As mining proceeds to deeper levels, higher in-situ stresses lead to more microseismicity caused by mining activities (DE SANTIS et al., 2019). This poses greater challenge to the safety of a mining operation. Mining-induced seismicity is influenced by a wide range of mining and geology parameters most notably, stope size and geometry, mining sequence, blasting rate, mining depth, ambient tectonic stress, and geological structures such as faults and dykes in the vicinity of the work areas (GUHA, 2000). Tracking and analyzing the root causes for mining-induced seismic events help better understand the influences of such parameters. It could also prove useful for both short and long term mine planning to control the occurrence of strong seismic events and to provide a safer work environment throughout the life of a mine plan (LEAKE et al., 2017).

This paper presents a case study of the Young-Davidson mine of Alamos Gold Inc. in northern Ontario, a gold mining operation using sublevel stoping method with delayed paste fill. The average production of the mine is 8,000 tpd. While deep excavations are normally expected to be associated with strong seismic activities, seismic events of magnitude M_N 2.0+ have been observed at mining depths of only between 600 m to 800 m below surface; see Figure 1. The occurrence of such large events at shallow depth is the key issue of this investigation. As can be seen from Figure 1, there is poor correlation between mining depth and the magnitude of seismic events (R²=0.026). Thus, the aim of this study is to analyze the microseismic database to discern the root causes for the unusually strong seismic activities recorded at such shallow depths. The effects of mining parameters and geological characteristics of the orebody host rocks on the seismic response are considered in this study.

For the mining parameters, the relation between blasting volume and induced seismicity is analyzed. The influence of a stiff dyke intersecting with the orebody strike is then considered. Furthermore, possible effects of stope extraction sequence and resulting in higher stress regime are examined using numerical modelling. In doing so, stopes are categorized into primary and secondary based on the production plan, and induced seismic rates are recorded for different stope categories and compared with computed mining-induced stresses, to help reveal the effect of mining sequence on seismicity.

It is shown from this investigation that mining sequence is critical, leading to high mining-induced stress in the last secondary stope in each sequence. This is especially true when the production sequence advances towards the dyke, and/or when the sequence advances towards a central pillar. While the findings learned from this research are specific to the case study mine, they could be used to shed light on mining-induced seismicity at other mines with similar mining and geologic conditions.