Cause of the Change of Focal Mechanisms of Wenchuan Aftershocks

The main shock of the earthquake(MS8.0,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.