汶川地震断裂带粒度分布特征:对地震碎裂机制的约束

CHARACTERISTICS OF PARTICLE SIZE DISTRIBUTION OF THE WENCHUAN EARTHQUAKE FAULT ZONE:INSIGHTS FOR CATACLASTIC MECHANISMS

断层岩的粒度分布包含岩石破裂机制、摩擦性质和地震能量分配等重要信息.筛分-称重和激光测量是分析断层岩三维粒度的2种有效方法,但每一种方法的测量范围仅有3个量级,难以全面反映断层岩的粒度分布特征.利用上述2种方法对汶川地震断层滑动带上的断层岩（简称断层岩）的粒度分布进行了测量,粒径测量范围从0.2μm至16mm,跨度达到5个数量级.结果显示：1）存在临界粒径dc（0.95～ 1.90μm）.粒度大于和小于dc的颗粒满足不同的颗粒数（Nd）-粒径（d）分布关系,表明该断层岩的粒径分布不具有自相似性特征.2）利用粒度大于dc的颗粒计算出的分形维数与断层岩类型有很好的相关性,即断层带边缘的角砾岩的平均分形维数为2.6,核部压碎角砾岩的平均分形维数约为3.0,中心断层泥的分形维数约为3.5.粒径小于dc的颗粒的分形维数为1.7～2.1.分形维数的突变反映出断层破裂机制的复杂性,即在不同的粒度域,岩石的破裂机制不尽相同.3）依据粒度分析结果,估算出汶川地震断层泥的单位破裂能（Es）为0.63MJ/m2.

Particle size distribution（PSD） of fault rocks has been widely used to investigate the fracturing mechanism, frictional property and energetic partitioning of earthquakes. Sieve-weighting and laser diffraction analysis are two methods of quantifying 3-1） PSD of fault rocks. However, each of them can only cover grain size in roughly three orders of magnitude, so they cannot reflect the full range characteristics of typical fault rocks, from nanometer scale to millimeter scale. This study performs measurements combining these two methods, on natural fault rocks collected from two surface exposures of the Wenchuan earthquake fault zone, with particle size measured ranging from 0.2~m to 16mm. Our measurements apparently show the following results： 1 ）There exists a critical particular size do（0.95 ~ 1.90~m） , and particles with larger and smaller size than do follow different power law relations between particle number （ Nd ） and particle size （ d）, indicating that the PSDs of the fault rocks studied are non-self-similar. 2）The fractal dimensions （D-value）obtained from the particles with d〉do are well relevant to the fault rock types. This means that the 1）-values obtained by sieve- weighting method and by laser diffraction analysis are consistent with one another. The D-values of fault rocks analyzed show an increase trend towards the slip surface, from 2.6 for fractured breccia, to 3.0 for cataclastic breccia, and to nearly 3.5 for fresh fault gouges. By contrast, particles with d〈do have low D-values of 1.7-2. 1. We infer that variation in fractat dimension for different particle size intervals reflects the change in cataclasis mechanisms. The critical particle size do, probably corresponds to the grind limit of the gouge material. 3）From the PSD results, the surface fracture energy calculated is 0.63MJ/m2 for the Wenchuan earthquake fault gouge.