Faulting (Chap. 8)
Anderson’s theory: assume no shear stress
parallel to Earth’s surfaceImplies one of the principal stress directions is
vertical near Earth’s surface:
Normal fault: 1 vertical, 2, 3 horizontal
Strike-slip: 2 vertical, 1, 3 horizontal
Thrust: 3 vertical, 2, 1 horizontal
Experiments indicate:
Faults form at ~ 30 deg. to 1.
Implies normal faults dip at 60o, thrusts at 30o
1
Correct only for homogeneous rocks. Bedding and
foliations control most fault orientations.
Listric faults
Dip decreases with depth (normal and thrust).
1 and 3 are curved due to increase in stress with
depth. Faults are also curved with depth.
Thrust fault paradox
When pushing a thrust block (> 1km thick), block
will fail internally first, before sliding because of
basal friction. But many large thrusts have
moved > 10 km (e.g. Blue Ridge).
2
Block will slide only if slope >30 deg.
Real detachments actually <1 deg.
Solutions:
1) Hubbert-Rubey hypothesis:
High fluid pressure acts as “lubricant”
Principal of effective stress: eff = n - Pfluid
Coulomb criterion:
 = C + n
C = 0 for basal detachment

 = n - Pfluid)
as Pfluid
n,   

Problem: need low permeability rock and seal
(e.g.shale)
Alternative hypothesis: blocks slide on weak rocks:
shales, mica schists, salt (Alps).
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Shear heating
V = Q + Eearthquake + Esurfaces
shear stress (N/m2)
Q = heat (W/m2)
V= slip velocity (1 m/s for earthquake)
Eearthquake + Esurfaces = small

Q= V
If Q and V are known,  can be estimated on faults.
Units:
W = Nm/s; W/m2 = N/ms
V = (N/m2) x (m/s) = N/ms
Pseudotachyltes; Coal metamorphism
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No heating observed on San Andreas fault
1) shear stress is small (weak fault hypothesis)
2) groundwater removes heat (strong fault
hypothesis).
Earthquakes and faulting
Sudden release of stored elastic energy
Inactive fault: no recent earthquakes
Active: earthquakes in recent past/present
Fault creep: aseismic (slow rate of slip)
Stick-slip: regular recurrence interval
Intraplate seismicity: New Madrid system
Plate boundary: Anatoli F, Alpine F, SAF.
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