Ten moderate earthquakes occurred in the Northwest of China after 1972 are selected,and the change of b value and the modulation shock ratio (the propertion of number of the shocks affected by earth tide to amount of all shocks) in their epicentral area is analysed in this paper.It is shown that before seven of the ten earthquakes occurred the b value of the epicentral region had obviously been correlative with the modulation shock ratio for years,and the correlation coefficients are much lager than corresponding correlativity examinati-on value for the confidence of 99 percent.The greater the earthquake is,the longer the correlation time is.
The correlation analysis of b value with the modulation shock ratio has been made in two regions,where seismic activity is more strong than other regions and seismic monitoring system is relatively perfect.After 1972 there were two periods when the correlation coefficient is larger than the correlativity examination value in above-mentioned regions,respectively.The periods correspond with the moderate earthquakes of the locality or near region.
In the light of research result of this paper,there may be the correlative change of some seismicity parameter with other ones before large earthquake occures.It might be made use of for earthquake prediction.