SIMS Method As shown in Figure 4, SIMS considers the following categories of events: • Events related to structures (or known weaknesses) in which one of the nodal planes matches the orientation of the weakness plane within some uncertainty in terms of the hypocenter location and the orientation of the nodal plane estimated. In this case, the direction of slip for the nodal plane needs to correlate direction of the shear traction in the weakness plane. Any angular difference can quantify the misfit between source mechanisms and the stress state. • Events close to tunnels with crush-type source mechanisms (with significant implosive component). In these cases, the convergence direction is controlled by the maximum principal stress in the plane orthogonal to the tunnel’s axis. Therefore, in this case, the misfit between the P-axes calculated and the maximum in-plane stress can be used to quantify the difference between the source mechanisms and the stress state. • Double-couple events not associated with a known structure and that are scattered spatially can be analysed in a similar way as with established techniques in this case (Gephart & Forsyth 1990; Hardebeck & Hauksson 2001). The smallest misfit between the shear traction and slip direction for both nodal planes is considered to quantify the difference between the mechanisms and the stress state. Figure 35 Seismic events recorded since 2020 in plan view (left) and looking North (right). Colour represents time. The size of the spheres is scaled according to moment magnitude. The red mesh represents a known structure. Several events locate away from mining.
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