We show that viscosity of crust-forming rocks makes the frequency curve of earthquake energy deviate from an empirical power law relation. To estimate the effect of viscosity, we consider a modified Burridge-Knopoff type spring-block model. The model consists of the blocks connected by Maxwell springs which represent the material property of crustal rocks. We applied a method of cellular automaton model to the system. The resulting earthquake sequences depend on the relaxation time &tgr; of the Maxwell spring. For relatively small 1/&tgr;, where elasticity dominates, the earthquake energy frequencies show the power law relation and deviate from the relation at large earthquake energies. The separation shifts to smaller energy with increase of 1/&tgr;. For large 1/&tgr;, the distribution does not show the power law relation. The decay of earthquake frequencies after major mainshocks is not observed. ¿ 1998 American Geophysical Union