We investigate the stability of tomographic analysis by comparing the results of two different methods of parameterizing the three-dimensional P-wave velocity variations in the vicinity of the 1989 Loma Prieta earthquake. A block inversion is implemented using 55 by 45 by 10 blocks of 1 km width and varying thickness to a depth of 25 km below the surface. Linear and nonlinear analysis are presented. The nonlinear analysis is achieved by iterating over three-dimensional raytracing and earthquake relocation relative to current three-dimensional models until solutions show only small improvements. A second parameterization is achieved by using cubic B-spline functions to span the space of the model, which is rotated by 46.5°. Nonlinear results are presented with several different starting models illustrating the robustness of the technique to the initial conditions. All the nonlinear results produced essentially the same final model, which was structurally the same as the model obtained by linear analysis using a reasonable starting model.