Abstract: Delay times of surface wave for different station pairs in Yunnan region have been calculated by ambient noise method, using continuous waveform records of 46 stations in Yunnan network from Jan., 2008 to Jan., 2010. Then, surface wave velocity distribution of different times in the same window length is obtained by tomography method. According to experiment, we apply window length of three months as the stack days to obtain noise cross-correlation function with stable signal-to-noise ratio (SNR). The results show that the wave velocity structure changes obviously in different times. These changes include ① the wave velocity in Chenghai fault increases due to energy accumulation after the Wenchuan M_S8.0 earthquake in 2008; ② Panzhihua region is located on the boundary of velocity rising and dropping before the Panzhihua M_S6.1 earthquake in 2008; ③ Wave velocity increases in the hypocentral area before the Yaoan M_S6.0 earthquake in 2009, and reduces after the shock. Comparing with conventional time-dependent seismic tomography, ambient noise method using the fixed rays in different times can reduce the error caused by uneven distribution and hypocenter location errors.

Key words: Ambient noise, Tomography, Surface wave velocity, Seismicity, Yunnan region