The ML=5.9 Whittier Narrows, California, earthquake of October 1, 1987, triggered a complex aftershock sequence. The aftershocks had many different focal mechanisms including thrusting on E-W striking, north dipping planes (similar to the main shock), right-lateral motion on NW-SE striking planes (similar to the ML=5.3 aftershock on October 4, 1987), thrusting on N-S striking planes, and left-lateral motion on N-S striking planes. I attempt to interpret these mechanisms in terms of a stress field that may have caused them. Previously, stress inversion methods based on focal mechanisms assumed that the stress field was spatially uniform. In order to test for the spatially varying effect of the main shock's dislocation on the aftershocks' focal mechanisms a new inversion technique was developed that solves for a uniform stress field that is superimposed on a given spatially varying stress field. A series of simulations were performed to test both the new method and an older method that uses only the uniform component of the stress field. These simulations show that both techniques work but will be limited by the amount of noise in the data and the accuracy to which the given spatially varying stress field resembles the actual spatially varying stresses.

Most importantly, the simulations show that the older methods do work when the stress field has a uniform component that is at least as large as the spatially varying component. The simulations also show that when applying the older stress inversion techniques, one can determine if the results are valid, and the amount of spatially varying stresses present, by examining the misfit between the data and the model. Application of these techniques to the Whittier Narrows aftershocks suggests that the uniform component of the stress field, and the complex faulting, corresponds to the response of an elastic halfspace to a simple regional N-S compression. The misfit between the data and the uniform model demonstrate the presence of spatially varying stresses at Whittier Narrows; however, their form, as shown by the simulations, can not be determined with this data set and these techniques.