关键词: 垂直位移, 定量研究, 不确定性, 正倾滑活动断裂

Abstract: 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.

Key words: Vertical displacement, Quantitative research, Unreliability, Normal dip slip active fault