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Discussion Questions- Strength Envelope
1) a.) The largest predicted values of differential stress are about 23 kbars at a depth of 20
km in the Eastern US and 2 kbars at 14km depth for the Western US.
b.) The stronger of the two is the thrust fault in the Eastern US, as shown in the Mohrs
circle, has a greater failure envelope.
2) a.) For the Western US my graphs predict a depth of 14km. For the Eastern US my graph
predicts the BDT just at 20km deep.
b.) The isotherm in the Western US is roughly 450 degrees C and 500 degrees C in the
Eastern US.
3) Increasing the strain rate will decrease the depth of the brittle plastic transition.
Increasing the strain rate creates a more brittle behavior in rocks, breaking more easily
at lower temperatures and more shallow depth.
4) Increasing the friction value would in effect create a larger Mohrs circle with a greater
failure envelope/steeper slope. This would move the BDT to greater depths.
5) a.) When introducing water to the brittle or EF regime, it weakens the rock and fault
strength. This brings the BDT to a more shallow depth and lower temperature.
b.) Introducing water to the Plastic or QP regime does not affect the rock strength.
Without shear strength in the plastic regime to begin with, the pore fluid pressure does
not change the rock strength.