  Earthquakes are one of the most devastating natural disasters that plague
society. A skilled, reliable earthquake forecasting remains the ultimate goal
for seismologists. Using the detrended fluctuation analysis (DFA) and
conditional probability (CP) methods we find that memory exists not only in
inter-occurrence seismic records, but also in released energy as well as in the
series of the number of events per unit time. Analysis of the conventional
earthquake model (Epidemic Type Aftershock Sequences, ETAS) indicates that
earthquake memory can be reproduced only for a narrow range of model's
parameters. This finding, therefore provides additional accuracy on the model
parameters through tight restrictions on their values in different worldwide
regions and can serve as a testbed for existing earthquake forecasting models.
