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

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