5. Stress

Question 1

What is the S.I. unit of stress? What are the units of stress in terms of N and m? What are the units of stress in base S.I. units (kg, m, s)?

Answer 1

Pascals (Pa)

1 Pa = 1 Nm^-2 = 1 kgm^-1s^-2

Question 2

What is the formula for vertical stress?

Answer 2

The vertical stress is usually set equal to the overburden given by

σ_v = ρ_rockgz

where ρ_rock is the average density of the rock column to a depth of z below the surface, and g is the acceleration due to gravity

In the continents ρ_rock ~2.7 g/cm³

Question 3

What is the definition of an over-pressured pore fluid? What is the definition of the pore pressure parameter, λ?

Answer 3

Over-pressured pore fluids are defined as pore fluids at a pressure greater than gz

p_fluid = ρ_fluid g z ~ g z

• If pores are not interconnected, then the pore fluids may be overpressured
• the pore fluid pressure varies between the hydrostatic condition

The pore pressure parameter λ ≡ p_fluid/σv

Question 4

In the Andersonian classification of stress regimes, which principal stress is vertical in areas that have (a) active high angle normal faults, (b) active low angle reverse faults, and (c) active vertical strike-slip faults?

Answer 4

a) σ₁
b) σ₂
c) σ₃