逆断层型地震地表破裂带滑动矢量计算方法探讨——以汶川地震为例

METHODS TO CALCULATING SLIP VECTORS OF REVERSE-FAULT EARTHQUAKE SURFACE RUPUTRE:A CASE STUDY IN WHENCHUAN EARTHQUAKE

大震地表破裂带滑动矢量及其分布特征,是震后调查需要获得的最重要的资料之一。由于逆断层型地震地表破裂带运动性质(正向逆冲或斜向逆冲)、破裂带走向的变化以及断错标志线、断层运动方向与断层走向间夹角的变化等等,均会导致视位移的产生,使得野外位移测量数据复杂化。滑动矢量作为一个三维空间的量,代表了各破裂点或破裂面在地表或近地表的运动轨迹,较之其各分量,能够更准确更直观地反应出破裂带的性质和运动学特征。文章讨论了滑动矢量的计算方法,重点讨论了利用两条不平行断错标志线计算滑动矢量的方法。利用这些方法,我们对汶川地震映秀-北川地表破裂带北段北川县城附近若干点的滑动矢量进行了计算,对地表破裂带的性质和运动学特征进行了讨论。

It is important to obtain surface slip distribution features for understanding the focal mechanism of rupture process, exploring geometrical and kinematic features of active structures triggering earthquakes, forecasting the likely size of future earthquake on faults that have not yet produced large historical earthquakes, and delimiting the width of set-back zone in engineering construction. Slip vector （SV） of reverse-fault surface rupture consists of three components： shortening （S） , lateral displacement（L）, and vertical displacement （V）. As a three-dimensional vector, SV is more accurate and perceivable to express kinematic characteristics of surface rupture than its components. However, precise calculation of SV is difficult because apparent shortening （S＇）, apparent lateral displacement （L＇）, and apparent vertical displacement（V＇） of marker line are not always consistent with S, L, and V,which may be induced by various behaviors of surface rupture（ such as pure thrust or oblique thrust）and the angles between marker line and fault and between motion direction and fault trend. In this paper,the following methods to calculating SV of reverse-fauh surface rupture are discussed： （a）If the fault surface is preserved,SV can be acquired through direct measurement and simple calculation using striations on fault surface. （b） If S can be acquired based on balanced cross-section technique using offset marker line or deformation strata in the trench,SV can be calculated further according to the relationship between S and SV（Fig. 1 and Fig. 3）. （c） If there is a pair of nearby non-parallel linear features, SV is the connection between the crossing point of marker lines at wall foot and that on the wall（ Fig. 2） in the three dimensional space, a method firstly proposed by Sharp（ 1975 ）. Using these methods,we calculated slip vectors of surface rupture at some sites near Beichuan County, north segment of Yingxiu-Beichuan surface rupture. At north of Beichuan County,we acquired 4 slip vectors（Table 1 ） according to striations on fault plane using method （1）. At Maoba Village （104°28′11.3″E, 31°50′2.6″N）, the magnitude,direction,and plunge angle of SV is 5.2±0.2m, NE 10°± 9° and 67°± 3°, which is calculated by method（2）. On a road near Lanjiadi Village （104°26′39″E, 31°48′50. 8″N）, three parameters of SV are 1.9± 0.2m,SE53°,and 18° ±7°,respectively. At a tea farm,they are 1.3±0.2m,SE 51°± 13°,and 39° ± 12°, respectively ,where the surface rupture was almost dominated by pure thrust. However, it changed to oblique thrust with right-lateral slip component in sites to the northeast.