Abstract: In this paper, based on the single scatter model (Aki), Coda Q_c was calculated from May 2003 to May 2007 in Yangjiang area, Guangdong Province, using vertical digital seismic wave recorded by Yangjiang earthquake station whose epicenter distance between 7 to 30 km. The average of Q_c with 24~27 s of elapsed time windows was 95, and 28~31 s is 115, and 34~44 s is 124. The M4.9 Yangjiang earthquake occurred on September 17, 2004. The temporal characteristics of Q_c between 24~27 s and 28~31 s of elapsed time windows show rough agreements from Feb. 2004 to Dec. 2004. Q_c would trend to decrease at first about half a year before the main shock occurred, then increased, a short time after the main shock occurred, Q_c would trend to decrease to the normal value 2~3 months after the main shock occurred. Q_c with 38~44 s of elapsed time windows didn't appear anomalous change. Q_c is an effective mean to observe the evolvement of earthquake sequence and predict the main shock, but the key point is how to select the length of the elapsed time windows. As an observational mean for seismic precursor, the elapsed time windows of Q_c should be about 24~31 s for Yangjiang areas.

Key words: Coda Q_c, M4.9 Yangjiang earthquake, Elapsed time windows, Digital seismic wave record