Average S-wave coda-Q (Qc) at 6 Hz and to about 10 sec after the S-wave arrival shows: (1) temporal variation positively correlated with the magnitude distribution parameter b for foreshocks and aftershocks of the 1979 Petatlan, Mexico earthquake; (2) spatial variation through the aftershock area, positively correlated with b, high Qc-b values at the two activity centers, intermediate values in between and landward, and low values toward the Middle America Trench, which remain low for two weeks after the main shock; and (3) five-foreshock average Qc=135¿1.6% and eleven-aftershock average Qc=175¿1.3% for the eastern asperity. The b values-and possibly Qc values-seem to correlate with the level of aftershock activity.