The Concepción-Constitución area [35-37°S] in South Central Chile is very likely a mature seismic gap, since no large subduction earthquake has occurred there since 1835. Three campaigns of Global Positioning System (GPS) measurements were carried out in this area in 1996, 1999 and 2002. We observed a network of about 40 sites, including two East-West transects ranging from the coastal area to the Argentina border and one North-South profile along the coast. Our measurements are consistent with the Nazca/South America relative angular velocity (55. 9°N, 95.2°W, 0.610 °/ Ma) discussed by Vigny et al., 2008 (this issue) which predicts a convergence of 68mm/yr oriented 79°N at the Chilean trench near 36°S. With respect to stable South America, horizontal velocities decrease from 45mm/yr on the coast to 10mm/yr in the Cordillera. Vertical velocities exhibit a coherent pattern with negative values of about 10mm/yr on the coast and slightly positive or near zero in the Central Valley or the Cordillera. Horizontal velocities have formal uncertainties in the range of 1-3mm/yr and vertical velocities around 3 to 6mm/yr. Surface deformation in this area of South Central Chile is consistent with a fully coupled elastic loading on the subduction interface at depth. The best fit to our data is obtained with a dip of 16° +/- 3°, a locking depth of 55 +/- 5km and a dislocation corresponding to 67mm/yr oriented N78°. However in the Northern area of our network the fit is improved locally by using a lower dip around 13°. Finally a convergence motion of about 68mm/yr represents more than 10 m of displacement accumulated since the last big interplate subduction event in this area over 170 years ago (1835 earthquake described by Darwin). Therefore, in a worst case scenario, the area already has a potential for an earthquake of magnitude as large as 8 to 8.5, should it happen in the near future.