Individual great earthquakes are posited to release the elastic strain energy that has accumulated over centuries by the gradual movement of tectonic plates. However, knowledge of plate deformation during a complete seismic cycle-two successive great earthquakes and the intervening interseismic period-remains incomplete. A complete seismic cycle began in south-central Chile in 1835 with an earthquake of about magnitude 8.5 (refs,) and ended in 2010 with a magnitude 8.8 earthquake. During the first earthquake, an uplift of Isla Santa María by 2.4 to 3 m was documented. In the second earthquake, the island was uplifted by 1.8 m. Here we use nautical surveys made in 1804, after the earthquake in 1835 and in 1886, together with modern echo sounder surveys and GPS measurements made immediately before and after the 2010 earthquake, to quantify vertical deformation through the complete seismic cycle. We find that in the period between the two earthquakes, Isla Santa María subsided by about 1.4 m. We simulate the patterns of vertical deformation with a finite-element model and find that they agree broadly with predictions from elastic rebound theory. However, comparison with geomorphic and geologic records of millennial coastline emergence reveal that 10-20% of the vertical uplift could be permanent.