In order to understand better the variations of mechanical properties close to the source and the failure process in the upper portion of the crust in a geologically very young subduction zone, the seismic S wave coda Qc from 3 to 20 Hz and P wave half pulse width Qp from 6 to 12 Hz were determined for selected foreshocks and aftershocks of the 1979 Petatlan, Mexico, Ms=7.6 earthquake. To limit the region being sampled, the quality factor of S coda, Qc, was obtained from the first 10 s following the S onset. Qc exhibited a strong spatial, temporal, and frequency dependence, and in the small foreshock portion of the aftershock area, Qc, and (to a lesser extent) Qp were significantly lower before the mainshock than afterward. The frequency dependence n, in Qc (f)=Qo fn, was practically constant for aftershocks in four different regions (n=1.32¿0.03) and for foreshocks (n=1.43¿0.07). Only in one peripheral region-station pair did we find a significantly different n (n=0.97¿0.09). Our observations indicate that Qc, and perhaps the frequency dependence n, are sensitive to the physical property changes in the source region and might be useful as predictors of large earthquakes in a shallow subduction zone environment. ¿ American Geophysical Union 1990