Abstract: The 14 November,2001 M_S8.1 West Kunlun Pass Earthquake is the largest event associated with the longest surface rupture that has occurred in the Tibetan Plateau since 1951. We made 291 surficial left lateral slip measurements and 111 net vertical slip measurements along the main fault zone. The displacement on the main fault strand is dominated by left lateral strike slip of 2.7m in average,with vertical slip component of mostly less than 1m. The maximum left lateral slip is 6.4m,with as much as 5.1m of vertical slip component. Sinistral surficial slip is quite variable along the main strand of the rupture at distance scales ranging from a few tens of meters to a few hundreds of kilometers,with slip gradient ranging between 10^-1～10^-4. The slip variations over short length scales (tens of meters to a few kilometers) might be caused by variations in thickness of unconsolidated sediments,fault strike and slip of the previous earthquake,distributed non brittle deformation and secondary fractures,complexities in fault geometry,and perhaps by measurement error. Despite this short wavelength variability,there is fairly regular long wavelength (tens to hundreds of kilometers) behavior to the east of the Buka Daban Peak. One notable characteristic of slip distribution along the faults is that very large surficial slips (as large as 5～6 meters) were observed at 5～6 sites located at different surface rupture segments in asymmetry to their left lateral slip functions. Slip on each of these rupture segments diminishes away from the highest slip site to its terminations with different slip gradients. This asymmetric distribution of slips may indicate the propagation direction of the rupture along the faults. This long wavelength variation in slip might be influenced by fault geometry,while the segmentation of the surface rupture zone might play a key role. It should be pointed out that the surficial slip (at both short and long length scales) is only a near field slip measured in the field by using tape measure. Therefore,it should be considered as a minimum value,and may represent the real variations in the amount of brittle slip on visible fractures at the surface,but it potentially underestimates the actual slip produced by the earthquake and slip distribution over the whole surface rupture due to the difficulty in identifying distributed non brittle deformation. This calls for caution in discriminating between one or multiple discrete events and in estimating the size of past and future earthquakes by using displaced deposits in trenches or offset geomorphologic features along strike slip prehistoric fault ruptures.

Key words: West Kunlun Pass earthquake, surface rupture zone, left-lateral strike slip, coseismic slip, slip gradient, surface rupture segmentation

摘要： 沿长约426km的2001年昆仑山口西M_S81地震地表破裂带共获得291个点的地表同震水平左旋位移数据,并在其中111个点获得了垂直位移数据。该地震总体以左旋水平位移为主,兼具一定的垂直位移。最大地表左旋水平位移值可达6.4m,平均水平位移约为2.7m,绝大多数测点的垂直位移均<1m。地表水平位移沿主破裂带走向位移梯度变化于10^-1～10^-4之间,这一起伏变化可能起因于野外测量误差、沿主破裂带岩性或松散沉积物厚度的变化、地表破裂带几何结构的不均匀性、地表破裂走向的变化、不同破裂段在昆仑山口西8.1级地震之前的地震中滑动量的起伏变化,以及大量非脆性变形、次级破裂的存在等。水平位移沿主破裂带的长波长(数十公里至数百公里)起伏变化较有规律,在布喀达坂峰以东表现为分别以5个水平位移峰值为中心而有规律地起伏变化。这5个位移峰值分别对应于不同的次级地震地表破裂段。各破裂段水平位移峰值均向阶区或拐点逐渐衰减,不同地表破裂段位移峰值向两侧衰减的速率是不同的,这种位移梯度的不对称分布可能指示了地震破裂的扩展方向。上述位移分布特征真实地反映了地表可见脆性破裂带上的同震位移状况,可能只是该断错事件的最小滑动量。在活动构造研究中,利用地貌断错量或占地震探槽中的地层断错量来区分一次或多次占地震事件,估计占地震事件的规模并预测未来地震的强度时需谨慎。

关键词: 地震地表破裂带, 左旋走滑, 同震位移, 位移梯度, 破裂分段, 昆仑山口西