The June 22, 1977, earthquake in the southern Tonga arc is a large normal faulting event with down-dip rupture propagation on a steeply dipping fault plane. We use a standard least squares inversion of P and SH waves for the source time function to determine a source consisting of two subevents located at depths of 40--55 and 105--125 km and a total time duration of 50 s. The ratios of the spectral amplitudes of the radial modes 0S0, 1S0 and 2S0 independently require a centroid depth of at least 100 km at their lower frequencies. The body wave analysis suggests a moment of 1.04¿1028, and the 0S0 spectral amplitude suggests a value of 1.8--2.0¿1028 dyn cm. The emerging picture is that of a complete rupture of the sinking lithospheric slab, starting from 40 km depth and propagating down to 125 km. This indicates that the decoupling of the overriding lithosphere from the slab in this area of the Tonga arc takes place by gravitational pull. ¿ American Geophysical Union 1988