Abstract:

The fault will experience different stress states in the process from compression to final rupture(earthquake outbreak). 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. Various rock deformation experiments in laboratory show that there are observable meta-instability stage and evident synergism activity of physical field. Observing the evolution characteristics and laws of relevant physical parameters is fundamental and helpful for identifying short-term signals indicating earthquakes. In this paper, based on the theoretical characteristics of meta-instability stage obtained in laboratory and the results of repeated absolute and relative gravity measurements in Longmen Mountains area during 1996—2007, we analyze the characteristics of gravity variation in the meta-instable state before the 2008 Wenchuan M_S8.0 earthquake and propose a basis or method for determining the meta-instability fault using gravity data. The results are as follows:
(1)Gravity field variations of Longmenshan fault zone before the Wenchuan M_S8.0 earthquake showed a normal-state change during 1996 to 2001, regional anomaly occurred in 2001—2004, then an obvious reverse change appeared in 2004—2007, but the change was small and the faults were in a locking state for a year before the earthquake. The 2008 Wenchuan M_S8.0 earthquake occurred at the high gravity change belts and the epicenter was located near the zero isoline of the gravity converting from positive value to negative value. The patterns of the gravity variation with time match well with the linear steady-state, the deviation from linear steady-state and the meta-instability state in the process of steady-state loading to instability stage of rock experiments in the laboratory. The dynamic changes of gravity reflect the local gravity anomaly near the epicenter, thus help to judge the instability location.
(2)The time series of gravity variation of some stations adjacent to the epicenter are obtained. Gravity time-series variations are disordered and irregular in the east side of the fault zone near the Sichuan Basin, but increased and decreased simultaneously since 2002 on the west of the fault zone in the observation stations over the Houshan Fault. The 2008 Wenchuan M_S8.0 earthquake occurred on the Houshan Fault and the distribution of aftershocks was consistent with the strike of the fault. This shows the Houshan Fault is the main instability position of Longmenshan fault zone. The instability of the fault led to the consistent change of the gravity field of the measuring stations in this area. The characteristic of this change satisfies the conditions for determining the degree of synergism which is an indicator for identifying the stress state. Combined with other research results we can confirm that Houshan Fault is in a critical state of meta-instability stage since 2002.
(3)The spatio-temporal variations of gravity in the measuring stations on the west of the Longmenshan fault zone showed a synergism activity process before the 2008 Wenchuan M_S8.0 earthquake. It began with the gravity rise centered on station 607 and 612 in 2002 and expanded to station 606, 607 and 612 in 2006. Station 606 and 607 are located near Yingxiu Town, closest to the epicenter of Wenchuan earthquake and also in the instability area.
In summary, the retrospective research on the regional gravity field before the 2008 Wenchuan M_S8.0 earthquake using the meta-instability fault theory indicates that the evolution of the time-varying gravity field corresponds well to the processes of rock deformation and instability in the laboratory experiment, and the time-series variation of gravity stations generally reflects the synergism characteristics of physical field, which is a key indicator to identify a fault in a meta-instable stress state. Seismicity evolution varies in different tectonic positions, so more earthquake cases should be investigated profoundly when we are trying to find the gravity anomaly related to the meta-instability stage. Next, we will carry out a profound study, such as gravity observation and analysis of fixed-point stations, intensive absolute gravity measurement, and research on gravity time series changes of measuring stations near the epicenter before and after strong earthquakes.

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

摘要：

文中应用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证据和方法。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。这种变化过程较好地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。汶川地震发生在后山断裂带上, 震后余震的分布和该断裂带的走向一致, 说明该断裂带是断层的主要失稳位置, 该断层的失稳导致该区域测点的重力场变化一致, 符合实验研究中断层由稳态进入亚失稳状态的物理场协同化演化的判定依据。3)重力剖面点的时空变化显示, 2008年汶川8.0级地震前在龙门山断裂西侧可观测到一次断层活动协同化过程。

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