鲜水河全新世断裂带的分段性、几何特征及其地震构造意义.

由五条左旋走滑的主要分支断层组成的鲜水河全新世断裂带,以惠远寺拉分区为界,可分为结构特征不同的两段:北西段结构较为简单;南东段则表现了由若干分支断层组成的复杂结构.这种断裂结构的分段性,造成了历史强震活动性的分段差异,同时也可能是断层近代滑动速率空间变化的主要原因. 该断裂带主要的几何特征之一是具有多重羽列性质.本文按阶区尺度的相对大小,作了羽列级别划分.其中,A级羽列不连续区伴有明显的地貌效应,是该断裂带分段的界限,其对历史上7级左右地震的破裂具有较明显的终止作用;B,C两级羽列不连续区也有一定程度的地貌显示,但对历史上大地震的破裂不具有明显的终止作用;更低级别的羽列几何则是在第四纪盖层中发育的地震地裂缝的主要组合型式. 另一种重要的几何特征是断层弯曲.无论沿整个断裂带还是在一些断层段上,均存在着不同程度的走向弯曲.局部弯曲的结果,可能是造成一些大地震时不对称破裂扩展和烈度衰减的重要几何影响因素,同时也可能是大地震或强震原地重复的构造条件之一.文中最后分析和讨论了两次历史大地震发震断层的立体模型. 

SEGMENTATION,GEOMETRIC FEATURES,AND THEIR SEISMOTECTONIC IMPLICATIONS FOR THE HOLO-CENE XIANSHUIHE FAULT ZONE

The Holocene Xianshuihe fault zone,which consists of five main branches with left-lateral strike-slip,can be divided into two segments of different structural styles,jointing at the pull-apart area of Huiyuan Monastery. The northwestern segment has a relatively simple structure. While the southeastern segment exhibits a complex structure composed of several branches. The segmentation of fault structure is the main cause of the different historic strong-earthquake activity,and perhaps,the spatial variation of recent fault slip-rates in different segments.The multiple-order en echelon pattern is one of the important geometric characteristics of the fault zone. Based on sizes of stepovers,orders of en echelon discontinuities have been relatively classified. Of which,a left-stepping discontinuity in the order A en echelon,which is apparently accompanied with topographic effect of pull-apart structure,has been taken as the dividing point of the segmentation of the fault zone. This discontinuity is also reflected in stopping surficial ruptures during historic earthquakes with approximate magnitude 7. Stepping discontinuities in en echelon faults of the order B and C also have topographic effects to a certain degree,but do not appear to have been significant in stopping large-earthquake ruptures. The earthquake ground-fissures developed within the Quaternary cover are mainly in forms of even lower order of en echelons.Bending is another important geometric characteristics of the fault zone. Along-strike bends at different degrees occur along the entire fault zone and some parts of faults. Localbends along a fault is the main geometric causes for unsymmetrical rupture-spreading and intensity-attenuating during large earthquakes,and perhaps,one of the tectonic backgroundsfor recurrences of large or strong earthquakes at the same localities. Finally,three-dimensional models for the source faults of two large historic earthquakes have been analysed and discussed.