Development of subcracks betweenen echelon fractures in rock plates

Abstract

Uniaxial compression tests on marble plates containing two prefabricated en echelon fractures were performed in this study. Photographs showing the typical characteristics of subcrack development were taken under direct scanning electron microscope (SEM) observation during the test. From these photographs, the effects of the length L of a single fracture, the separation distance d and overlapping proportion a/L between two prefabricated en echelon fractures on the development of subcracks were analyzed. The results show that the interaction between en echelon fractures strengthened with decreasing d. For d ≥ L, there was little interaction and the development of subcracks near one of the en echelon fractures was unaffected by the existence of the other. Whereas for d < L, the number of subcracks in the area intermediate between en echelon fractures tended to increase with increasing a and decreasing d. In order to ascertain whether the experiment can furnish some implications for research on earthquake prediction and neotectonic activity, the test results were compared with the spatial-temporal development of foreshocks and ground-water anomalies before the 1975 Haicheng earthquake and the structural framework of the Ganzi pull-apart basin. The results of the comparison are encouraging. According to the similarity between the test results in the laboratory and the natural phenomena in the field, the fault on which the Haicheng earthquake would occur, could be inferred immediately from the relative geometry of the two sub-parallel active faults in the area. Thus, it is considered that the test results would advance our understanding of the process of neotectonic activities and give us inspiration for earthquake prediction.