地震断层带破碎非均匀程度研究——以新马德里地震带Reelfoot断层为例

Study on heterogeneity of fracture in the seismic fault zone——Taking the Reelfoot fault in the New Madrid seismic belt as an example

地震断裂带深部破碎的非均匀程度对于该地震断裂带滑动特性至关重要,而对于地震断裂带深部的破碎非均匀程度研究是目前地震地质比较薄弱的环节.本研究以美国新马德里地震带的Reelfoot断层为例,采用震源机制求解应力场的方法来揭示断层的破碎程度.首先采用小震位置拟合出地震断裂的几何形态,然后沿断裂走向,利用中微震震源机制求解非均匀应力场的应力形因子R值和应力方向,并分析了应力形因子在空间的不均匀性,得到如下结论:(1)Reelfoot断层受到近水平方向上的NEE-SWW向挤压应力作用为主,而断层中部张应力轴方位出现较大的随机性,表明断层中部的构造应力得到较为充分的释放,介质松散度和破碎程度较高;(2)Reelfoot断层两端张应力轴接近垂直方向,跟周围的逆冲型应力场一致,表明断层端部的破碎程度相对较低,能够承担加载于其上的构造应力;(3)Reelfoot断裂带的非均匀应力方向和应力形因子表现为由断层中部向两端逐渐过渡的结果,表明了应力场反演结果的连续性.本研究提供了揭示断层破碎程度的一种研究思路,并且实例结果对进一步研究该区域的断层活动特性和地震动力学过程有一定的参考意义.

The heterogeneity of fragmentation degree in the deep part of the seismic fault zone is crucial to the sliding characteristics of the fault zone.The study on heterogeneity of fragmentation degree in the deep part of the seismic fault is a relatively weaker field in seismology and geology.In this study,the Reelfoot fault in the New Madrid seismic belt of the United States is taken as an example to reveal the fragmentation degree of the fault zone by stress field determination from focal mechanism data.Firstly,the geometric parameters of the seismic fault are estimated by the position of the small earthquake.Then,along the fault strike,the stress shape factor R value and stress direction of the heterogeneity of stress field are solved by the focal mechanism of medium and small earthquakes,and the spatial heterogeneity of the stress shape factor is analyzed.The following conclusions are obtained:(1)The Reelfoot fault is mainly affected by the NEE-SWW compressive stress with nearly horizontal direction,however,the random orientation of the tensile stress axis in the middle part of the fault zone,indicating that the tectonic stress in the middle part of the fault zone is completely released,and the medium is looser and more fragmentized.(2)The tensile stress axis at both ends of the Reelfoot fault is close to the vertical direction,which is consistent with the surrounding thrust stress field,indicating that the fragmentation degree at the end of the fault zone is relatively weaker and enough to bear the tectonic stress loaded on it.(3)The heterogeneity of stress direction and stress shape factor of the Reelfoot fault are the results of gradual transition from the middle part of the fault zone to both ends,indicating the continuity of the inversion results of the stress field.This study provides an idea to reveal the fragmentation degree in the fault zone,and the example results have certain significances for further studying the fault characteristics and seismic dynamic process in the study area.