Abstract: Focal depth is one of the most basic seismological parameters, showing where the earthquake occurred in the earth, which will be of great significance to investigate the physical and chemical condition of seismic generation and occurrence, the tectonic background for concentration and release of seismic energy. In this paper the focal depth of the Balinzuiqi M_S5.9 earthquake occurred on Aug. 16, 2003 was determined using the PTD earthquake location algorithm. The result is 20 km, which is deeper than the average focal depth (11±5) km in Northeast China block. Then this paper analyzes the result error, and gives solution. At last, it discusses the velocity structure of P wave of the crust in the earthquake region combined with correlative results, and two layers homogeneous velocity structure of P wave in the earthquake region is given.

Key words: Balinzuiqi M_S5.9 earthquake, PTD earthquake location, Focal depth, Head wave, Direct wave