Systems, methods, and instructions encoded in a computer-readable medium can perform operations related to generating a model of a subterranean formation based on a probabilistic earth model. An earth model including a probability distribution for a property of a subterranean region is received. A subterranean formation model is generated based on sampling the probability distribution for the property. The subterranean formation model includes information on boundaries of rock blocks of a formation in the subterranean region. The subterranean formation model may be used for simulating an injection treatment applied to the formation, for example, to predict fracture propagation in the formation. In some implementations, the subterranean formation model may be used for designing an injection treatment for the formation.