Repeated geodetic measurements with the Global Positioning System (GPS) have provided direct measurements of displacement due to active plate motions in Central America and northern South America, a tectonically active area of complex interaction between the Nazca, Cocos, Caribbean and South American plates. The displacements observed in the period 1988-1990 are obtained from the results of the first two CASA (Central And South America) GPS experiments. With these measurements are used, to determine the convergence rates across the Colombia-Ecuador and Mid-America trenches, and the spreading rate across the Galapagos Rise.

A network of 30 stations was occupied in the CASA UNO experiment (1988), but only five stations were reoccupied in 1990. The complete network was recently reoccupied, in early 1991. Special attention has been paid to the analysis of all possible sources of systematic errors, especially those which might arise from differences between the two networks. The two experiments had different tracking networks, used different equipment, and the 1990 network was much sparser in the CASA region. The potential effects of these differences are formally accounted for in the uncertainties.

The Nazca-South America convergence measured by GPS is slightly faster than predicted by the NUVEL- 1 plate motion model. However, the station on South America is actually on the independent North Andes block, and if the GPS displacement rate is corrected for the predicted motion of the North Andes block, the NUVEL- 1 rate would lie within the 95% confidence ellipse of the GPS estimate. The measured convergence rate between the Cocos and Caribbean plates is 57 mm/yr, only 60% of the 93 mm/yr predicted by the NUVEL- 1 model. The convergence direction measured by GPS is almost identical to that predicted by the plate motion model. The observed convergence rate is consistent with the hypothesis that plate convergence is slowing south of the Cocos Ridge, or with the hypothesis that the displacement measured on this baseline was affected by the temporal effects of the earthquake cycle. Alternatively, there could be localized tectonic motion at the site on the Cocos plate. Spreading across the Galapagos Rise was significantly different than the motion predicted by NUVEL-1, with a difference of 40 mm/yr in the east component. However, both the north and vertical displacement rates are within 1 mm/yr of the NUVEL-1 model predictions. There is no clear explanation for the difference, although again there is the possibility of anomalous motion of the site at Isla del Coco.

With only two measurements of the GPS network, the displacements measured with GPS results raise more questions than they answer and at this point it is more appropriate to speculate than to draw firm conclusions about the tectonic significance of the GPS results. Additional measurements of these baselines will significantly increase the confidence level of the conclusions which can be drawn from them. The second complete occupation of the network (CASA 1991) should provide a more robust solution, and its much greater redundancy will make it simpler to isolate possible local motions at particular sites.