The size and source complexity of the great (Ms= 8.1) Michoacan, Mexico earthquake can be attributed to both a restriction of the downdip width of the seismic interface between the overriding and downgoing plates, and a uniform distribution of asperities. The seismogenic evidence of strain accumulation during the 20 years prior to the 19 September, 1985 mainshock closely resembles that observed prior to the smallerMW= 7.6 earthquakes in the adjacent Colima (1973) and Petatlán (1979) regions: the subsequent rupture zones are seismically quiescent (mb≥4.0) for ∼ 2.5 ‐ 4 years prior to the mainshocks; the locked thrust interface is loaded by aseismic slip and normal faulting within the downgoing plate below 25‐30 km. There is evidence that the 1981 Playa Azul earthquake (MW= 7.3) is an integral stage of the evolutionary process of strain accumulation and release in the Michoacan area and could thus be considered a foreshock to the great 1985 ea