One or more earthquakes with abnormally large numbers of aftershocks at the beginning of their aftershock sequences are possible long-term precursors of a stronger earthquake. This precursor, named pattern B (burst of aftershocks), was tested together with two other possible premonitory patterns, S and &Sgr;, described previously. Pattern S (swarm) consists of the spatial clustering of earthquakes at a time when the seismicity of the region is above average. Pattern &Sgr; consists of an increase in the sun of earthquake energies to the 2/3 (roughly) power, in a sliding time window. These three patterns were tested by retrospective long-term prediction of earthquakes of Southern California, 1932--1977, with magnitudes ?6.5. The total duration of identified periods of elevated probability of occurrence is about 14 years. Five out of six earthquakes occurred during these intervals. However, the patterns do not indicate the exact location of future strong earthquakes with the region. All three patterns seem to represent different projections of the same general pattern, of 'bursts of seismicity,' i.e., an abnormal clustering of earthquakes in the time-space-energy domain. Pattern B also precedes strong earthquakes in New Zealand and Italy; Pattern &Sgr; precedes strong earthquakes in New Zealand.