摘要
2008年5月12日汶川大地震之后,在青藏高原东缘龙门山地区实施了汶川地震断裂带科学钻探,这是目前世界上最快回应大地震的科学钻探,为地学家探索地震成因机制提供了极好的机遇.汶川地震断裂带科学钻探工程(WFSD)沿产生同震地表破裂的两条断裂带——龙门山的映秀—北川断裂和灌县—安县断裂共实施了6口科学群钻.其目标在于对钻孔的岩心、岩屑和流体样品进行多学科观察、测试和研究,揭示汶川地震断裂带的深部物质组成、结构、产出以及构造属性;探索地震过程中的岩石物理和化学行为、能量状态与破裂演化过程;认识汶川地震发生的应力环境、巨大的地震破裂产生及传播原因、地下流体在地震的孕育、发生、停止过程中的作用,从而检验和深入理解地震断裂发震机理.目前,汶川科钻项目已取得的部分重要成果如下:(1)查明了汶川地震断裂带结构、组成;(2)揭示了汶川地震过程中"热增压"为重要断裂弱化机制,提出断裂带内石墨可作为判断大地震发生的标志;(3)发现目前世界上最低的断层摩擦系数,并首次记录到大震后断裂带快速愈合信息;(4)重建龙门山的构造格架,提出汶川大地震发生的新的成因模式;(5)通过对汶川地震余震的精确定位、钻孔附近的地震台阵观测,确定了地震活动与龙门山断裂带不同区段的空间关系;揭示断裂带深部流体特征与地震活动的关系,为确定大震孕育过程提供深部流体活动行为的科学依据.
After the devastating Wenchuan earthquake on May 12,2008,the Wenchuan earthquake Fault Scientific Drilling project(WFSD)was carried out in the eastern Tibetan Plateau,which is the most rapid scientific drilling response to large earthquakes in the world.The WFSD project provides an excellent opportunity for geoscientists to explore the formation mechanism of earthquakes.Intotal,6 drilling boreholes were drilled along the Yingxiu-Beichuan and Guanxian-Anxian faults(Longmen Shan,China),which both ruptured during the Wenchuan earthquake.The WFSD project aims to reveal the composition,structure,morphology and tectonic properties of the Wenchuan earthquake fault zone at deep depths based on multi-disciplinary observations,measurements and analyses,to explore the physical and chemical behavior of the faults,their energy states,as well as their rupture processes during the Wenchuan earthquake.Thus,we aim to further understand the stress condition,the causes of nucleation and propagation of the rupture,the role of fluid in earthquake nucleation,propagation,and cessation,and the seismogenic mechanism of seismic faults.So far,some important research results have been achieved:(1)We know the structure of the Wenchuan earthquake fault zone.(2)We suggested that thermal pressurization was an important fault weakening mechanism during the Wenchuan earthquake,and that graphite in fault zone can be considered as an indicator to determine the occurrence of large earthquakes.(3)We obtained the lowest fault′s frictional coefficient in the world and recorded the information about fault healing quickly after this large earthquake for the first time.(4)We reconstructed the tectonic framework of the Longmen Shan and presented a new formation model of the Wenchuan earthquake.(5)We determined the spatial relationship between seismic activity and the different sections of the Longmen Shan thrust belt by accurately locating the aftershocks of the Wenchuan earthquake and by observing the seismic array near the boreholes,and revealed the relationship between the characteristics of fluid and seismic activities in the fault zone at deep depth,which provides the scientific basis for determining the formation process of large earthquakes.