In some aspects, a number of subsystem models is accessed by a computer system. Each subsystem model represents dynamic attributes of a distinct physical subsystem in a subterranean region. At least one of the number of subsystem models represents dynamic attributes of a mechanical subsystem in the subterranean region. A discrete fracture network (DFN) model representing a fracture network in the subterranean region is accessed at the computer system. The DFN model includes junction models. Each junction model represents interactions between a respective set of subsystem models associated with the junction model. Junction variables of the junction model can be defined based on dynamic attributes of the respective set of subsystem models associated with the junction model. A stimulation treatment for the subterranean region can be simulated by operating the DFN model including the junction variables.