Crustal shortening in the Andes: Why do GPS rates differ from geological rates?

GPS data indicate 30-40 mm yr(-1) present-day crustal shortening across the Andes, whereas geological evidence shows crustal shortening concentrated i n the sub-Andean thrust belt at a much lower rate (<15 mm yr(-1)). We recon cile the discrepancy between the geodetic and the geological crustal shorte ning using geodynamic modeling that includes timescale-dependent crustal de formation. The GPS velocities reflect the instantaneous deformation in the Andes that includes both permanent deformation and elastic deformation that will be recovered during future earthquakes, whereas the lower geological rates reflect only the permanent deformation. The three-dimensional viscoel astic model predicts nearly uniform short-term velocity gradients across th e Andes, similar to the GPS results, and concentrated long-term crustal sho rtening in the sub-Andean thrust zone, consistent with geological observati ons.