We examined the P wave displacement spectra of nine aftershocks (M=2.5~3.6) of the 1989 Loma Prieta earthquake recorded on PASSCAL/IRIS instruments to search for changes in the spectra that are indicative of heterogeneity in the source region. Temporal variations of spectral characteristics have been observed from the records of two events (♯7 with M=2.7 and ♯8 with M=2.5) that occurred at an almost identical location with a time interval of 16 min and recorded at common stations. The different spectral characteristics can be attributed to the differences in the source time functions of these events. Also, the spectrum of event ∥2 (M=2.5) that occurred near San Francisco, is substantially richer in high frequency than those from events of a comparable size in the immediate aftershock area. This could be an indication of progressive stress concentration beyond the present aftershock area. ¿ American Geophysical Union 1990