The Playa Azul earthquake broke the central part of the Michoacan gap along the Mexican subduction zone. The aftershock area estimated from field data for a 6-day period beginning 19 hr after the main shock is 40 km × 20 km. Preliminary focal mechanism of the main shock is consistent with a shallow-dipping thrust fault. Although there is some evidence of seismic quiescence for 512 yr followed by a foreshock about 3 months before the main shock and an above average microseismic activity in the western part of the epicentral region during the last 412 months or more, the data is not sufficient quality to assert this conclusion. The Michoacan gap was previously assigned category 3 (Singh et al., 1981) based on the classification of McCann et al. (1979). The seismic potential of the unbroken segments of this gap (∼45 km to NW and ∼35 km to the SE of the Playa Azul aftershock area) remains uncertain. Corresponding to a rupture length of 40 km, each segment could generate an earthquake of similar magnitude to the Playa Azul earthquake. Unfortunately, the recurrence period of large earthquakes in the region is not known and great/major earthquakes cannot be ruled out.

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