汶川余震震源机制变化的原因

摘要

The main shock of the earthquake (A/s8.0, May, 12, 2008) at Wenchuan is reverse dip slip with mainly press stress axle NW-SE. And then the focal mechanism solutions of some aftershocks are the same with the main shock and the others are obviously different, have progressively changed to strike slip type from the south to the north. The main shock and some of the aftershocks with the same focal mechanism solutions of the main shock are reverse dip slip earthquakes that caused by glue slipping along the fault plane between the upper and lower plates of the reverse fault that is induced by the pushing and cover body of the Longmen mountain which rushed against the massif of Sichuan directly formed by the tectonic stress field. So the three axles of its focal mechanism solutions are in basically conformity with the tectonic stress field. While aftershocks of strike slip type are not the result directly formed by tectonic stress field. It is that this type of earthquakes do not broke out along the fault plane of the reverse fault so that it is not glue slipping, but is due to the transform stress field, formed by the quicker rock of different slipping speed rocks with small lump of being bad integrative (or body formed with different mediums) pressing the ahead neighbouring rock and pulling out the back neighbouring rock along the upper and lower plates of the reverse fault which plained with bad mixed matter, different frictions and binding. The focal mechanism solutions of aftershocks changed regularly is formed by the new shear fracture occurred in the ahead and back neighbouring rocks on the effect of this kind of some stress field.%2008年5月12日,汶川M8.0主震是一个主压应力轴NW-SE的逆倾滑型的地震,而之后的余震震源机制解有的与主震震源机制解一致,有的发生了明显变化,由南至北逐步变成走滑型地震.主震和同主震震源机制解一致的部分余震,在构造应力场直接作用下,龙门山推复体向四川地块逆冲,致使在逆断层的上下盘之间的断层面上产生粘滑而发生逆倾滑型的地震,故其震源机制解的三轴基本上与构造应力场一致.而走滑型余震则认为不是构造应力场直接作用的结果.即,这类地震不是发生在龙门山逆断层的断层面上,故而不是粘滑,而是在逆断层的上盘或下盘滑动的岩块中,由于断层面上的介质不均匀,摩擦力大小不一致或粘结的程度不同,致使盘面上不成一体的断裂小岩块体(或不同介质构成块体)滑动快慢不一,滑动快的小岩块对前方邻接岩块产生挤压,对后方邻接岩块产生拉张的转换应力场.在这种局部应力场作用下,导致前、后邻接小岩块体中发生新的剪切破裂,因而引起余震震源机制解有规律的变化.