Figure 1 shows the seismicity recorded since 2020. Over 13,000 events were recorded with moment magnitudes in the range of mW -2.4 to mW 1.6. Source mechanisms were calculated for many events, and about 800 of these were selected for the SIMS analysis (Figures 2 and 3). The mechanisms observed are of both crush type and slip type. The crush-type events occur near the excavations and are interpreted to be sudden episodes of dynamic stress fracturing in the rock mass (Malovichko, 2020). The slip-type or doublecouple events can be interpreted as sudden slip along existing weaknesses or shear rupture of the intact rock mass (Malovichko and Rigby, 2024). The red lines in Figure 2 represent the P-axes (pressure axes) of the moment tensor solutions. Typically, P-axis orientations are assumed to be parallel to the orientation of the maximum principal stress, σ1, and T-axes parallel to the minimum principal stress, σ3. However, this is not necessarily accurate, as σ1 can be anywhere within the dilatational quadrant of the focal sphere (McKenzie, 1969). This means that the P-axes can only be used as a proxy for σ1. The CVLD components observed are mostly just attributed to noise in the data, rather than tensile cracks which are not likely to happen in a mine setting. A group of several source mechanisms can be used to estimate the orientation of principal stresses (σ1, σ2 and σ3). A few procedures for this analysis have been proposed in the literature (Angelier, 1984; Gephart and Forsyth, 1984; Michael, 1984; and Harmsen and Rogers, 1986). However, to use those procedures, some assumptions are required: the stress field is homogeneous within the area of analysis, source mechanisms are slip type, seismic events are not controlled by a structure (slip planes need to have random orientations) and that the direction of slip is parallel to the direction of shear traction on the slip surface. It is worth noting that method by Vavryčuk, V. (2014) inverts jointly for stress and fault orientation. In practice, a large portion of the seismicity recorded at mines violates one or more of these assumptions. As such, applying these existing methodologies can only be applied to a carefully filtered subset of the seismic catalogue. To overcome this, Malovichko and Rigby (2024) proposed a novel technique in which clusters of seismicity with different patterns observed in mines (events associated with structures, or tunnels, or events attributed to random oriented planes of slip) can be used to invert for the far-field stress (or depending on the area selected, the total stress: in-situ stress + induced stress by a large excavation). The method is described in detail by Malovichko and Rigby (2024), but a brief summary is provided in the next section.
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