state friction. ICGT refers to the Imperial College Geomechanics Toolkit, a finite-element based computational software package written in C++. It's used in research and academic work, particularly within the Department of Earth Science and Engineering. Three-dimensional quasi-dynamic finite element approach is used for the simulation of seismicity on a heterogeneous fault with rate-and-state friction. The coupled nonlinear hydro-mechanical equations of the fault and surrounding matrix are solved monolithically to obtain the fluid pressure and displacement fields. The augmented Lagrangian method is used to apply the contact constraints. The fault is represented as a surface in the model using zero-thickness interface elements. Software is tested for operating system Windows Server 2022 Version 21H2 using AMD EPYC 7H12 64-Core processor. Note: ICGT uses Ansys ICEM CFD (2022) for mesh generation.To start the simulation, run the Benchmark_problem.bat input file.This input file can be modified using user-defined parameters.