Abstract: We determined the 2003 Dayao M6.2 and M6.1 earthquakes sequence location and velocity structure of the hypocentral area by a simultaneous inversion method. First the velocity structure is determined by the method, and then earthquake location is performed on the basis of velocity structure. The results show that the M6.2 earthquake sequence can divided into two segments, the relatively dense southeast segment and the relatively sparse northwest segment, and there is a dip offset between the two segments,characterized by a turn. At the transitional zone of high and low velocity structures. The distribution of the M6.2 earthquake sequences shows a “V” shape along the depth, which corresponds to the reversing triangle shape of low velocity structure, namely, the aftershocks of M6.2 are distributed along the transitional zones of high and low velocity structures. The rupture zones of the M6.2 and M6.1 earthquakes are located on the southeast side of the reversing triangle shape of low velocity structure. The mainshock of M6.2 ruptured downwards and the mainshock of M6.1 ruptured upwards along the transitional zones of high and low velocity structure. The velocity structure at the depth of 6km shows that there is the quadrant distribution of high and low velocity structures and the aftershocks of the M6.2 and M6.1 earthquakes are distributed along the northwest transitional zones of high and low velocity structure. The value of low velocity structure at the southeast side of the M6.2 earthquake is higher than the value of low velocity structure at the northwest side, which is disadvantageous to the rupture southeastward for the M6.2 earthquake and is advantageous to the energy accumulation and occurrence of the M6.1 earthquake.

Key words: Dayao earthquakes, Simultaneous inversion, Earthquake location, Velocity structure