E4 Failure and Fracture Mechanics

Topic

A geological fault is a plane of weakness within the Earth's crust—a plane along which the crust eventually ruptures and slips. A well-established mechanical failure criterion determines exactly when that rupture occurs: a fault ruptures when the shear stress acting upon it exceeds the sum of its own cohesion and its frictional resistance—a resistance that depends, in turn, on the difference between the normal stress compressing the fault and the pressure of the fluid filling the rock's pores.

This pore-fluid pressure reduces the effective normal stress experienced by the fault, and this reduction can trigger an earthquake even under nominal stress levels that would never cause a rupture in completely dry rock; this same mechanism explains the seismicity induced by fluid injection into deep wells.

Geological faults are classified according to the tectonic regime under which they formed: a normal fault occurs under extension, with the upper block moving downward relative to the lower block; a reverse fault occurs under compression, with the upper block moving upward over the lower block; and a strike-slip (or transcurrent) fault produces predominantly horizontal movement between the two blocks.

Fault rupture during an earthquake propagates at speeds of between two and four kilometers per second—always slower than the speed of shear waves—and the resulting slip scales empirically with the total length of the rupture: the longer the rupture, the greater the slip that typically accompanies it. An earthquake's scalar seismic moment—the product of rock rigidity, average fault slip, and the total rupture area—is the most physically rigorous measure of the energy released by that earthquake; this same scalar seismic moment is used to calculate moment magnitude, a scale that, unlike older ones, does not lose sensitivity or saturate for the largest earthquakes, thereby closing the link between the fault rupture mentioned at the start of this description and the final measure of the energy that rupture releases.

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