Study On Seismicity Characteristics And Mechanism In PingLiang Area

In rencently years,with the quickly development of mine , the optimizational renovation of digital seismograph network and the enhanced monitoring ability of small earthquake,the activity of small earthquake increases obviously in large scale mine ,such as Pingliang coal mine,Jinchuan nickel minerals,Jingtieshan iron ore,Tianzhu gesso mine.Take the coal mine at Huating in Pingliang area as example,since 2001, the earthquakes over ML1.0 have happened more than1023,the biggest is ML4.2 on Oct.2 2005,and brought some losses of wealth and people.Therefore,it is very necessary to do reseach in large scale mine.In this article , taking the increasing seismicity of small -moderate in PingLiang area as object,we do the further study in order to find seismicity mechanism in this area.We definited focal depth by locating method based on genetic algorithm, calculated focal mechanism by moment tensor inversion, according to seismic spatiotemporal distribution,seismic phase and regional geological structural environment, finally, we deduced the seismic mechanism in PingLiang area.We get following conclusions:The seismicity in PingLiang area increases form Dec.2001,reaches the maximum during March to August and the monthly frequence arrives at about 20. The seismicity appears densely in space and the earthquakes are located 30~40km area,The structure of magnitude is primarily composed of ML1.0-3.9,the earthquakes over ML3.0 distribute in Huating and its surroundings in the west,the earthquakes between ML1.0-2.9 distribute in Chongxin and its surroundings in the east.After the comparison of seismic phase with blasts,earthquakes,we reached a conclusion that seismic phase of the small-morderate earthquake in Pingliang area are similar with the mining-induced earthquake .The mainly phase is (?),(S|-),the period is very long,the wave shape is very smooth ,single and regular, the frequece is lower,and the weakening speed is very quickly; the time of P wave first motion is nearly with peak value,the S is full. We definited focal depth in Pingliang area and the western region- Liu pan mountain by locating method based on genetic algorithm ,the results showed that the localization in space keeped unchangeable,but the focal depth changed obviously,the focal depth in pingliang area is mostly inside 5km and the focal depth in Liupan mountain is mainly between 10-30km.We calculated focal mechanism in Pingliang area and the western region- Liu pan mountain by moment tensor inversion ,the types of focal mechanism is mainly compressional-thrust fault , others are the normal slip fault and the strike-slip fault ,and the types is mixed.The dominant direction of principal compression axis is NW, the dominant inclination angle is 15o-20°, the dominant direction of tensile compression axis is NE, and the dominant inclination angle is 30°.The types of focal mechanism in the western region- Liu pan mountain is mainly strike-slip fault.The dominant direction of principal compression axis is is NE, the dominant inclination angle is about 50°, the dominant direction of tensile compression axis is NEE, the dominant inclination angle is60°-80°.Based on the characters of space-time seismicity , seismic phase,focal depth, focal mechanism,the possible seismic mechanism in Pingliang area are as the follows:(1)Among 1023 earthquakes more than ML1.0 from 2001 to 2007, there were 481 earthquekes between 1.0-1.9 which were mining-induced earthquakes by coal body. The kinds of initial mechanism is the dynamic phenomenon that coal structure losed the stability and destructed under thecompression load.After the underground mining,duing to the concentrative strain, the strain exceed the strength limit near the coal wall on coal face,the coal material become unstable under strain softing,but the deep area which have seldom influences by mining keeped flexible stage.With the mining area enlarging ,the deformed area is largening after coal mass get into peak strength, the strength of strain softing intensified,and the coal structure transform from stable to unstable.When they became unstable ,the system lost the stability and destructsed under the external force. (2) There were 493 earthquekes between 2.0-2.9 which are mining-induced earthquakes by work face .The kinds of initial mechanism ,that is lose the stability at extreme value point, with the load increasing , the balanceable route t is firstly steaty , when the load arrived at local maximum.the structure will loss the stability .There were 48 earthquekes between 3.0-3.9,1 earthquake between 4.0-4.9 which are mining-induced earthquakes by fault rockburst. The kinds of initial mechanism is the system whice made up by fault media and wall rock loss the stability under the mining activity.The direct reason is the coal mining, ,the fault and wall rock are dynamic balanced system before the mining,and the coal mining reduce to the fault strain distributes again.