Temporal fluctuations in seismicity of southern California have been investigated by means of two methods, the Allan Factor Analysis and the Detrended Fluctuation Analysis, which are used to detect the scaling behaviour of an nonstationary observational time series. Performing the time-clustering analysis varying the depth of the events, we found that the seismicity of the area reveals a tendency toward Poissonian behaviour near the depth ranges 14--16 km. ¿ 2001 American Geophysical Union