后半夜是太阳和月亮的引力最小的时候。潮汐是这样形成的，矿山的冒顶事故也符合这个规律。

English:

The Earthquake Happen at Latter Midnight 

The latter midnight is the time when the sun and the moon have gravitational force of smallest.

This is form of the tide, and most of the mine roof accidents also occurred in latter of midnight.

（Tsaokuohua=曹国华=320602520830201）