Far‐reaching transient motions after Mojave earthquakes require broad mantle flow beneath a strong crust

Geodetically observed postseismic surface displacements in the 7 years following the 1999 Hector Mine earthquake demonstrate a previously unrecognized broad pattern of transient deformation throughout southern California and into Nevada, more than 200 km from the epicenter. Unlike previous postseismic observations in which trade‐offs between postseismic mechanisms and the depth of flow lead to non‐unique solutions, this deformation pattern can only be explained by viscoelastic flow in a region of the mantle 100s of km wide and below a depth of 40 km. This result enables two robust conclusions regarding the nature of lithospheric strength in this region: the mantle is weaker than the lower crust, and flow occurs over a wide region of mantle as opposed to within a narrow shear zone beneath the fault.