Abstract:
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
ML3.9 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.