This paper presents a numerical methodology that can simulate rupture (unstable or seismic slip) and aseismic slip along a fault and estimate seismic energy radiated if conditions for instability emerge. The methodology is developed in the Universal Distinct Element Code (UDEC) using its explicit time-stepping scheme. Evaluation of rupture energetics is first demonstrated by a direct shear test that simulates some unbound fraction of a fault with slip-weakening behaviors. This same approach is extended to model rupture along a shallow, strike-slip, fault that is constrained by surrounding elastic rockmass. The model mesh geometry and loading conditions are calibrated by simulating idealized fault activation and then checking the calculated rupture area, slip (cumulative displacement discontinuity), seismic moment, and radiated seismic energy against available analytic solutions. Loading the idealized fault by non-uniformly distributed single couples and mesh geometry that follows the 1:40 ratio of the mesh size to rupture length are found to generate results within 5% error of the analytical solutions. As an example of the methodology application, relationships between rock rigidity (shear modulus), slip-weakening behavior, rupture length, slip, seismic moment, and radiated seismic energy are then discussed through a set of parametric studies. Globally recorded variations between seismic energy and the seismic moment for shallow strike-slip earthquakes are also used to verify seismic energy calculations in each case of parametric studies. Results show that the developed methodology and modeling approach provide a useful platform for mechanistically studying earthquake physics.