Abstract For complicated types and distribution of the aftershock focal mechanism solutions，we give a concrete thought of inverting stress by aftershock focal mechanism with human factors as little as possible，according to the characteristics of ZMAP package.Based on aftershock focal mechanism solutions of the 1994 Northridge earthquake，using the stress tensor inversion method by Michael to analyze the tectonic stress state and its distribution in source region and preliminarily discuss the variations of stress state and its links to structure.The results show that the maximum principal stress in source region is nearly horizontal and trends NNE-SSW，which is an explicit characteristic of tectonic stress in Northridge earthquake source region.After Northridge earthquake，tectonic stress in deep and shallow area of source region are not entirely consistent.Even though the maximum principal axis is NNE both in deep and shallow area，the stress regime are thrust-faulting regime in deep area and strike-slip regime in shallow area.And the inversion result of deep data is consistent with region tectonic stress field.At same time，the difference of stress between deep and shallow area in source region is probably related to earthquake structure and movement mode of the Northridge earthquake with a stress regime of thrust-faulting.After the earthquake occurred, stress continued to release and blind fault kept on with thrusting movement under the action of regional tectonic stress. The thrusting movement of fault caused several thrusting earthquakes and stress state in deep area has no significant change. The thrusting movement of Northridge fault probably causes variations of stress and fault’s movement in the shallow area of source region.
