1. A method of an injection treatment of a subterranean region, the method comprising: defining a common solution vector for multiple distinct subsystem models, each subsystem model representing a distinct subsystem of an injection treatment system of a subterranean region, wherein the multiple distinct subsystem models comprise at least a block model, a joint model, and a fracture flow model, wherein the common solution vector includes variables for each of the multiple distinct subsystem models, wherein the multiple distinct subsystems models are tightly coupled to allow for solving the multiple distinct subsystems models together to produce the common solution vector; updating, by a numerical root-finding technique, parameters of the subsystem models based on the common solution vector according to predefined relationships between the common solution vector and the parameters of the subsystem models, wherein updating parameters of the subsystem models based on the common solution vector comprises generating a complete data set for all the subsystem models to operate in parallel on the common solution vector without iterating solutions between individual subsystem models; operating, by data processing apparatus, the subsystem models based on the common solution vector and the updated parameters to perform a simulation of an injection treatment of the subterranean region; and performing the injection treatment of the subterranean region using the simulation.