AGU: Geophysical Research Letters

 

Editors' Highlight

Inferring crustal and mantle strength: Observations after a Mojave earthquake

The strength of Earth's outer layers and their ability to resist deformation are important factors in determining earthquake hazards in seismic zones. To study this, Freed et al. (2007) 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 km 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 after-slip. Only flow in the upper mantle below a depth of 40 km 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.

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