Abstract: Faulting process in soft sediments usually did not leave any macroscopically observable trace,so the occurrence of the fault as well as its activity and mode of motion cannot be identified by naked eyes in the field.Available data have shown that microstructural and micro-sedimentologic analyses are effective method for identifying obscured fault and paleoearthquake event in soft sediment.Recently,a large-scale trench has been excavated across the Jiaocheng Fault by the Institute of Crustal Dynamics and Earthquake Administration of Shanxi Province.They believe that the trench reveals 3 paleoearthquake events of M>7.0.In this study,we attempt to seek for the microstructural and micro-sedimentologic indicators of obscured fault and paleo-seismic event in soft sediment,through the microscopic analysis of this trench.Microstructural observation reveals that the fault in this trench microscopically appears as micro-fracture zone,slip surface,deformation bands and the bands of clay fillings,while deformation microstructures can be observed in the samples collected from the vicinity of the fault.Micro-sedimentologic analysis shows that the angularity,grain-size and particle size distribution of the samples collected from the fault differ to some extent from those of the samples collected outside the fault.The fractal dimension of the particle size distribution of the sample collected from the fault is D=2.0,which indicates the existence of cataclasis by controlled particulate flow during faulting process.Microscopic indicators comparable to the macroscopic indicators of seismites,such as ball and pillow structure,load structure,fluid flow,mass flow and the floatation of organics,are observed widely in the samples studied.All the results show that microstructural and micro-sedimentologic analyses are effective approaches to the identifying of obscured fault and paleo-seismic event in soft sediments.

Key words: soft sediment, obscured active fault, paleo-seismic event, microstructural analysis, micro-sedimentologic analysis, Jiaocheng Fault, Shanxi Province

摘要： 松散沉积物中的断层常常没有留下明显的宏观形迹,单靠肉眼观测往往难以确定其存在,即使存在,也很难判定其活动性质和活动方式。最近,中国地震局地壳应力研究所和山西省地震局在太原市西张村横跨交城断裂开挖了大规模探槽,认为该探槽揭示了3次7级以上地震事件。文中通过对该探槽开展微观分析,来探索松散沉积物中宏观形迹不明的断层或隐形断层及其伴随的古地震事件的显微构造及显微沉积学标志。研究结果表明,该探槽中的断层在微观上表现为微破碎带、滑移面、变形条带和泥质充填带等多种形式,同时断层附近样品也有一定的变形现象。断层和外围样品中的碎屑颗粒在棱角度、粒度大小和分布特征等方面均有不同,断层样品粒度分布的分维值D=2.0。这些结果表明松散沉积物中断层作用有颗粒流动也有碎裂作用。同时发现了与震击物(seismite)宏观标志非常相似的诸多微观标志,如球状体、负荷构造、流体通道和物质流动等,从微观上证明该断裂活动伴随有古地震事件。

关键词: 松散沉积物, 活断层, 古地震事件, 显微构造, 显微沉积学, 山西交城断裂