Page 177 - Reservoir Geomechanics

160 Reservoir geomechanics

a.
Double-Couple Model

Fault
plane

Auxiliary
plane

b. View from Side c. View from Above
“Beach Ball”
EARTH’S SURFACE
FAULT Auxiliary
Depth PLANE plane

Auxiliary P T
plane

Fault
FOCAL SPHERE plane
Fault-plane
projection
Figure 5.11. (a) Schematic illustration of the radiation pattern and force-couple associated with
earthquakes as the basis earthquake focal plane mechanisms. An east–west striking, vertical
right-lateral strike slip fault intersecting a half space is shown. The polarity of the P-waves defines
the compressional and dilatational quadrants. (b) Cross-sectional view of the nodal planes,
radiation pattern and P- and T-axes associated with an east-dipping normal fault. The radiation
pattern does not uniquely distinguish the fault plane from the auxiliary plane. (c) Lower hemisphere
stereonet representation of the normal faulting focal mechanism.

Earthquake focal mechanisms associated with normal, strike slip and reverse faults
are illustrated in Figure 5.1. Note that while the conjugate shear faults are at angles
±30 on either side of the maximum principal stress, the focal mechanism illustrates
◦
orthogonal nodal planes, one of which is the fault. As a point of historical interest, focal
plane mechanisms were instrumental in establishing the theory of plate tectonics as they