Analysis of Seismicity in Mines and Development of Re-Entry Protocols

Abstract

Following a large seismic event in hard rock mines, there is a short-term increase in aftershock activity, which over time decays to background levels. During this time of elevated seismic activity the risk of aftershocks with sufficiently high magnitude to cause damage is also high. Workers are therefore restricted from re-entering affected areas for a specified time period. This is the re-entry protocol. The objective of this research has been to produce practical, standardized guidelines for the development of re-entry protocols. This thesis is divided into two parts. In Part I, current re-entry practices are evaluated based on the results of a survey of 18 seismically active mines, mostly in Ontario. Based on this compilation, a complete set of best practice guidelines are proposed. Part II of this thesis presents an in-depth study of the temporal evolution and characteristics of aftershocks and their implications for re-entry protocol development. Through this part of the thesis, eight mining and two crustal seismicity catalogues are used to study the statistical properties of aftershock sequences. These include the modified Omori’s law, Båth’s law, the Gutenberg-Richter frequency-magnitude relation, the Reasenberg and Jones model, and the Epidemic Type Aftershock Sequence (ETAS) model. Statistical procedures for identifying the most consistent parameters of these relations were developed and tested. Despite the site specific nature of mining seismicity, consistent statistics were identified and used to develop guidelines for re-entry protocol development in Ontario mines without the requirement for previous intensive calibration. These simple recommendations are intended for those mines with limited historical seismicity, and to serve as a first approach guide for developing a re-entry protocol. Their applicability is, however, limited to single aftershock sequences. For mines with a significant seismic database a probabilistic approach for setting a family of decay curves (seismic envelopes) and estimating seismicity rate thresholds for re-entry protocol development is proposed. With this formulation it is possible to interpret the decay behaviour of new aftershock sequences and quantify the degree of confidence of the re-entry protocol decision making process in real-time. This is the recommended method for the standardization of re-entry protocol development.