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

Figure 4.21: Tectonic model of yielding for (A) elastic model and (B) plastic models. Faults are assigned
a cohesion of 0 MPa and a friction angle of 35 degrees.
4.6 Modelling Rock Mass Degradation
Seismic events occur as a result of slip on existing fractures or through the creation of new
fractures when stress exceeds the strength of the rock mass. Seismicity can thus signal the onset
of rock mass damage and degradation. Damage begins as a process of fracture initiation and
progresses to a state of fracture growth and interaction as the rock mass progressively yields.
Thresholds for both rock mass damage and yield in brittle rock can be defined by stress and
through seismic monitoring (Falmagne, 2001). This is the same process defined using acoustic
emission monitoring at the lab scale (Eberhardt et al., 1999). The onset of microseismicity
indicates that stress exceeds the damage threshold and that fracture initiation has commenced;
continued emission signals fracture propagation (Falmagne, 2001). When seismic events begin to
cluster, this denotes that stress levels exceed the yield threshold of the rock mass. At this stage,
fractures are numerous and long enough to interact and coalesce, causing an overall change in
rock mass strength properties (Falmagne, 2001). High event rates as well as dense event
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