A Comparison of Mechanical Behavior and Frequency-Energy Relations for Two Kinds of Echelon Fault Structures Through Numerical Simulation

摘要

Heterogeneities of faults and rock blocks were included in the present numerical simulation to study differences between the compressive and extensional echelon fault structures. Released elastic strain energy due to shear and tensile failures of elements was calculated. An event is defined as a failed element releasing energy. For calculation of the frequency-energy relation, whose negative slope was labled b _0 value, a cut-off factor, used to delete a few larger events, and a classification factor, used to group the remaining events into several grades, were introduced. The evolution of b _0 value at different stages was obtained. Three kinds of changes in b _0 value were found. In the compressive echelon fault structure, no remarkable change in b _0 value is observed during the outward propagation of tensile failure zones, also during the jog failure in the extensional fault structure. This is due to the fact that tensile failure only increases the number of small events that are originally larger. In the compressive echelon fault structure, a rapid decrease of b _0 value is found during the jog failure since a shear failure zone links two fault tips, leading to the occurrence of larger events. For both kinds of echelon fault structures, b _0 value is found to decrease continuously during the fault reactivation. This is related to stress or strain enhancement over a larger area. In the extensional fault structure, a recovery in b _0 value in the vicinity of the stress peak is found after a global minimum, which is similar to phenomena caused by aftershocks. Moreover, evolution of b _0 value at different stages for the same echelon fault structure and that at the same stage for different echelon fault structures are compared and analyzed. It is found that the change in fault structure may not lead to the change in b _0 value. To obtain a more continuous and less fluctuant b _0 value curve, the suggested values of the cut-off and classification factors are 1/8 ~ 1/2 and 3 ~ 4, respectively. The proposed method can also be used to analyze the mechanical behavior and frequency-energy relations for other typical or complex fault structures. Our work can contribute to identify the regions where abnormalities or precursors can be monitored and to assess the positions of larger events.