汶川地震最大垂直位移处同震滑移带特征及其高磁化率意义

THE CO-SEISMIC SLIP ZONE AT THE MAXIMUM VERTICAL DISPLACEMENT SITE OF WENCHUAN EARTHQUAKE AND ITS HIGH MAGNETIC SUSCEPTIBILITY IMPLICATIONS

本文根据映秀-北川地表破裂带最大垂直位移观测点（31°50′21．2″N，104°28′09．7″E）测量数据，结合同震滑移面特征、擦痕产状等，重新讨论了汶川地震地表破裂带展布特征、过程、最大同震位移值等基本参数。该观测点对应约7.0m的垂直位移，水平位移约为5．0m。同震断层滑移面上只发育一组侧伏向为SW、侧伏角约55°擦痕，说明汶川地震破裂在映秀-北川断裂北段主要经历了一次破裂过程。使用U形槽连续采集同震滑移带断层泥样品，在2G-760岩石磁学系统自动控制平台上利用BartingtonMS2磁化率仪的环形探头，按每1cm间距采集数据。测试结果说明同震滑移带断层泥具有高磁化率特征，增强机制可能是由于地震滑移作用产生的高摩擦温度导致断层岩中高磁化率新矿物的形成所致。本研究对于鉴定断层岩经历摩擦生热作用提供了一种定量分析新尝试。

The Wenchuan earthquake （Mw7.9, 12 May 2008） produced two surface rupture zones along the Yingxiu- Beichuan and Anxian-Guanxian faults, respectively. The Yingxiu-Beichuan rupture zone is characterized by thrusting with dextral slip. Aiming at understanding the distribution characteristics of surface rupture zone, faulting process, and the maximum offsets, in-situ measurements were performed at the maximum offsets observation site along the Yingxiu-Beichuan surface rupture （31°50′21.2″N, 104°28′09.7″E） The dextral displacement is ca.5.0m according to the ca.7.0m vertical displacement at the Shaba Village, Qushan Town, Beichuan County. Co-seismic fault striations, striking SW with pitch angles of ca. 55°, can be seen on the rupture plane at this site. These reveal that the northern segment of the Yingxiu-Beichuan fauh is characterized by oblique thrust motion with strike-slip component and experienced one rupture process. After removing the weathered rock, we systematically sampled fault gouge by U channels （2~ 2cm plastic box）. Magnetic susceptibility of the U-channel sample was measured continuously every 1.0cm using Bartington MS2 magnetic susceptibility system mounted on 2G-760 cryogenic magnetometer autotracking rail. The rock types control the greatest proportion of the magnetic susceptibility. Moreover, possible correlations between the magnetic susceptibility anomalies and the occurrence of fault gouge was shown by the high magnetic susceptibility phenomenon of the fault gouge. The principal mechanism responsible for the high magnetic susceptibility of the gouge is most likely caused by the production of new magnetite from iron-containing silicates or clays. This study shows that magnetic analysis can help to quantitatively recognize frictional heating events in specific fault rocks. Fault gouges with high magnetic susceptibility in the same wall rocks can be considered as a proxy for earthquakes or seismic signatures.