Abstract: The waveforms of microseismic events recorded during rockburst contain abundant precursor information of the rockburst. Based on the attenuation characteristics of microseimic wave in the deep tunnel, the maximum effective amplitude is modified, and the relative effective amplitude and maximum effective frequency are used as analysis parameters of frequency spectrum. Case studies reveal that the evolution characteristics of frequency spectrum during instant rockburst induced by the tunnel boring machine (TBM) and the drilling and blasting (D&B) are similar. As there no rockburst occurs, the magnitude of relative effective amplitude corresponding to the largest radiated microseismic energy for daily event is often at a level of 10-6 m/s or even less; and most of the maximum effective frequencies are greater than 300 Hz. Before the medium intensity rockburst happens, the magnitude of relative effective amplitude maintains at a level of 10-5 m/s, and the maximum effective frequencies are between 200 Hz and 300 Hz. (3) Before the intense rockbust occurs, the magnitude of relative effective amplitude increases and reaches 10-4 m/s; and the maximum effective frequencies are almost below 200 Hz. For a whole process rockburst, the relative effective amplitude reach its maximum value and the maximum effective frequency become minimum on the occurrence of rockburst. The evolution of frequency spectrum of microseismic signals can provide reference for early warning the occurrence time and degree of instant rockburst.

Key words: deep tunnel, instant rockburst, frequency spectrum, TBM, drilling and blasting (D&B), early warning