New evidence of repeated earthquakes alongWenchuan earthquake fault zone
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摘要: 虹口乡八角庙出露完整的映秀-北川断裂带剖面断层岩,高分辨率磁化率测试揭示出多个具有高磁化率特征的断层岩带。系统的岩石磁学分析证明一层褐色断层岩相对围岩具有最大的磁化率值,存在新生成的磁铁矿和拥有相似的天然剩磁(NRM)和非磁滞剩磁(ARM)强度衰减过程。高磁化率特征是含铁顺磁性矿物受到断层滑移过程产生摩擦生热作用生成磁铁矿所致。同时断层岩还获得了热剩磁,记录了地震活动磁学信息。结合汶川地震科学钻探项目1号孔(WFSD-1)磁化率和岩石磁学研究结果,说明映秀-北川断裂带包含多层具有高磁化率特征的断层岩,暗示了多次强震的发生。具有高磁化率特征的断层岩可以作为判定地震活动的标志之一。Abstract: High resolution magnetic susceptibility and detailed rock magnetism studies of the Yingxiu-Beichuan fault zone were conduced on the field section at Bajiaomiao, Hongkou Town. The systematic analysis reveals the existence of quite a few high magnetic susceptibility fault rock zones. A brown layer fault rock has the following features:(1) the highest magnetic susceptibility, (2) significant characteristics of magnetite new formation, and (3) similar demagnetization behavior of natural remanent magnetization and anhysteretic remanent magnetization. The principal mechanism responsible for the high magnetic susceptibility of the brown fault rock was most likely caused by the production of new magnetites from iron-bearing paramagnetic minerals. These new magnetites might be formed by frictional heating on slip planes along a seismic fault. Combined with the analytical result of the core from the first drill hole of the Wenchuan Earthquake Fault Scientific Drilling Project (WFSD-1), the authors hold that the presence of high magnetic susceptibility fault gouges in the same country rock can be considered to be an indicator of earthquakes or seismic signatures. Quite a few layers of high magnetic susceptibility fault rocks indicate that strong earthquake repeatedly occurred alongWenchuan earthquake fault zone.
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图 1 青藏高原东缘活动构造及汶川地震同震地表破裂分布图(据文献[30, 31]修改)
(F1—汶川—茂县断裂；F2—映秀—北川断裂；F3—安县—灌县断裂)
Figure 1. Geological map of Longmen Mountain and the western Sichuan basin area,showing the position of theWenchuan Earthquake
(F1,Wenchuan-Maoxian Fault; F2,Yingxiu-Beichuan Fault; F3,Anxian-Guanxian Fault). Cross sections across the central LMS (modified after [30] and [31])
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图 2 虹口乡八角庙剖面断层岩代表照片
Figure 2. Typical photographs of fault rocks on section at Bajiaomiao, Hongkou Town
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图 4 八角庙剖面代表断层岩质量磁化率
Figure 4. Mass magnetic susceptibility of typical fault rocks on Bajiaomiao section
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图 5 八角庙地表破裂探槽表面磁化率
Figure 5. Surface magnetic susceptibility of trench crossing the rupture at Bajiaomiao
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图 6 断层岩摩擦生热作用的岩石磁学证据
Figure 6. Magnetic evidence revealing frictional heating from fault rocks
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