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An earthquake which follows a larger earthquake or main shock and originates at or near the focus of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency with time. So, in 1965, after a strong earthquake in the Aleutian Islands archipelago for 24 days was 750 small earthquakes or aftershocks; After Suusamyr 9 magnitude earthquake in 1992 in 22 days - 473 aftershocks.