Broad-band data from IRIS and ORFEUS data centers, the ARCESS array and SCP station are analyzed to infer plane geometry and the character of rupture during the October 17, 1989, Loma Prieta earthquake. Interference of P and sP seen in the P wave forms is consistent with the 18 km source depth inferred from local observations. A minimum source depth of 14 km is inferred from interpretation of the S wave arrival time, relative to instrument trigger, using horizontal strong motion accelerations observed at Corralitos, California. P, sP, S and sS polarities and relative amplitudes are used in a comprehensive grid search to infer that oblique right-lateral faulting occurred on a steeply dipping (56¿--60¿) section of the San Andreas fault with an equal amount of thrust component consistent with aftershock locations. A suite of point moment tensor inversions for source depths spanning the observed depth of aftershock hypocenters yields similar mechanisms. The best moment tensor in terms of wave form fit and minimum non-double couple component occurs for a source depth of 8 km, substantially shallower than the locally inferred hypocenter. Seismic moment for this model is 2.3¿1026 dyn-cm. The slow growth of the P displacement pulse with time and impulsive sP suggests a working model for fault rupture where rupture initiates at 18 km and propagates upwards and outwards along the fault. The P wave forms show the effects of variations in rupture along the plane through observation of a cascade of successively larger subevents. ¿ American Geophysical Union 1990