We found evidence that four episodes of large shallow (h60 km).

The time intervals between extensional and subsequent compressional events range from 1.6 to 9.9 years with an average of 6.3¿3.7 years. This apparent pattern which iniates at depth was further documented for final 2/3 of the last seismic cycle. From the spatiotemporal distribution of seismicity during this final period, we identified three general stages os stress release. Stage I, from 1964 to January 1980, was characterized by scattered seismicity. Stage II, from February 1980 to June 6, 1982, showed a significant increase in downdip seismic activity while the shallow zone became quiescent. (There was also a period of apparent seismic quiescence from mid-1975 to early 1978). Most seismic energy in stage II was released on October 24, 1980, by a downdip normal fault earthquake at intermediate depth (MS=7.0, h=65 km). Stage III gegan on June 7, 1982, with the occurrence of two large interplate thrust fault earthquakes at shallow depths (SS=6.9, h=18 km, and MS=7.0, h=19 and 23 km). The documentation of stage III through June 1986 shows subsidence of previous downdip activity and an aftershock zone which is spatially coincident with the shallow quiet zone identified in stage II. The spatiotemporal characteristics of the the 1982 main shock-aftershock sequence suggests that rupture propagated to the northwest toward the site of the 1957 (MS=7.5) Acapulco-San Marcos earthquake. This propagation is of interest because the latter site is presently considered to possess high seismic potential. We suggest that downdip normal fault seismic activity at intermediate depths reflects a symptomatic state of stress associated with some basic mechanism of plate interaction prior to interplate slippage in a large earthquake. This mechanism may have an important bearing on the generation and possible intermediate-term forecasts of large shallow interplate earthquakes along the subduction zone in Mexico. ¿ American Geophysical Union 1988