余震震源机制变化的原因

大震以后,跟随着一系列的余震,其震源机制有的和主震一致,有的则大不一样,这表现在P波初动的反向,反应了震源应力场的变化。 余震震源机制变化的原因,既不是岩块的错动过头,也不是液体流出的下陷,而是发生大震的滑动岩块对其前后邻接岩块产生的转换应力场所引起的剪切破裂。

CAUSE OF THE CHANGE OF FOCAL MECHANISMS OF AFTERSHOCKS

Some focal mechanisms of aftershocks are generally similar to that of the mainshock while others are remarkably different. This difference is shown by the change of direc-tions of the P-wave first motions of the aftershocks and implies a change of stress field in the focal region.As regards to the cause of this change of focal mechanisms of aftershocks,it is neither due to rock mass overshoot nor due to subsidence following an outflow of pore fluid,but is due to the shear fracture that is induced by the transformed stress field when the slipped rock mass triggred the main skock and extended its effect to the neighbouring rock mass.