AN EXPERIMENTAL STUDY ON THE PROCESS OF DEFORMATION AND INSTABILITY FOR EN-ECHELON FAULTS
LIU LIQIANG, MA JIN AND WU XIUQUAN (Institute of Geology State Seismological Bureau Beijing, China)
Twelve models of en-echelon faults with different geometries were designed to study the process of deformation.The spatial and temporal process of acoustic emission, fault displacement, orientation and magnitude of principal stress in the staggered area were observed during the fracturing of the specimen. It was found that AE sequence, AE distribution and precursory form for mode LL (Left lateral shearing, Left stepping offset) were quite different from mode LR (Left lateral shearing, Right stepping offset). The main differences are as follows:1) For mode LL, foreshocks swarm in the staggered area, where the main earthquake will take place, and migrate gradually towards the epicenter of the main shock. As opposed to this the foreshocks for mode LR occur around the staggered area without the migrating trend towards the epicenter of the main shock and an AE gap is delineated in the center.2) For mode LL, the foreshocks are frequent and their magnitude greater. The energy of the main shock is only about 10 times as large as the total energy of the foreshocks. For mode LR, the foreshocks are rare and the energy of the main shock is about 100 times as large as the total energy of the foreshocks. The energy of the main shock for the mode LR is one magnitude greater than that for mode LL.3) The fault displacement of mode LL accelarated gradually and the acceleration becomes much larger just before the main rupture. At the tip of the fault the acceleration is the largest. The displacement rate of mode LR keeps constant almost during the whole process.4) The staggered area of mode LL is in a strong tensile stress and weak shear stress state. As a result of microcraking, the axis of principal stress rotates about 45°90° in the staggered area just before the main shock. At the same time, the value of the tensile stress decreases. On the contrary, the staggered area of mode LR is in a strong compress and shear stress state. The axis of stress is steady before the main shock, but the compret.sive stress intensifies violently.