Long-period seismograms from the GDSN, RSTN, and IDA network are used in a centroid-moment tensor analysis of the September 19, 1985 Michoacan, Mexico, earthquake (MS=8.1). The long-period data are also used together with broad band displacement P-waveforms in a joint very broad band inversion for the moment tensor and the moment rate distribution in time and space. Results are that the earthquake had a total scalar moment of 1.1¿1028 dyne-cm and that the moment release occurred mainly in two subevents, the second of which was located approximately 70 km southeast of the first. Observations of Earth's normal modes at ultra long periods (600 to 2000 sec) are shown to be consistent with the obtained scalar moment and source mechanism.