Sources of seismic energy based on butterfly failure theory

The source of greatest energy released during an earthquake has not yet been determined. Considering the basic characteristics of frequent occurrence of strong earthquakes at the intersection of two sets of crustal rupture zones,a model for energy analysis by taking a soft anisotropic body as the center was established and the calculated method of seismic energy was proposed on the basis of butterfly failure theory. From the viewpoint that the surrounding rocks of a soft anisotropic body were subjected to plastic failure resulting in the change in energy released,the authors numerically analyzed the influence of the presence of a soft anisotropic body on energy accumulation in surrounding crustal rocks during an earthquake,and discussed the general law of natural earthquake triggering. Based on the geo- logical background of the epicenter location of the Ludian ML6. 5 earthquake in the southwestern part of China,the variation law of source energy and distribution characteristics of earthquakes with or without a soft anisotropic body was further analyzed. The calculated results showed that the soft anisotropic body in the crust redistributed the stress in the surrounding rock mass and formed the stress concentration around the soft anisotropic body. With the increase of com- pression and tensile structural stress,the rock masses around soft anisotropic body were subjected to plastic failure leading to an accumulation of energy with the epicenter of butterfly-shaped damage zone,which was the main source of seismic energy. The soft anisotropic body changed the energy accumulation characteristics of the surrounding rock mass exponentially,and the butterfly-shaped energy concentration area was formed during the evolution of butterfly plastic damage in the surrounding rock of the soft anisotropic body,indicating the transition from the steady state to unsteady state of energy concentration. The range of plastic failure and the state of accumulated energy of rock mass around the soft anisotropic body determined the magnitude of earthquakes. In the period of intense expansion of the butterfly- shaped failure,minor stress disturbances can cause the catastrophe of the whole energy system of surrounding rock and trigger large-magnitude natural earthquakes.