Normally, when one sees the main seismic event as a spike on the furthest left (as shown here) a
Type I series of events has the primary aftershock following soon after with a Richter scale measure
of 75-80 percent of the original. Subsequent aftershocks map closely to a decaying exponential.  
Type II series are less common and lack the primary aftershock. We have identified some Type III
events where there are significant (> 4.9 Richter) events that serve as a prelude to the main shock,
so that an asymmetric bell-curve forms, but these are rare.