Studies of the Earth's response to large earthquakes can be viewed as large rock deformation experiments in which sudden stress changes induce viscous flow in the lower crust and upper mantle that lead to observable postseismic surface deformation¹. Laboratory experiments suggest that viscous flow of deforming hot lithospheric rocks is characterized by a power law in which strain rate is proportional to stress raised to a power, n (refs 2, 3). Most geodynamic models of flow in the lower crust and upper mantle, however, resort to newtonian (linear) stress–strain rate relations^{4, 5, 6, 7, 8, 9, 10}. Here we show that a power-law model of viscous flow in the mantle with n = 3.5 successfully explains the spatial and temporal evolution of transient surface deformation following the 1992 Landers¹¹ and 1999 Hector Mine¹² earthquakes in southern California. A power-law rheology implies that viscosity varies spatially with stress causing localization of strain, and varies temporally as stress evolves, rendering newtonian models untenable. Our findings are consistent with laboratory-derived flow law parameters for hot and wet olivine—the most abundant mineral in the upper mantle—and support the contention that, at least beneath the Mojave desert^{5, 6}, the upper mantle is weaker than the lower crust.

1. Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907, USA
2. Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, California 94720, USA

Correspondence to: Andrew M. Freed¹ Email: freed@purdue.edu

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