Study On The Seismogenic Fault Of Lushan Ms7.0Earthquake And Focal Mechanisms Of It’s Aftershocks

The micro and small aftershocks of the2013Ms7.0Lushan earthquake were recorded by29stations around the mainshock region for more than about one year. We firstly located these aftershocks, and then calculated their focal mechanisms. Results show that the aftershocks laterally distribute along Shuangshi-Dachuan branch fault and its surrounding regions, and vertically concentrated on two intersecting belts between about5to20km depth. One aftershock belt dips towards NW, and its dip angle becomes steeper at depth of smaller than～12km. The extrapolation of this belt coincides with the buried fault between Shuangshi-Dachuan branch fault and Xinkaidian Fault at surface. The other aftershock belt dips towards SE. The extending position of this belt to the ground is close to the Shuangshi-Dachuan fault. However, the dipping direction is opposite to Shuangshi-Dachuan fault. The hypocenter of Lushan main shock locates close to intersect of the two aftershock belts. Two nodal planes of focal mechanism of the main shock correspond well with the geometry of two aftershock belts, respectively. This seismogenic structural feature implys that the Lushan earthquake probably is a result of simultaneous activities of two faults along the two aftershock belts. The focal mechanisms of the main shock indicates that the fault activity is thrusting from NW to SE, while focal mechanisms of aftershocks mainly display normal-slip and strike-slip fault activities. The results of focal mechanisms of aftershocks show that the direction of horizontal maximum principal stress changed from NW before the Lushan main shock to NNE after the main shock, indicating that the regional stress might be completely released and tectonic stress field was strongly changed during the main shock.