Abstract: A parameter, the strong earthquake modulation rate, which represents the modulation effect of the rotation of the earth and the tide force to the strong earthquakes during the strong earthquake activity in group in China's continent, and its significance for earthquake prediction are studied in this paper. The strong earthquake activity is the basic pattern and feature of the seismically active period. The occurrences of some strong earthquake groups w ere modulated by the rotation of the earth and the tide force. The degree of modulation is represented by a parameter K , w here K_t and K_t are the strong earthquake modulation rate of the rotation of the earth and the tide force respectively. K_t= N_r /N_total , R_t = N_t /N _total, N _total, N _r, N_t are the total number of the earthquakes in a strong ear thquake group, the number of the earthquakes modulated by the rotation of the earth a nd the tide force respectively. The strong earthquake modulation rate is acharacteristic parameter. Because i t represents the degree of the modulation resulted from the rotation of the earth and the tide force as well as some feature of the seismogenic structure and the timing of seismicity, it is of some practical value to earthquake prediction.

Key words: Earthquake group, Rotation of the earth, Tide force, Strong earthquake modulationrate