Slip Tendency
Learning objectives
- Read the slip-tendency stereonet: every fault orientation colored by how close it sits to slipping
- Identify the critical orientations, the hot band near 60 degrees dip, and the safe ones
- Locate the canon fault (dip 30) in a cool, low-slip-tendency region
- Show how rising pore pressure heats the whole net, turning more orientations critical
Every Orientation at Once
Section 9.1 rotated one fault and watched its slip tendency change. The stereonet shows every orientation at once. Each point on the net is the pole to a fault plane, and it is colored by that plane's slip tendency: cool where the fault is far from slipping, hot where it rides the frictional limit. One picture answers, for a given stress state, which faults are dangerous and which are safe, the map that turned slip-tendency analysis (Morris and colleagues, 1996) into a standard screening tool.
The hot zones are the critically-oriented faults, those dipping about 60 degrees with the right strike; the cool interior holds the low-angle and misaligned planes. Find the canon fault, dipping 30 degrees: its pole sits in the cool region at slip tendency 0.35, a spectator. This is exactly how a geomechanicist screens a mapped fault population, overlay the real faults picked from seismic and read which of them fall in the hot zones, because those are the ones that will move first when pressures change.
Heating the Net
Now raise the pore pressure with the slider. The whole net warms. Added pressure lowers every effective normal stress and lifts every slip tendency, so orientations that were safe drift toward the hot band and the hot band itself widens. This is induced seismicity as a single image: an injector does not create new stress, it lowers the effective stress everywhere, and faults that sat quietly for millions of years find themselves critically stressed. On the Ogbon-1 model, with its razor-thin margin, even a modest pressure rise turns a wide swath of the net red. The next section puts a number on how much pressure a given fault can take.
Reading a Real Field
The power of the stereonet is that it separates the stress question from the geology question. The coloring is fixed by the stress state alone, computed once; the faults are the field's own, mapped independently. Lay one over the other and the hazard reads off directly: a field can hold many faults and be perfectly safe if they all fall in the cool zones, or hold a single fault in the hot band and be one injection away from a felt earthquake. Orientation, again, is the hinge. Part 9 now turns from which faults are close to how close, the reactivation pressure, and then to how big the resulting event can be.
References
- Morris, A., Ferrill, D. A., & Henderson, D. B. (1996). Slip-tendency analysis and fault reactivation. Geology, 24(3), 275-278.
- Ferrill, D. A., Winterle, J., Wittmeyer, G., et al. (1999). Stressed rock strains groundwater at Yucca Mountain, Nevada. GSA Today, 9(5), 1-8.
- Zoback, M. D. (2007). Reservoir Geomechanics (ch. 5). Cambridge University Press.