  The statistical properties of earthquake aftershocks are studied. The scaling
relation for the exponents of the Omori law and the power-law calm time
distribution (i.e., the interoccurrence time distribution), which is valid if a
sequence of aftershocks is a singular Markovian process, is carefully examined.
Data analysis shows significant violation of the scaling relation, implying the
non-Markovian nature of aftershocks.
