【摘要】 2008年5月12日在青藏高原东缘龙门山地区发生了毁灭性的汶川地震(Ms 8.0),并沿映秀—北川断裂和灌县—安县断裂分别产生约270 km和80 km长、并具不同运动性质的地表破裂带。大地震后的断裂带科学钻探是研究地震机制的有效方法,为更好地了解汶川地震过程中的断裂机制、岩石的物理、化学变化和特征,2008年11月4日(汶川地震后的178 d)快速实施了汶川地震断裂带科学钻探项目(WFSD),该项目在这两条断裂带的上盘布置深浅不一的五口群钻(500～3000 m深)。笔者以汶川科钻一号孔(WFSD-1)岩心为主要研究对象,通过对岩心的岩石学研究和构造分析,识别出映秀—北川断裂带中的不同断裂岩分布和组合,以及确定了汶川地震主滑移带位置,为进一步研究汶川地震断裂机制提供了基础。更多还原

【Abstract】 The May 2008 Ms 8.0 disastrous Wenchuan earthquake occurred in the eastern margin of Tibetan Plateau, producing 270 and 80 km-long different kinematics features coseismic surface ruptures along the Yingxiu-Beichuan and Anxian -Guanxian faults, respectively. Drilling in active fault zones, especially after large earthquakes, is an effective way to study earthquake mechanisms, in order to better understand the fault mechanisms during the earthquake process as well as physical and chemical characteristics of rocks. The Wenchuan earthquake Fault Scientific Drilling project was carried out on November 6, 2008 (178 days after the Wenchuan earthquake). Five boreholes were ultimately drilled along the Yingxiu-Beichuan and Anxian-Guanxian faults, ranging from 600 m to 3000 m in depth. This paper focuses on the cores of the first hole (WFSD-1). Petrological and structural analysis of the cores allowed the identification of the distribution and combination of fault-related rocks in the Yingxiu-Beichuan fault, and the Principle Slip Zone (PSZ) location of the Wenchuan earthquake was determined, which provided a basis for further research on earthquake mechanism. The detail analysis of the cores reveals the existence of a fault zone, which is about 100 m wide from 575m to 759 m in depth and consists of gouge, cataclasite, and fault breccia. From the top to the bottom there exist mainly cataclasite zone, fault gouge and fault breccias mixed zone, breccias zone and fault gouge zone. This fault zone lies between the Neoproterozoic Pengguan complex and Triassic Xujiahe sedimentary rocks, which is the Yingxiu-Beichuan fault zone, mainly distributed in Xujiahe Formation. The Principle Slip Zone (PSZ) of the Wenchuan earthquake was found at ~589 m with ~1 mm fresh fault gouge, which lies in the upper part of Yingxiu-Beichuan fault zone, as evidenced by logging data, and clay mineral analysis, in combination with the microstructure of the gouges. There exist similar structures between Pengguan complex and Triassic Xujiahe Formation in Hongkou outcrop with the thickness of about 190m, lying at the bottom of the Yingxiu-Beichuan fault zone. Hence, The Wenchuan earthquake fault slip cut the Yingxiu-Beichuan fault zone obliquely, indicating that Wenchuan earthquake fault might be a new high-angle thrust fault with a dip angle of 62°-65°, which did not totally follow the slip zone of ancient earthquake and might be related to the earthquake magnitude. It is the result of the long term earthquake activity and evolution from about 10-15 million years ago to the present, which formed the 100~190 m-wide Yingxiu-Beichuan fault zone.更多还原