Abstract: The main purpose of paleoseismic study is to distinguish or reveal deformation evidence of large earthquakes recorded by geologic and geomorphic features,and obtain corresponding seismic parameters such as timing,recurrence behavior,and coseismic displacement of large earthquakes. To achieve the aforementioned scientific aim,whether a trenching site preserves evidence of a complete paleoseismic sequence since late Quaternary and contains multiple measurable samples or not,and whether it can accurately identify paleoseismic events and collect well-constrained samples on events or not,all of these problems are directly responsible for reliabilities on assessment of future large earthquake hazard. Due to special displacement styles on strike-slip faults,good trenching sites are not widespread. Through comprehensive analysis on characteristics of coseismic surface ruptures and influencing factors on several trenching cases,we suggest micro-landforms such as depressions,basins,troughs,sag ponds,successive-offset channels,continuous scarp-derived deposit and multiple geomorphic surfaces are likely to be good trenching sites for paleoseismic studies on strike-slip faults. Multiple trenching or three-dimensional trenching should be the primary layout on strike-slip faults. Offsets of micro-landform across a fault,young stratigraphic units overlying on faulted units,locally distributed scarp-derived colluvial deposits,filled fissures,abrupt increases or decreases in displacement of different stratigraphic units on a fault,warping in different degrees,and multiple periodic paleo-sag ponds accumulation,all of these deformation evidences are good indicators for identifying paleoseismic events. To narrow uncertainties of paleoseismic studies,we should base on an organized research process and make a technical proposal and sophisticatedly conduct trenching work. Conclusions need to be repeatedly checked and widespread discussed,and also we should pay much attention on details and use more evidence to support or supplement analysis.

Key words: Mainland China, strike slip fault, paleoseismic studies, trench sites, paleoseismic indicators

摘要： 古地震研究的主要目标是识别或揭露地质地貌记录的大地震变形遗迹,确定大地震发生的时间、复发特征、同震位移量等参数。要实现这样的目标,探槽开挖地点能否完整记录晚第四纪发生的古地震事件、能否获取大量的测年样品以控制事件发生的年代、能否正确地识别这些事件等是关键,并直接关系到未来大地震危险性评价的可靠性。由于走滑断裂位移发生的特殊性,好的探槽研究地点并不普遍。文中在综合分析走滑断裂同震地表变形特征、影响因素的基础上,通过案例分析,总结出可能成为走滑断裂古地震研究和探槽开挖的候选地点,如洼地、盆地、槽谷、断塞塘、被同步位移连续错开的冲沟床、连续的坎前堆积地层和多级地貌面连续变形等。组合探槽或三维探槽应该是走滑断裂探槽布设的首选。跨断层微地貌位错、断错地层以及上覆更新地层、局部坎前堆积和裂缝充填堆积、不同地层单元沿断层面位移量的突然增加或降低、不同程度的弯曲变形、不同期次的古断塞塘(坑)堆积等,可以作为重要的事件识别依据。要降低古地震识别的不确定性,需要理顺研究程序,明确技术方案,逐一精心实施。认识和结论的得出需要反复推敲,并广泛讨论,同时需要注意细节以及各种信息的相互补充和印证。

关键词: 中国大陆, 走滑断裂, 古地震研究, 探槽地点, 事件识别