关键词: 正断层, 环境影响, 古地震识别, 关键技术

Abstract: Normal faults, developed within extensional environment, are widely found in North China. Given the varieties in surface ruptures of different earthquakes and their depositional environment, some issues are needed to be paid much attention to in exposing the actual and complete history of paleoseismic events occurring along normal faults. In this paper, based on the existing knowledge about surface rupture characteristics of large earthquakes and indicators of normal fault, combining the cases study in China and the factors of geological, geomorphologic and climatic environment, some key techniques and methods in paleoseismic study on normal faults in mainland China are recommended as follows: (1)Choosing appropriate trenching sites according to local conditions. In the area where the faulted surface deposits are mainly alluvial-fluvial materials of piedmont or river and lake sediments, the trenching sites should try to meet following conditions: the geomorphy can reveal multiple fault events with not too large single displacement, the erosion(or denudation)of external force and the accumulation processes maintain relative balance, the sediments are medium-fine grained, and the samples for dating are easy to be collected. The sites where the faulted sediments are mainly composed of loess or secondary loess or sandy loam should be avoided to excavate trenches for paleoseismic study, however, if it cannot be avoided, the areas with weaker erosion and accumulation near small gullies are the choices to be considered, because these areas may have different deposits from upstream of the gullies, and some supplemental information such as tectonic landform are needed to substantiate the paleoseismic analysis.(2)Recording and analyzing the trench profiles in detail in the field. For the deposits(e.g. loess)with no stratification, the key observation point is the slight change in the color, grain and orientation, which may indicate the stratigraphic boundary. Indicating the scarp-derived deposits units such as colluvial wedge is the key to analyzing paleoseismic events, and the indicated elements conclude the messy configuration and nodules in the collapse facies, and the soil developed in the upper of the erosion facies. When the scarp-derived deposits are difficult to distinguish from normal strata, we should, by "brushing", "jabbing" or "microscopic analysis", try to analyze the color, grain, non-loess materials(e.g. small gravel, plant roots, etc.)and the enrichment degree of calcareous materials(e.g. calcium-mod, calcium-nodule, calcium-dot, calcium-filament, etc.), to identify the stratigraphic boundary.(3)Synthetically analyzing and checking the paleoseismic results combining other information. The appearances of the scarp-derived deposits revealed by trench are often obscure, so supplemental information from geomorphology and multiple trenches are necessary. Some techniques and methods, such as progressive constraining method of paleoseismic events, fault displacement constraining method, correlating method between multiple trenches, inversion and reconstruction of fault events, etc., are helpful for judging whether the paleoseismic results are actual and complete.

Key words: normal fault, environment impact, paleoseismic identification, key techniques