Abstract: Researchers have defined the fault in meta-instability stage as the stress changes from quasi-static release to irreversible quasi-dynamic release. Meta-instability stage is the last stage of instability before earthquake and means earthquake is sure to break out. To recognize meta-instability and evolution characteristics of correlated physical fields is helpful of identification of short-term and the signal indicating earthquakes. In this paper, based on the observed results of repeated absolute and relative gravity measurements in Longmen mountain area during 1996~2007, we analyse characteristics of gravity variation before the 2008 Wenchuan MS8.0 earthquake in meta-instable state, to propose a basis or method for determining the meta-instability fault using gravity data. The results show that: 1)Gravity field variations of Longmen mountain fault zone before WenchuanMS8.0 earthquake are positive normal change during 1996 to 2001, and occur regional anomaly in 2001-2004, then exist an obvious reverse in 2004-2007, but change little(lock model)for a year before the earthquake. The patterns of the gravity variation with time are well reflected the process of steady state loading to instability stage of rock experiments in the laboratory: linearity extends steadily-deviated from linearity-meta-instability. 2) The time series of gravity variation of some points in the adjacent area of the epicenter are obtained. Gravity time-variations are disorderly and irregular in east side where near Sichuan basin, but increase and decrease simultaneously from 2002 which observation points are distributed mainly over Houshan fault. The characteristic of this change satisfies conditions of the degree of synergism which is an indicator of the stress state. Combined with other research results we can condition Houshan fault is in a critical state of meta-instability stage since 2002. 3) Gravity field before the 2008 Wenchuan MS8.0 earthquake has the characteristics of synergism activity, which is helpful to identify the meta-instability state. Seismological evolution varies in different tectonic positions, so more earthquake cases should be researched profoundly when we are trying to find gravity anomaly related to the meta-instability stage.

Key words: meta-instable stage, the Wenchuan earthquake, Longmenshan faults, gravity change, synergism activity

摘要： 本文应用1996-2007年龙门山断裂带上的重力观测资料，分析了汶川地震前亚失稳阶段的重力场变化特征，根据震前失稳过程的重力场观测证据探讨应用重力场识别断层进入亚失稳阶段的依据和方法。研究结果表明：1）汶川地震前沿龙门山断裂带的时变重力场在1996-2001期间呈正常态变化，2001-2004期间出现区域性区重力异常，2004-2007期间出现较明显的反向变化，震前一年变化较弱，呈现闭锁状态。这种变化过程较好的对应了岩石变形实验中由稳态加载阶段到失稳阶段中的线性稳态、偏离线性稳态、亚失稳状态的过程。2）龙门山断裂东侧接近四川盆地上的测点时序变化平缓无序，西侧后山断裂带上的测点在2002年开始出现一致性较好的“同升同降”变化特征，符合实验研究中断层由稳态进入亚失稳状态物理场协同化演化的判定依据。结合其它资料可认为龙门山断裂带上的后山断裂在2002年开始进入亚失稳态。3）汶川地震前重力场的时空演化具有协同化特征，这种特征的识别利于断层亚失稳态的判定。由于每次地震的孕育发展发生在不同的构造结构，建议在寻找与亚失稳相关的重力异常时，还需更多的震例分析总结。

关键词: 亚失稳阶段, 汶川地震, 龙门山断裂带, 重力变化, 协同化活动