Techniques for determining a reservoir characteristic include determining, with an analytical solution, a change to at least one control point of a boundary of a control volume defined in a subterranean formation, the change to the at least one control point caused by a hydraulic fracture formed in or adjacent the subterranean formation; determining, with a numerical solution, a fluid pressure change of the control volume based on the change to the at least one control point; and determining, with a mechanical model based on the analytical solution and the numerical solution, at least one reservoir characteristic based at least in part on the determined fluid pressure change of the control volume.