The transformation of olivine to spinel in a subducting slab gives rise to thermal strain, transformation strain, and buoyancy forces inside the slab. We use a finite element method to solve the mechanical equation, and we follow the evolution of the stresses inside the slab during its descent. We use two different rheologies: elastic and viscoelastic with a viscosity that depends on pressure, temperature, and grain size. The stresses due to buoyancy are found to be negligible compared to the internal stresses due to the phase transformation. When the viscoelastic rheology is used, a zone of very high differential stress coincides with the olivine-spinel phase boundary. This zone is associated with large downdip compressive stresses inside the metastable olivine wedge, and large downdip tensile stresses in the spinel region, along the wedge. If we assume that the failure criterion can be related to the differential stress, our model is in agreement with the patterns of deep seismicity. However, patterns of deep seismicity are unlikely to reflect buoyancy stresses, which in the presence of a metastable olivine wedge, are negligible compared to the internal stresses.