Abstract

This study focuses on the present-day deformation mechanisms of the south central Chile margin, at the transition zone between two megathrust earthquake segments defined from historical data: the Valdivia and Concepción sectors. New GPS data and finite-element models with complex geometries constrained by geophysical data are presented to gain insight into forearc kinematics and to address the role of upper plate faults on contemporary deformation. GPS vectors are heterogeneously distributed in two domains that follow these two earthquake segments. We find that models which simulate only interseismic locking on the plate interface fail to reproduce surface deformation in the entire study area. In the Concepción domain, models that include a crustal-scale fault in the upper plate better reproduce the GPS observations. In the Valdivia domain, GPS data show regional-scale vertical axis rotations, which could reflect postseismic deformation processes at the edge of the Mw 9.5 earthquake that ruptured in 1960 and/or activity of another crustal fault related to motion of a forearc sliver. Our study suggests that upper plate faults in addition to earthquake cycle transients may exert an important control on the surface velocity of subduction zone forearcs. © 2008 by the American Geophysical Union.