CHARACTERISTICS OF V_P/V_S RATIO IN THE SERIES OF JIUJIANG-RUICHANG EARTHQUAKE

Through analysis on the characteristics of V_P/V_S ratio in the series of Jiujiang-Ruichang earthquake we found:(a) there is a difference in the mean V_P/V_S ratio 10km beneath the surface,which is just dividing layer of upper crust and middle crust in the area,and it is consistent with the depth of Jiujiang-Ruichang M_S(5.7 earthquake) that occurred on November 26,2005 in the region.It indicated that there were some variations in the physical and chemical character of the medium in the vicinity of hypocenter,which may be closely relevant to the development and occurrence of the earthquake.(b) along northwest of the series' dominant direction in the vicinity of hypocenter,the mean V_P/V_S ratio at both side is different,it indicates there is difference in geological structure and rock character.As the dividing line of difference is consistent with the northwest strike of one group nodal planes in the focal mechanism solution,we guess that Jiujiang-Ruichang M_S5.7 earthquake may be caused by a northwest active fault.(c) Before the M_L3.9 aftershock that occurred on December 3,2005,the V_P/V_S ratio appeared continuously high value anomalies,which went beyond the warning line.After the V_P/V_S ratio recovered below the mean value,the aftershock occurred.At present,the V_P/V_S ratio keeps in high value anomaly,we should be alert the occurrence of felt earthquakes when the mean value drop.