An analysis combining historical triangulation and recent Global Positioning System (GPS) survey measurements in west and north Sumatra, Indonesia, reveals a detailed slip history along the central part of the Sumatran fault. The arc-parallel components of the combined velocity field are consistent with slip rates inferred from GPS data, ranging from 23 to 24 mm/yr. Between 1.0 ¿S and 1.3 ¿N the Sumatran fault appears to be characterized by deep locking depths, of the order of 20 km, and the occurrence of large (MW~7) earthquakes. The long-term (1883--1993) strains show simple right-lateral shear, with rates similar to GPS-measured, 1989--1993 strain rates. Coseismic deformation due to the 1892 Tapanuli and 1926 Padang Panjang earthquakes, estimated from triangulation measurements taken before and after the events, indicates that the main shocks were significantly larger than previously reported. The 1892 earthquake had a likely magnitude of MW≈7.6, while the 1926 events appear to be comparable in size to the subsequent (M~7) 1943 events and an order of magnitude higher than previously reported. ¿ 2000 American Geophysical Union