image: Principle strain state at fault tips in first stage of crack propagation. For a clear observation, this type of strain map is all magnified by 200 times compared with measurement value in this paper. (a) Strain field as wing crack growth at the upper tip; (b) strain field as wing crack growth at the lower tip. view more
Credit: © Science in China Press
Faults movement and development are the most challenging issues in tectonic geology. The classical theory and experimental studies have been carried out with a penetrative fault (2D state). However, the formation and evolution of faults in the crust is basically three-dimensional (3D) such that the fault extends from the ground surface to a certain depth of the crust only. The results from 2D analysis are difficult to transfer to the 3D case. Recently, some researchers have tried to describe the 3D fault growth process using different theoretical model and experimental studies. However, there are some difficulties in establishing a theoretical model to explain the whole surface fault growth process, and the experimental studies were not systematically continued.
Recently, a large number of the experiments on 3D surface fault (flaw) growth study were carried out in the Rock Mechanics Laboratory of The Hong Kong Polytechnic University with varied material, including PMMA, gypsum and real rocks of granite, gabbro, sandstone and marble samples. Although these studies provide some foundational understanding on surface fault growth, the rupture development from surface fault is not yet fully understood.
This study used three different observation methods to study the rupture conformations of a surface fault under biaxial compression. The granite sample is used in this study with the size shown in Fig. 1a). The deformation around the fault is measured and fully analyzed with a high-density Multi-Channel Digital Strain Gauge (MCDSG Figs. 1e-g) and Digital Speckle Correlation Method (DSCM) which is an images analysis of visible light. Simultaneously the micro-fractures involving the fault formation is observed, using a 3D acoustic emission (AE) location system with a set of multi-channel whole-wave record equipment (Fig. 1e-g). The result of this paper represents the first comprehensive investigation on the 3D rupture conformations of a surface fault with reliable and detailed observation and analytical method. The experimental studies were carried out at State Key Laboratory of Earthquake Dynamics, China Earthquake Administration, Beijing. The analysis of multi strain system (MCDSG) and 3D AE location were carried out respectively by Liu LiQiang (Figs. 2 &3) and Liu PeiXun (Fig. 4) of State Key Laboratory. The speckle correlation analysis (SCDM) was carried out by Ma ShaoPeng (Fig. 5) of Beijing Institute of Technology.
The three main conclusions reported by the investigators.
- The crack growth process around a fault could be divided into three stages. The first stage is tensile cracks (wing cracks Figs. 3 and 4d, 5) induced at the fault tips, which is similar to the experimental results obtained from a fracture of a 2-D flaw. The second stage is a conversion stage where petal cracks (Fig. 4d) grow along the fault edge of the interior of rock sample and propagate with strong deformation and rupture activities near the internal border of the fault. The last stage is when shell-shaped cracks appear on the sample surface. They are so-called anti-wing cracks because their growth direction is opposite to the wing cracks (Fig 5). The anti-wing cracks are developed by the growth of petal cracks from the interior of the rock extending to the rock surface.
- The property of anti-wing crack has not been reported by other researchers. Furthermore, the crack growth activities from petal cracks especially are of vital importance during surface fault propagation. This is also a key conversion state and marks an intrinsic difference between 2D and the 3D state in fault development.
- From spatial morphology viewpoint, a three-dimensional structure model, abstracted from a natural fault, is of course better than a two-dimensional one, but the rupture pattern of a 3D flaw is much more complicated than that of the two-dimensional case.
This study is accepted for publication in Science in China Series D: Earth Sciences because it is not only to provide a fundamental understanding of the rupture development from surface fault but also to enrich the knowledge on the determination of 3D location of acoustic emission (AE). It will have a significant impact on the future study of the active fault movement.
The research team includes four institutions: (1) The Hong Kong Polytechnic University; (2) State Key Laboratory of Earthquake Dynamics, Institute of geology, China Earthquake Administration, Beijing; (3) Beijing Institute of Technology; and (4) Shandong University of Science and Technology. This study, however, was carried out only in the first three institutes.
The study is financially supported by the Hong Kong Polytechnic University to RHC Wong (PolyU 5049/01E , PolyU5285/05E, A-PG46, and G-YF88), and by the State Key Laboratory of Earthquake Dynamics; (LED0501) Institute of Geology and the China Earthquake Administration, Beijing, the National Science Foundation of China to W.S. Zhu (40212120, 90510019 and 50579033), and the National Basic Research Program (973 Program) of China to W.S. Zhu and S.C. Li (2002CB 412705).
Reference:
Journal Papers
Wong, R. H. C., Huang, M. L., Jiao, M.R. and Tang, C.A., Crack propagation from brittle solid containing surface fracture under uniaxial compression, Journal of Rock Mechanics and Engineering, 22 (supp.1) pp.2145-2148, 2003 (in Chinese).
Wong, R.H.C., Law, C.M., Chau, K.T. Zhu, W.S. Crack propagation from 3-d surface fractures in PMMA and marble specimens under uniaxial compression, Int. J. of Rock Mech. & Min. Sci., 41/3 pp. 360, 2004.
Wong, R.H.C., Huang, M.L. Jiao, M.R., Tang, C.A. and Zhu, W., T he mechanisms of crack propagation from surface 3-D fracture under uniaxial compression, Key Engineering Materials, 261-263, pp.219-224, 2004.
Guo Y.H., Zhu W.S., Wong R.H.C., Li S.C. and Lin C.J. The mechanism investigation on propagation and coalescence pattern of three internal plane flaws in brittle materials, Key Engineering Materials Vols. 324-325 (2006) pp. 739-742.
Guo, Yan-shuang, Wong, R.H.C., Zhu Wei-shen1 Chau K.T. LI Shu-cai, Fracture Pattern Study of Open Surface-flaw in Gabbro, Journal of Rock Mechanics and Engineering, Vol. 26, No.3 , pp525-531, 2007.
Liu Liqiang, Liu PeiXun, Wong H.C., Ma ShaoPeng, Guo YanShuang, Experimental Investigation of Three-Dimensional Propagation Process from Surface Faults, 2008 Science in China Series D: Earth Sciences (in press)
Conference Papers
Wong R. H. C., Huang, M.L. Kung, W.H. and Chong, M.K. Growth and coalescence of three dimensional cracks under uniaxial compression, 39th U.S. Rock mechanics symposium, Soil/Rock America, June, Vol. 1, pp.901-908, 2003.
Wong R.H.C., W.H. Kung, Shape and boundary effect on growth and coalescence of three dimensional cracks under uniaxial compression, 2003 Mechanics and Materials Conference, Scottsdale, June, pp. 184-185, 2003.
Wong, R.H.C., Huang, M.L., Jiao, M.R., Tang, C.A. and Zhu, W., The mechanisms of crack propagation from surface 3-D fracture under uniaxial compression, Conference on Fracture & Strength of Solids, October, Japan, pp206, 2003.
Wong R.H.C., Guo Y.S.H., Chau K.T., Zhu W.S. and Li S.C., Anti-wing crack growth from surface flaw in real rock under uniaxial compression" 16th European Conference of Fracture (ECF16), July 3-7, p.825, Alexandroupolis, Greece, 2006.
Wong R.H.C., Guo Y.S.H., Chau K.T., Zhu, W.S., and Li S.C., The fracture mechanism of 3-D surface flaw with strain and acoustic emission measurement under axial compression, Asian Pacific Conference on Fracture and strength, 2006 Sanya, Hainan Lsland, China, November 22-25, pp.336, 2006.
Guo Y.S.H., Wong R.H.C., Chau K.T., Zhu, W.S., and Li S.C., Crack growth mechanisms from 3-D surface flaw with varied dipping angle under uniaxial compression, Asian Pacific Conference on Fracture and strength, 2006 Sanya, Hainan Lsland, China, November 22-25, pp.335, 2006.
Wong, R.H. C. Li, T.C. Chau, K.T. Li, S.C. and Zhu, W.S., 2007, Crack growth study of a 3-D surface fracture under compressions using strain and acoustic emission measurements, 1st Canada-U.S. Rock Mechanics Symposium, May 27-31, 2007, pp565-573.
http://www.wjgnet.com/1007-9327/13/4873.asp