Tectonic Plates and Faults - Earthquake Engineering for Structural Design - page 164

164 Earthquake Engineering for Structural Design

5.3.4 Fracture of Weak Compressed Crust

Produced exclusive in the Earth's crust, the faults in diffuse zones due to the
fracture of crust have different features in comparison with the tectonic plate
boundaries: no divergence, no subduction, as in the case of tectonic plate
boundaries. There are the following fracture types:
- Fracture in the compressed crust.
- Fracture in the tension crust.
- Fracture in the sheared crust.
In the followings, the differences between these fracture types will be presented.
Fracture of weak compressed crust (Fig. 5.19). Active plates and microplates
are stressed by forces applied in the adjacent plates, forces applied at the plate
boundaries due to the motions of plates. If the compressed crust presents a
weakness a fracture of crust occurs, forming a reverse fault (Fig. 5.20a). In case of
low-angle (average < 45 degree), the fault is called as thrust fault and it consist
often of fault planes parallel to sedimentary layers, forming ramps. The hanging
wall moves up relatively to the footwall (Fig. 5.20b), producing a shortening of the
crust. It is very important to notice that, after some time, an erosion of hanging
wall peak occurs (Fig. 5.20c). Figure 5.21 shows the map of Eastern USA and
adjacent Canada, with compression stresses produced by the Mid-Atlantic fault

Fracture of weak
compressed crust

Figure 5.19 Fracture of weak compressed crust