Abstract: According to the Guangdong Digital Seismic Network records, the focal mechanism is obtained from the earthquake (M_S4.9) in Yangjiang on September 17, 2004. The strike is 57°, the dip angle is 47° and the rake is 69° of the nodal plane, the azimuth and the dip angle of principal compressional stress axes P is 342° and 0°, respectively; the azimuth and the dip angle of principal extensional stress axes T is 252° and 75°, respectively, the type is reverse faulting. The seismic moment is 1.69×10¹⁵ N·m, the stress drop is 1.7 MPa, the source radius is 876m of this earthquakes. Some relations are obtained by analyzing source parameters of the 22 moderate-small earthquakes in Yangjiang area. The descent trend of corner frequency of S wave is showed by the changes of the time before the earthquake (M_S4.9), the azimuth of axes P of Yangjiang sequence show that the stress has begun to be identical in four months before this event.

Key words: Source parameter, Focal mechanism, Yangjiang earthquake (M_S4.9), Corner frequency