前震活动特征及其识别的研究（Ⅰ）

分析了近２０年来中国大陆强震（Ｍ≥７．０）和华北地区中强地震前广义前震序列时间和空间分布的多分形特征，计算其广义分维谱Ｄｑ和标度指数谱Ｆ（α），并探讨了它们随时间的变化．结果表明：（１）对广义前震活动，在所研究地区的时、空分布，均呈现多分形结构；（２）大地震发生前半年至一年，地震活动时、空分布的高次分维Ｄ∞呈下降趋势．这种降维特征，亦在实验室含水大样本岩石破坏前，声发射序列Ｄ∞的变化中观测到；（３）与中强地震相比，大地震孕育区地震活动的空间高次分维偏低，表明地壳应力分布的高度不均匀性，是板内大地震孕育环境中的本质因素之一．

STUDY ON THE CHARACTERISTICS OF FORESHOCK AND ID IDENTIFICATION

In the paper,the multifractal features of foreshock in broad sense have beenanalysed before and after the strong and moderate earthquaks occurred in ChineseContinent during past 2l years. Generalized fractal dimension Dq and correspondingsingularity spectrum f),as well as its variations with time for the spatialtemporal distributions of seismicity are calculated by using Fixed-mass and Fixed-radius methods.The results show,1) The spatial and temporal distributionsof foreshock in broad sense in the regions under study have illustrated amultifractal pattern,despite of whether the strong earthquakes with M≥7.0occurred or not in the regions.（ 2） Dq （ q→ ∞） both in space and time showsdecreasing prior to the occurrence of strong earthquakes about half a year to oneyear. The phenomenon of decreasing dimensions of acoustic emissions beforefractures of rocks has also been observed in rockfailure experiment with large scalespecimens（ 0. 5m× 0. 5m×1. 0m）.（ 3） The tectonic environment for preparingstrong earthquakes（M ≥ 7. 0） is quite different compared with that for moderateearthquakes.The high degree of heterogeneity of crustal stress distribution mayplay an important role in preparing large intra-plate earthquakes.