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强地壳地震的发生,往往伴随有许多前震和余震,其震源与主震断层密切相关。但是,有时主断层周围前震和余震的空间分布通常型式不同。研究欧洲的一些地震表明,余震不仅沿主震断层分布,而且在主断层周围的某一区域内也有分布。这种异常的地震能量释放型式就是所谓的断层外前震和断层外余震。此类地震通常可形成小震群,其位置明显地偏离主断层线,有时甚至离主断层线较远。Das和Scholz(1981)曾试图用静态剪切应力场的变化解释断层外地震现象,这种应力场的变化是由与主断层有关的二维面内格里菲斯剪切破裂所造成。Sanders和Kanamori(1984)把发生断层外前震和断层外余震的地方称作地壳的相对软弱部位,它相当于一个检测震前和震后阶段主断层应力变化的“应变仪”。
Strong crustal earthquakes are often accompanied by many foreshocks and aftershocks, and their sources are closely related to the main shock faults. However, sometimes the spatial distribution of foreshocks and aftershocks around the main fault is usually of a different type. Studying some of the earthquakes in Europe shows that aftershocks are not only distributed along the fault of the main shock but also within a certain area around the main fault. This type of anomalous release of seismic energy is known as pre-fault and fore-fault aftershocks. Such earthquakes can generally form small swarms that deviate significantly from the main fault line and sometimes even away from the main fault line. Das and Scholz (1981) attempted to explain the phenomenon of extra-seismic earthquakes using changes in the static shear stress field caused by two-dimensional Griffith shear fracture associated with the main fault. Sanders and Kanamori (1984) referred to the place where the fault’s pre-fault and aftershock aftershocks occurred as the relatively weak part of the earth’s crust, which is equivalent to a “strain gauge” that detects the stress changes in the main faults before and after the earthquake.