正倾滑活动断裂垂直位移定量研究中相关问题的讨论

当探槽开挖长度未跨过断层变形带时，得到的断层垂直位移将偏离断层活动的真实情况，在缺少依据帮助确定断层陡坎原始下坡角的具体位置时，通过断层陡坎高度获得的断层垂直位移也将与实际情况有较大的偏离，文中对此进行了讨论。并讨论了应用断层两侧近水平地层累积变位量的分解确定古地震事件期次的方法，以及探槽剖面中断层两侧同层地层厚度差异是断层活动事件的反映等问题。引用了内蒙古大青山山前断裂和狼山山前断裂、北京平原夏垫断裂和南口－孙河断裂及日本丹那断层探槽开挖的实例。

DISCUSSION ON QUANTITATIVE RESEARCH OF VERTICAL DISPLACEMENTS ON NORMAL DIP-SLIP ACTIVE FAULTS

Vertical displacement is an important parameter for quantitative research on normal dip slip active faults and seismic risk assessment. It bears a close relation to the magnitudes of earthquakes,dislocation rate of fault, and the average recurrence interval of strong characteristic earthquakes, while the difference of vertical displacements in different horizons of the same profile is related to the number of paleoseismic events. Therefore, if this parameter can not be determined properly, then the error will be carried into the quantitative researches of the other parameters. In this paper we discuss the following cases. If the length of a trench does not cover the entire deformation zone of a fault, then the vertical displacement estimated from this trench represents only a part of the real displacement on the fault. If the two sides of a fault scarp consist of geomorphic planes of different ages, then the vertical displacement estimated from the height of the fault scarp will deviate significantly from the real displacement. For example, two large trenches were dug across the Daqingshan Piedmont Fault in Inner Mongolia. In these two trenches, the displacement rates of the fault estimated from the heights of fault scarps and from combining fault scarp data with trench log data are 1.1mm/a and 1.9mm/a, as well as 1.13mm/a and 1.6mm/a respectively. The revised values of the displacement rates of the fault from trench log data are about one third or half of the original value obtained from fault scarp data in this example. Moreover, a method for determining the number of paleoseismic events is proposed on the basis of the resolution of accumulative vertical displacements of nearly horizontal strata on both sides of the fault. We find that the vertical displacement of a fault may lead to the difference of the thickness of deposits on both sides of the fault. All these phenomena can be observed not only in log of trench across the fault but also in log of borehole on both sides of the fault. The boreholes on both sides of the Xiadian and Gaoliying Faults in Beijing Plain, as well as the trench across the Tanna Fault in Japan can be taken as examples.