What is the reasoning behind the statement "Faulting will occur along the plane where the shear stress is the highest"?

Question

In "An Introduction to Seismology, Earthquakes and Earth Structure", authors Seth Stein and Michael Wysession made the statement: "Faulting will occur along the plane where the shear stress is the highest". However, no further clarification was given. What are the implications of this statement? Is the statement true? What are the processes that cause faulting to occur along the plane of maximum shear stress?

Answer 1

Faulting occurs along the plane where shear stress is highest, because that is where the most force is being applied. If you grab a banana from either side and move your right hand away from you, and your left hand towards you, the banana will break in the center between your hands, where the most shear stress is occurring. Faulting occurs from the movement of tectonic plates. If two tectonic plates lay adjacent to each other, and the right one starts moving North while the left one moves South, shear stress occurs where the two plates meet. The tectonic plates, which constitute Earth's crust, lie on the upper mantle. The upper mantle is fluid because it is magma (molten rock). Within this upper mantle, hot magma rises up while the cooler magma gets pushed down again, creating a convection current. This allows the crust on top to move around, just like if you floated a chip in a bowl of boiling water.