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

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