We present a mechanism for shear‐wave generation from buried explosions as part of the Source Physics Experiment (SPE) series. The SPE series includes sensitized heavy ammonium nitrate/fuel oil sources of various sizes detonated in a borehole in the jointed Climax stock granite. The cylinder‐shaped shots were grouted in the borehole to couple the energy to the rock. A high‐fidelity site model—with explicit inclusion of the cylindrical explosive, the grout‐filled borehole, and site joint sets—was included in a numerical simulation that mimics the near‐field velocity environment measured by an array of in‐ground accelerometers. This approach was accommodated through a coupled Euler–Lagrange code that allows simultaneous solving of a Euler domain to model the high‐deformation source region and a Lagrange domain that includes the complex geology with full contact. Specific laboratory‐measured geomechanical properties for the rock and the joint sets were included in the model. The simulations compare favorably to the data and provide a possible physical mechanism for unexpected shear motion through the release of stored shear strain on the joints. This research will advance our understanding of explosion‐generated shear‐wave energy from low‐yield nuclear tests.