ABSTRACT: Shear sliding along a fault plane leading crack propagation can be classified as a strike-slip fault movement. The cracks propagated from the extensional quadrant or compressional quadrant of fault is named wing crack and anti-wing crack, respectively. This paper focuses on the experimental study on nucleation and growth of anti-wing crack from a surface flaw under biaxial compression. Four observation systems were used including multi channel digital strain gauge (MCDSG), video camera, digital speckle correlation method (DSCM) and acoustic emission (AE). In this study, two types of anti-wing crack development were found which was not reported by the previous studies. One of the crack types was formed by the growth of petal cracks extending from the interior of the specimen to the surface, whiles the other crack type was formed by the growth of tensile cracks from the specimen surface extending into the interior of the rock. Both types of anti-wing crack are the mixed mode pattern of tensile and compressive crack. Furthermore, the growth behavior of wing crack and anti-wing crack can be explained by the result of strain record. The result of this study provides more knowledge and understanding on the crack development of the strike-slip fault.
1. INTROUDUCTION
Faults are one of the common tectonic structures. Cracks can propagate from the termination of a fault plane under external loading. Such kind of fault movement is called Strike-slip fault. Experimental and theoretical analyses [1-2] are demonstrated that tensile stress and shear stress concentrate around the fault (or flaw) tips during the shearing process. Kim and Sanderson [3] divided three quadrants around a tip of the strike-slip faults: shear quadrant, extensional and compressional quadrant (Fig.1). Crack propagated from the extensional quadrant, compressional quadrant and shear quadrant are called wing cracks, anti-cracks [4] or anti-wing cracks [5] and shear cracks, respectively. In general, wing cracks sprout first from the flaw tip, due to the tensile strength of rock being lower than that of the shear or compressive strength, it then follows by shear cracks or anti-wing cracks. However, in some of cases, only anti-wing cracks are observed at the tip of the strike-slip faults. For example: anti-wing cracks with the size of 0.2m to 0.5m appear at the strike-slip fault in limestone from Somerset, U.K. [6] (Fig. 2a). Thrust faults (similar to the crack pattern of anti-wing cracks but in a much large scale) of 1-2km at the termination of Chelungpu fault in Taiwan [7] (Fig. 2b), and thrust fault with the size of 50km to 70km induced at the tip of strike-slip fault of Gurvan Bogd Mountains in the eastern Gobi-Altay [3] (Fig. 2c). A number of studies had been done on the growth mechanisms of wing crack. However, the nucleation and growth of the anti-wing crack from the compressional quadrant of fault (or flaw) is not known. Wong and her co-authors [5, 8-11] carried out a series of experiments to investigate crack growth mechanisms of a 3-D surface flaw in PMMA and real rock specimens under uniaxial compression with changing the flaw angle ±, flaw length (2C), the specimen thickness (t) and flaw cutting depth (d).
