The strength of Earth's outer layers, their ability to resist deformation, is an important factor in determining earthquake hazards in seismic zones. To study this, Andrew Freed, assistant professor of earth and atmospheric sciences, sought to infer the strength of the lithosphere under California's Mojave Desert by treating local earthquakes as large rock deformation experiments. Using a wide-ranging network of GPS instruments spanning more than 200 kilometers from the epicenter of the 1999 Hector Mine earthquake, the authors found that a broad pattern of transient deformation occurred in the 7 years following the earthquake. These data were compared with predictions from numerical models of various postseismic relaxation mechanisms including viscous flow in the lower crust and upper mantle, rebound associated with crustal fluids, and afterslip. Only flow in the upper mantle below 40-km depth was able to generate the pattern of displacements seen at large distances. This comparison pointed to only one possible scenario: the mantle is weaker than the lower crust (a "crème brûlée" structure) and the mantle flows over a wide region rather than within a narrow shear zone beneath the fault.