Research on the Effect of Seismogenic Structure with Complex Spatial Relationship on Earthquake Mechanism with the Wenchuan Earthquake as An Example
GUO An-ning,GUO Zheng-jian,ZHANG Jin,ZHAO Cheng-cheng,ZHANG Wei-chao(Lanzhou Institute of Seismology,China Earthquake Administration,Lanzhou,Gansu 730000,China)
The influence of seismogenic structures with complex geometric relationships,in which the seismogenic structure is not parallel in the planar projection,on the mechanism of earthquakes is studied.Seismogenic structures that influence each other have a damping effect on regional earthquakes after their occurrences.However,the mechanical mechanism research shows that when the fault distribution is of complex geometry,the increasing and decreasing effect of earthquakes differ.Perpendicular intersectional faults are known as perpendicular faults;those equivalent to the bottom line are known as bottom faults.If an earthquake has occurred on the bottom fault,the perpendicular fault will have damping effect,which is known as the occurrence of an earthquake on a bottom fault following the occurrence of an earthquake on a perpendicular fault.If a strong earthquake has occurred on the perpendicular fault,a strong earthquake occurring on the bottom fault is known as the occurrence of a smaller earthquake on a perpendicular fault following the occurrence of an earthquake on a bottom fault.When earthquake occurs on a bottom fault,the relation between magnitude M and damping distance R to the perpendicular fault islg R=0.49 MS-1.97(1) where R is the damping distance in kilometers and MS is magnitude.When an earthquake occurs on a perpendicular fault,which is nearly at a right angle with the bottom fault,energy accumulates in the bottom fault.This result is conducive to damping and is more conducive to the accumulation of energy for earthquakes.When the distance between the top and the end parts of a seismogenic fault on a perpendicular fault is approximately 2~3 times the length L of the seismogenic fault on the perpendicular fault,energy may be superimposed on the bottom fault.The relation between length L and magnitude M on the perpendicular fault islog L=0.48M-1.57(2) Using the relationship of the occurrence of a smaller earthquake on a perpendicular fault following the occurrence of an earthquake on a bottom fault,we investigated the influence of the 1933 Diexi M7.5 earthquake and the 1976 MS7.2 earthquake in Songpan and the effect of energy superposition of the Longmen Mountain seismic structure,which later led to the Wenchuan MS 8.0 earthquake. The seismogenic structure of the Diexi and Songpan earthquakes is approximately orthogonal to that of the Wenchuan earthquake.These earthquakes are more conducive to earthquake occurrences than to shock absorption because of the accumulation of energy. In addition,the complexity of the model of earthquakes occurred in Wenchuan was discussed.Some authors determined that the thrust of the Wenchuan earthquake was caused by the Bayan Har block moving southeastward,which squeezed the Longmen Mountain seismogenic structure.We discussed the complexity of mechanical contradiction for this explanation. In this study,according to China Probabilistic Seismic Hazard Analysis(CPSHA) and the complex geometric relationship of seismogenic structures,we demonstrated the manner in which the seismic zone of the potential seismic source distribution function fi,mj,is identified,and we provided an index of intricate for evaluation.