Abstract: A study of shear wave splitting is made through the recordings of 6.2 and 6.1 aftershocks in Yunnan 2003. The results show that the preponderant polarizations of fast shear wave is N33°W in Dayao, it accords with the regional predominant direction of principal compressional stress. Through contrasting polarizations and time delay of two aftershocks, we can see evident stress relaxation after 6.1 earthquake. Before strong aftershocks, we observed that the time delay basically would increase. However the time delay abruptly decreased immediately before several strong aftershocks. It accords with the conclusion that long-term stress accumulation and short-term stress relaxation is existed before earthquakes. It is significant for the short-term earthquake prediction that the time delay will decrease before strong earthquakes. The result suggests the parameters of shear wave splitting may reflect the dynamic change of region stress field and provide useful information for the study of stress field and earthquake prediction.

Key words: Shear wave splitting, Sequence of Dayao earthquake, Time delay