Review on Early Warning Methods for Rockbursts in Tunnel Engineering Based on Microseismic Monitoring  
[PERSON]  
1 School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China; [EMAIL_ADDRESS] (S.  
between rock physical and mechanical parameters, stress state, and rockburst level and risk, the rockburst trend can be predicted and evaluated, providing a basis for engineering design. Different criteria for evaluating the rockburst tendency have been established on the basis of various rockburst theories and prior knowledge, mainly including the stress concentration factor (the ratio of the maximum tangential stress to the uniaxial compressive strength of the surrounding rock: \(\sigma_{\theta}/\sigma_{c}\)), the rock brittleness index (the ratio of the uniaxial compressive strength to the tensile strength of the rock: \(\sigma_{c}/\sigma_{t}\)), and the elastic energy index [7; 8; 9]. On this basis, considering the factors influencing the rockburst tendency (such as the stress, lithology, geological structure, and construction scheme) and with the help of mathematical methods and tools (such as intelligent algorithms, numerical simulation methods, fractal theory, and nonlinear chaos theory), there has been a gradual shift in research, i.e., from the use of rockburst tendency criteria to rockburst trend prediction of engineering rock masses. However, limited by the fineness of survey data, the prediction of the rockburst trend is associated with fuzziness and limitations. In the construction stage, with the refinement of data pertaining to the geological conditions and stress state, the precision and accuracy of rockburst trend prediction have been significantly improved. However, for projects with a high risk of strong and extremely strong rockbursts, predicting the rockburst trend alone cannot meet the requirements of dynamic early warning. Therefore, it is necessary to utilize all the available types of monitoring instruments and equipment to monitor rockbursts in real time. On-site real-time monitoring methods include the pressure method, photoelastic method, resistivity method, electromagnetic radiation method, and microseismic monitoring method. Microsiismic monitoring has been under development since the 1960s, drawing lessons from the rock mass structure monitoring and analysis technology based on geophysical methods. It has been widely used in mining, petroleum, large-scale structural stability monitoring, and other fields. Compared with conventional methods, such as stress monitoring [10] and displacement monitoring, microseismic monitoring has the advantages of real-time and regional performance, and is more suitable for monitoring the stability of brittle rock masses and structures. The monitoring and early warning of rockbursts can be realized in principle owing to the following three reasons [11]:  
1. Although a rockburst is related to the regional stress field, it mainly depends on the local inhomogeneity of the rock mass (faults, joints, weak zones, and other defects). For underground engineering construction, the geological conditions within the scope of on-site construction are generally investigated in detail before construction, which has important reference value for the early warning of rockbursts. 2. Rockburst is a gradual failure process of brittle media from local damage to a macroscopic abrupt change, with evident microfailure precursors. The rockburst precursor area is limited and can be effectively covered by monitoring methods. 3. A rockburst has a short formation period and is evidently disturbed by construction. The short-period repeated rockburst activities in tunnel engineering provide conditions for mastering the law of rockbursts. In the process of a rockburst, microseismicity shows evident evolution characteristics from disorder to order. [PERSON] et al.