DETERMINATION OF NORMAL FAULT ORIGIN AND EVOLUTION BASED ON STRAIN ENERGY

Normal fault origin and evolution, a hot research topic in rock mechanics, geology and seismology, not only plays an important role in effective and safe petroleum production but also is helpful to understand related shallow-focus earthquake. This study proposed a method to determine normal fault origin and evolution based on strain energy. Following similarity principle, we designed and conducted 3 physical simulation experiments using normal faults at Ordos basin, China, and acquired dip angle, displacement, and strain energy via velocity profile recorded by high-resolution Particle Image Velocimetry (PIV). Fault origin and evolution stages were identified using strain energy and elastic rebound theory. The density of strain energy was determined using surfer 12 to describe spatial variation and internal structure of fault zone. Strain energy release rate and fractured zone derived from fault growth were obtained through calculating scale and central position of strain energy with material point method. Earthquake generated by the releasing of strain energy was classified with the Richter magnitude scale and was further verified with Bath’s law. Results show that normal fault is originated from one point and grows by connecting and merging multiple points. Releasing energy from several centralized strain energy regions is responsible for the unstable fault line and related strata. Small-scale strain energy releasing from multiple regions indicates the formation and evolution of normal faults since energy releasing was typically along with the growth of normal faults. However, low energy release rate does not represent weak fault activity, whereas increasing rate is a good indicator of fault stabilizing. The determination of normal fault origin and evolution suggests that the variation of central position results in both tension and shear fractures in fracture zone, which demonstrates the valid application of strain energy in normal fault. Residual strain energy mainly distributes in the hanging wall after the formation of normal fault; this is why aftershock of shallow-focus earthquake commonly occurs in the hanging wall. Therefore, this study can provide insight into shallow-focus earthquake associated with normal fault.