Time Fractal Behavior of Microseismic Events for Different Intensities of Immediate Rock Bursts

Abstract

A fractal calculation method was developed to investigate the self-similarity of the time distribution of microseismic events during the evolution of rock bursts. Microseismic data, which were utilized for back-analysis after the rock burst, were collected corresponding to different intensities of immediate rock bursts, occurring in the deeply buried headrace tunnels of Jinping-II Hydropower Station in Sichuan Province, China. The overburden depths of the tunnels are between 1,8 and 2,5 m, where the maximum principal stress reaches 65 MPa in a rock mass mainly comprising Baishan Group marble. The results are as follows: The time distribution of microseismic events during the evolution of immediate rock bursts was found to display fractal properties. If the intensities of rock bursts were lower, the time fractal dimensions were smaller (weak rock bursts < moderate rock bursts < intense rock bursts). The time fractal dimensions can be used as a basis for evaluating rock-burst intensities. Time fractal dimensions correspond to intense rock bursts (>1.6), moderate rock bursts (1.1–1.6), and weak rock bursts (<1.1). The daily time fractal dimensions of microseismic events increased during the process of immediate rock bursts but were significantly reduced before the rock burst occurred. On this basis, the time distribution of microseismic events has the potential to be used as a rock-burst predictor, and the microseismic events time fractal dimension can provide a basis for establishing a warning index and reduce the risk of rock bursts in deep tunnels.