Tectonic Plates and Faults - Earthquake Engineering for Structural Design - page 169

Fracture of sheared crust. In some cases, during the plume rising, instead to
produce normal faults, the shear faults occur. There are two types for shear faults
(Schettino, 2007):
- Symmetrical pure shear type (known as McKenzie model) (Fig. 5.26a), which
considers the ductile behavior of the lower crust and the brittle behavior of the
upper crust. Due to the shear stresses, a symmetrical system of normal faults
results.
- Asymmetrical shear type (known as Wernicke model) (Fig. 5.26b), which is
a low-angle fault that crosses through the lower crust and determines an
asymmetric tectonic structure of faults in the upper crust.
From the fault point of view, in both cases, a whole series of fault plane can
occur by the rotation of a series of micro-blocks (Fig. 5.28), separated by fault
planes.
The main example for this crustal fault is the New Madrid-Missouri (central
USA) fault system, producing the largest historically documented earthquake in
the United States (Fig. 5.29)
(a)
(b)
Figure 5.26 Sheared crust: (a) Symmetrical pure shear type;
(b) Asymmetrical shear type (modified after Schettino, 2007)