海丰震群发震构造型式的讨论
STUDY ON THE TYPES OF EARTHQUAKE FAULTING OF THE HAIFENG EARTHQUAKE SWARM
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摘要: 海丰震区发育着规模宏大的 NE 向海丰——梅陇断裂及一系列与其平行的次级断裂.同时,还存在与其共轭的断续分布的 NW 向断裂.震群中的三个子群的主震的震源机制解非常类似,其中一节面为 NE,另一节面为 NW.从余震的空间分布及极震区的长轴方向来看,2月26日 ML3.3和4月9日 MLL,4.2地震的断层面为 NE 向;而3月14日 ML,3.4地震,其断层面应取 NW 向.又据137个小震的四个台的 P 波初动符号的组合特征,可划分为八个类型并作出相应的迭加震源机制解.综上所述,本区破裂过程,主震及Ⅰ、Ⅳ 类地震是岩块沿NE 及 NW 向共轭构造的粘滑;Ⅱ——Ⅴ、Ⅶ、Ⅷ 类地震是岩块在粘滑过程中对前后邻接岩块引起平行滑动方向的挤压(前)和引张(后)的转换应力场所产生的剪切破裂.Abstract: The major Haifeng-Meilong fault trending NE and a series of small faults parallel to it are distributed in the Haifeng earthquake region. Besides, there also exist disconnected conjugate faults trending NW. The nodal planes of the focal mechanisms of the largest shocks of each of the three sub-swarms are quite similar and one of the nodal plane is trending NE and that of the other NW. According to the space distribution of the aftershock sequences and the direction of the longer axis of the maximum intensity region, the faults causing the earthquake ML 3.3 on February 26 and ML 4.2 on April 9 should strike NE while that associated with the event ML 3.4 on March 14 strikes NW. In addition, by examining the data of signs of first motion of 137 small earthquakes recorded by four seismic stations, we found that there are eight types of combinations from which composite focal mechanisms can b eobtained. Therefore the fracturing process associated with the main-shock as well as theⅠ, Ⅵ type small earthquakes are stick-slip of rock masses along conjugate faults trending NE and NW. The small earthquakes of the Ⅱ, Ⅲ, Ⅳ, Ⅴ, Ⅶ, Ⅷ types are concerned with the. shear fractures caused by a transformed stress field resulted from compression on adjacent rockmasses at the front and tension on the rear and parallel to the direction of slip during the stick-slip of rockmasses.
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[10] 魏柏林,余震震源机制变化的原因,地球物理学报,23, 1, 1980.
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