It is well known that the detection rate of aftershocks is extremely low during the period immediately after a large earthquake due to the contamination of arriving seismic waves. This has led to considerable difficulties in obtaining estimates of the empirical laws of aftershock decay and magnitude frequency immediately after main shocks. This paper presents an estimation method for predicting the underlying occurrence rate of aftershocks of any magnitude range, based on a magnitude frequency model that combines the Gutenberg-Richter law with the detection rate function. This procedure enables real-time probability forecasting of aftershocks immediately after the mainshock, when the majority of large aftershocks are likely to occur.