Abstract: On the condition of near-source, the Brune model was employed to determine the apparent stress of 439 earthquakes with M_L≥3.0 occurred in Sichuan during May 2000 to April 2008 with seismic waveform data recorded by the regional digital telemetric seismic networks in Sichuan area. The spatial distribution image of the apparent stress in Sichuan before the M_S8.0 Wenchuan earthquake on May 12, 2008 had been given, and the variation process of the apparent stress with time in Sichuan, the Longmenshan Fault Zone and the dense zone of small earthquakes in the Longmenshan Fault Zone was analyzed. The results were as follows: ① High apparent stress was mainly concentrated in the Sichuan-Yunnan boundary region before the Wenchuan earthquake; ② Variation of the apparent stress with time in Sichuan from 2001 to 2006 showed a process of abrupt rise and fall, and the high-value points of the apparent stress increased gradually. The apparent stress declined slowly at a low level from 2007 to the beginning of 2008, and had little increase before the Wenchuan earthquake. ③ Apparent stress in Longmenshan Fault Zone showed an obvious increase-decrease process from the midyear of 2004 to the beginning of 2005. The variation of Dsapp value which showed two increase-decrease processes was relatively complex. ④ Apparent stress in the dense zone of small earthquakes in the Longmenshan Fault Zone showed two obvious increase-decrease processes from 2003 to 2005, and increased abruptly before the main earthquake. The Dsapp value increased at the beginning and then decreased from 2003 to 2004, and increased at a certain level before the main shock.

Key words: Apparent stress, Temporal and spatial variation, the 2008 M_S8.0 Wenchuan earthquake, Sichuan area