The 4 April 2010 Mw 7.2 El Mayor-Cucapah (EMC) earthquake provides the best opportunity to date to study the lithospheric response to a large (>M6) magnitude earthquake in the Salton Trough region through analysis of Global Positioning System (GPS) data. In conjunction with the EarthScope Plate Boundary Observatory (PBO), we installed six new continuous GPS stations in the months following the EMC earthquake to increase station coverage in the epicentral region of northern Baja California, Mexico. We modeled the pre-EMC deformation field using available campaign and continuous GPS data for southern California and northern Baja California and inferred a pre-EMC secular rate at each new station location. Through direct comparison of the pre- and post-EMC secular rates, we calculate long-term changes associated with viscoelastic relaxation in the Salton Trough region. We fit these velocity changes using numerical models employing an elastic upper crustal layer underlain by a viscoelastic lower crustal layer and a mantle half-space. Forward models that produce the smallest weighted sum of squared residuals have an upper mantle viscosity in the range 4–6 × 1018 Pa s and a less well-resolved lower crustal viscosity in the range 2 × 1019 to 1 × 1022 Pa s. A high-viscosity lower crust, despite high heat flow in the Salton Trough region, is inconsistent with felsic composition and might suggest accretion of mafic lower crust associated with crustal spreading obscured by thick sedimentary cover.
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