中文摘要：

本文根据Brune模式,在近震源条件下,利用中国数字遥测地震台网观测的波形资料,测定了2000年5月至2008年4月四川地区439次ML≥3.0地震的视应力值,给出了2008年5月12日汶川8.0级地震前四川地区视应力空间分布图象,分析了四川地区、龙门山断裂带以及龙门山断裂带上小震频次高值区视应力随时间变化过程。结果表明：①汶川地震前,视应力较高的地震主要集中在四川南部的川、滇交界地区;②2001—2006年,四川地区视应力随时间的变化表现为突升突降型变化形态,且视应力高值点有逐渐增大的现象,从2007年初至2008年初视应力在较低水平呈缓慢下降,临近汶川地震前有小幅升高变化;③龙门山断裂带上视应力在2004年中至2005年初曾出现过较为明显的、短暂的上升—下降变化过程。而Δσapp值的变化则相对复杂,出现过二次上升—下降变化过程;④龙门山断裂带上小震频次高值区视应力在2003—2005年间,曾出现过二次较为明显的上升—下降变化过程,临震前,视应力出现突然上升。而Δσapp值随时间的变化则主要表现为2003—2004年的上升和其后的下降过程。临近地震发生前,Δσapp值也有一定幅度的上升变化。

英文摘要：

On the condition of near-source,the Brune model was employed to determine the apparent stress of 439 earthquakes with ML≥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 MS8.0 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.