关键词: 松散沉积物, 活动断层, 显微构造分析, 显微沉积学分析, 顺义-前门-良乡断裂

Abstract: The faults developed in unconsolidated soft sediments are often macroscopically obscured,so that their occurrence and mode of motion are difficult to be identified only by macroscopic observation.Recently,a lot of data have shown that microscopic analysis is an effective approach to the identification of obscured faults and paleo-earthquake event in soft sediments.Our purpose,therefore,is focused mainly on the exploration of the micro-structural and micro-sedimentological indicators of obscured faults and paleo-seismic events in soft sediments.A case study on a trench dug across the Shunyi-Qianmen-Liangxiang Fault at Shunyi,Beijing is presented in this paper.Microstructural observation reveals that the obscured fault in soft sediments can be traced distinctly under microscopic scale.Microscopically,the fault can be identified by its various appearances,such as distinct slip surface,micro-rupture zone,deformation bands,deformation band shear zone,band of clay infillings,and band of concentration and orientation of clay minerals or clastic particles.Micro-sedimentological analy～sis shows that the disturbance of micro-bedding,as well as the changes of angularity,size and distribution pattern of clastic particles from the beds outside and inside the fault can be taken as the micro-sedimentological indicators of obscured fault in soft sedi～ments.The obtained high D value(～2.5)of the particle size distribution(PSD)is believed to represent sudden seismic faulting event.Our microscopic observations have also revealed that the macroscopic indicators of seismites,such as balls and pillows,loading structures,mass flow,liquid flow and floatation of organics,are also reflected in microscopic scale.We believe that these microscopic indicators of seismite can also be used as the indicators of paleo-seismic event in soft sediments.All the results show that microscopic analysis is an effective method for identifying obscured fault and paleo-seismic event in soft sediments.

Key words: soft sediments, active fault, micro-structural analysis, micro-sedimentological analysis, Shunyi-Qianmen-Liangxiang fault