This paper presents the results of a detailed study of an intermediate depth earthquake beneath southern Tibet (September 14, 1976; 0643:51.6 UT; 29.81 ¿N, 89.57 ¿E; focal depth 90¿10 km; mb = 5.4). From the differences in arrival times of P, pP, and sP and from Rayleigh wave spectral densities and focus is estimated to be at depth of 90¿10 km and therefore in the uppermost mantle. From the polarities of the first motions of P waves, inversion of the relative amplitudes of P, pP, and sP, and synthesis of their waveforms, the source mechanism is constrained to be predominantly normal faulting with the T axis approximately horizontal and trending east--west. This mechanism is similar to those of shallow earthquakes in Tibet. The occurrence of an earthquake at depth of 90 km is consistent with rapid deformation at a temperature of about 700 ¿ to 800 ¿C in the mantle beneath the Tibetan crust. The seismic moment of an equivalent double couple point source is estimated to be about 9¿1024 dynecm (with an uncertainty factor of 1.3) from the sP amplitudes in the period range near 10 s, and 5¿1024 dynecm (also with an uncertainty factor of 1.3) from the spectral density of Rayleigh waves with periods of 50 s.