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title of the thesis - Department of Geology - Queen's University

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propagation as confining stresses are relaxed. Beyond this exists a damage zone where yielding<br />

occurs with some localization along failure planes. High stresses occur on <strong>the</strong> periphery <strong>of</strong> <strong>the</strong><br />

yield zone as <strong>the</strong> rock transitions from a fractured rock mass into a stronger, more intact rock<br />

mass. This parallels <strong>the</strong> rock mass model <strong>of</strong> an inner yield zone, damage zone and outer intact<br />

zone as discussed in Chapter 3.<br />

When <strong>the</strong> faults are weaker than <strong>the</strong> rock mass, right-lateral slip is induced on <strong>the</strong> Plum Shear<br />

Zone and <strong>of</strong>ten on <strong>the</strong> Return Air Raise Shear Zone to <strong>the</strong> south <strong>of</strong> <strong>the</strong> excavation. Remote to <strong>the</strong><br />

excavation, no slip occurs along <strong>the</strong>se or o<strong>the</strong>r discontinuities. The deflection <strong>of</strong> stress around <strong>the</strong><br />

excavation may act to enhance <strong>the</strong> normal stress, and thus <strong>the</strong> frictional resistance, along<br />

discontinuities remote from <strong>the</strong> excavation, preventing slip. Closer to <strong>the</strong> excavation maximum<br />

principal stresses are reduced but oriented at a low angle to <strong>the</strong> Plum Shear Zone, increasing <strong>the</strong><br />

shear stress on <strong>the</strong> fault and instigating slip in both plastic and elastic models. This, however,<br />

occurs within <strong>the</strong> yield zone and may not contribute to seismogenic slip.<br />

4.7.1 Syn<strong>the</strong>sis: Stress, Seismicity and Structure<br />

The relationship between structure and seismicity in <strong>the</strong> Creighton Deep is intimately linked to<br />

stress. The spatial distribution <strong>of</strong> seismicity closely corresponds to zones <strong>of</strong> high stress. A<br />

comparison <strong>of</strong> seismicity and stress is shown in Figure 4.25. This similar distribution supports <strong>the</strong><br />

idea <strong>of</strong> <strong>the</strong> presence <strong>of</strong> a yield zone in proximity to <strong>the</strong> excavation where low stress and no<br />

seismicity occurs; a damage zone <strong>of</strong> high stress where dense seismicity occurs, and beyond this an<br />

intact zone where low to intermediate stress is associated with little seismicity, as discussed in<br />

Chapter 3. Both <strong>the</strong> distribution <strong>of</strong> stress and seismicity appear to be modified to align with <strong>the</strong><br />

strike <strong>of</strong> <strong>the</strong> 118-System shear zones, but not within <strong>the</strong> shear zones <strong>the</strong>mselves. The exception is<br />

102

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