An unusual foreshock sequence that began with an earthquake of moment 2.0¿1021 N m and rupture length of about 150 km preceded the great 1960 Chilean earthquake. Earthquake relocations of the sequence show a progression of seismic activity toward the initiation of the main shock. The rupture length of the great 1960 Chilean earthquake is estimated from the distribution of aftershocks and crustal deformation to be 920¿100 km. The source mechanisms of a foreshock and two aftershocks are determined and used to constrain the source mechanism of the main shock. The first event in the sequence ruptured the segment of the Peru-Chile Trench between 37.03 ¿S and the 38.74 ¿S, whereas the great 1960 Chilean main shock ruptured the adjoining segment to the south. The great 1960 Chilean main shock initiated within the rupture area of the first event and terminated near the intersection of the Chile Ridge with the Peru-Chile Trench (46.5 ¿S). ¿ American Geophysical Union 1989