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大地震会产生地面重力变化在50年前已被公认,中国开展重力变化与地震活动关系的研究已有30多年。中国地震局第二监测中心重力研究人员2006年发现,在中国的南北地震带存在明显的重力变化,认为2007～2008年四川汶川地区存在发生大地震的可能,并提出了中期预测意见。这一研究表明,把重力变化作为主要的地震前兆是可行的。本文中,我们给出了地震监视区重力测量的方法,以及分析地震测量数据的程序,还有汶川5·12地震之前重力变化的特征。尽管汶川震中附近存在明显的重力变化,但重力变化是否能被看成是汶川地震的前兆,还需要更多的研究。本文讨论了重力变化的不确定性,包括重力测量误差、地壳垂直运动及水文重力效应等。观测数据还存在其他局限,重力观测站点密度小以及观测时间间隔长(2～3年),不能捕捉到孕震过程中出现的完整前兆信息。基于这些,我们对重力数据采集提出几点建议,以期加强未来用重力监测数据研究地震的能力。
Over 50 years ago, the gravity change caused by the Great Earthquake has been recognized. More than 30 years have passed since China began carrying out studies on the relationship between gravity changes and earthquakes. Gravity researchers at the Second Monitoring Center of China Seismological Bureau discovered in 2006 that there is a clear gravity change in the north and south seismic zones in China. They think there is a possibility of a large earthquake in Wenchuan, Sichuan Province from 2007 to 2008, and put forward medium-term prediction opinions. This study shows that it is feasible to use gravity changes as a major earthquake precursor. In this paper, we present the methods of gravity measurement in the seismic monitoring area, as well as the procedures for the analysis of seismic data and the features of gravity changes before the Wenchuan May 12 earthquake. Despite the obvious gravity changes near the Wenchuan epicenter, whether gravity change can be regarded as a precursor to the Wenchuan earthquake needs more research. This article discusses the uncertainty of gravity changes, including gravity measurement errors, vertical crustal movement and hydrological gravity effects. There are other limitations of the observed data. The gravity observing site is small and the observation interval is long (2-3 years), which can not capture the complete precursor information that appears during the seismogenic process. Based on these, we make some suggestions on gravity data collection in order to strengthen the future ability of using gravity monitoring data to study earthquakes.