Double plane shear failure

The double plane shear mode of failure is graphically shown in Figure 7.6. The parameters associated with this failure are shown in Figure 7.7. 

FIGURE 7.6 Double plane shear mode of failure. 

Single- and double-lap and single- and double-strap joints have stress concentrations at the ends of the overlap when loaded with in-plane loads. These are the most probable locations for the failure initiation. A t3rpical shear stress distribution is shown in Figure 5.27. 

Earthquake Focal Mechanisms Earthquake focal mechanisms describe the nature of deformation at the earthquake source. Most VT earthquakes are double-couple, indicative of shear motion on a fault plane. The orientation of focal mechanism describes the stress field at the source. Rotations of the focal mechanism with time from, for example, strike-slip to reverse, have been reported before some eruptions and interpreted in terms of magma pressurization changing the orientations of the principal stresses in the vicinity of the magma body (Roman and Cashman 2006). Non-double-couple earthquakes indicate a more complex source, including tensile failure and events associated with finite volume changes, such as explosions or collapses. In Iceland, for example, such sources have been associated with tensile failure during the opening of subsurface cracks. 

VT earthquakes are interpreted as resulting from brittle failure or shear fracturing along a fault plane and are therefore indistinguishable from ordinary double-couple tectonic earthquakes. Since VT earthquakes possess the same characteristics as tectonic earthquakes, if they are recorded at sufficient stations, standard seismo-logical methods and tools can be used to determine their hypocenters, magnitudes, and focal mechanisms. 

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