倾斜断层不对称分布引起的几何效应-上下盘效应.

首先介绍了具有加权平均意义的均方根距离Drms. 与断层距Drup和发震断层距Dseis相比， 均方根距离可以真实地反映观测点与倾斜断层的整体靠近程度. 然后利用断层距、 发震断层距和均方根距离， 通过回归分析分别对1999年集集地震加速度峰值的上下盘效应进行研究. 残差分析表明上盘的加速度峰值明显大于相同断层距或发震断层距处下盘的加速度峰值, 而相同均方根距离处的上下盘观测点的加速度峰值没有明显差异. 这说明上下盘效应是一种由于倾斜断层的不对称分布引起的几何效应， 因此进行地震动衰减分析时， 如果采用均方根距离作为观测点与断层之间的距离标准， 上下盘效应对近断层地震动的影响可以忽略. 

Geometric effects resulting from the asymmetry of dipping fault: Hanging wall/footwall effects

Root-mean-square distance Drms with characteristic of weighted-average is introduced in this paper firstly. Comparing with rupture distance Drup and seismogenic distance Dseis, Drms can exactly reflect general proximity of a site to a dipping fault plane. Then, the hanging wall/footwall effects on the peak ground acceleration (PGA) during the 1999 Jiji(Chi-Chi) earthquake are evaluated by regression analysis using Drup, Dseis and Drms, respectively. The logarithmic residuals illustrate that the PGA on hanging wall is much greater than that on footwall at the same Drup or Dseis. In contrast, there is no significant difference between the PGA on hanging wall and that on footwall at the same Drms. The result indicates that the hanging wall/footwall effect is a geometric effect caused by the asymmetry of dipping fault. Therefore, the hanging wall/footwall effect on the near-fault ground motions can be ignored in the future attenuation analysis if root-mean-square distance is used as the site-to-source distance measure.