Inferred rheological structure and mantle conditions from postseismic deformation following the 2010 Mw 7.2 El Mayor-Cucapah Earthquake

The 2010 Mw 7.2 El Mayor-Cucapah earthquake provides a unique target of postseismic study as deformation extends across several distinct geological provinces, including the cold Mesozoic arc crust of the Peninsular Ranges and newly-formed, hot, extending lithosphere within the Salton Trough. We use five years of GPS measurements to invert for afterslip and constrain a 3D finite element model that simulates viscoelastic relaxation. We find that afterslip cannot readily explain far-field displacements (more than 50 km from the epicenter). These displacements are best explained by viscoelastic relaxation of a horizontally and vertically heterogeneous lower crust and upper mantle. Lower viscosities beneath the Salton Trough compared to the Peninsular Ranges and other surrounding regions are consistent with inferred differences in the respective geotherms. Our inferred viscosity structure suggests that the depth of the LAB is ∼65 km below the Peninsular Ranges and ∼32 km beneath the Salton Trough. These depths are shallower then the corresponding seismic LAB. This suggests that the onset of partial melting in peridotite may control the depth to the base of the mechanical lithosphere. In contrast, the seismic LAB may correspond to an increase in the partial melt percentage associated with the change from a conductive to an adiabatic geotherm.