In this paper

the geologic structure of north depression of South Yellow Sea where cluster earthquakes occurred is analyzed and the variation of ambient stress during earthquake clustering is calculated and studied.In addition

the background of and mechanism for the M

L

5.1 earthquake on July 23

2002 is discussed.It is indicated that there is a correlation between the pause of clustering process and medium-large earthquake

and that the average ambient stress value during initial term and middle term of cluster earthquake formation is higher than that before cluster earthquake formation but lower than that in the eve of occurrence of M

L

5.1 earthquake and the average ambient stress value during upper term of cluster dropped because of the occurrence of M

L

5.1 earthquake.The results of this research can provide a significant reference for analyzing and forecasting the occurrence of medium-large ear

thquakes by using cluster earthquakes and the variation of ambient stress values.