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Why study the friction of rock? - A Primer on Laboratory Friction Studies

Why study the friction of rocks? Most earthquakes are caused by sudden movement on preexisting faults. An understanding of the frictional forces between rock surfaces is important for understanding earthquake behavior. 


Sketch Illustrating Stick-Slip Friction
Sketch illustrating stick-slip friction. (Public domain.)

A simple analogy would be a block and spring experiment: A block rests on a flat surface, and a force is applied to it by pulling the end of the attached spring slowly. The block can respond in different ways. In one scenario, the block may slide forward smoothly, with a constant force in the spring. This stable sliding of the block is equivalent to creep along an active fault. Alternatively, a force is applied to the block by the spring, and at first the block does not move. The spring keeps pulling until suddenly, the block slips forward and the force in the spring drops. The cycle begins again with the force applied by the spring increasing until the block suddenly slips again. This is called stick-slip behavior. Many phenomena are caused by stick-slip behavior, such as the music produced by bowed instruments, the noise of car brakes and even the sound from grasshoppers. In rock friction experiments, stick-slip behavior is the laboratory equivalent of the earthquake process.

Both types of deformation can occur in the block and spring model depending on characteristics of the sliding surface and the spring. Likewise, both types of deformation occur along faults such as the San Andreas in California. The northern and southern portions of the San Andreas Fault are locked for long periods of time as stresses build up in the earth due to the relative motion of the Pacific and North American Plates. When the fault can no longer support the increasing stress, it suddenly slips, releasing stored up energy and causing an earthquake. In Central California, however, the San Andreas Fault slowly creeps, causing numerous microearthquakes but no large events.

The simple block and spring model is not adequate to simulate the complexities of fault behavior at depth in the earth, so several types of experimental apparatus have been devised for more realistic laboratory experiments.

Biaxial and Triaxial Experiment
Sketch of the three directions of stress exerted on rock samples in the Rock Physics Lab. (Public domain.)

In a biaxial experiment, a test geometry in which forces are applied in two directions, rectangular blocks of rock are squeezed together, and at the same time made to slide past one another due to an applied vertical force. Displacement of the blocks and the force necessary to cause slip between the blocks are recorded. From this, the frictional shear strength of the fault surfaces can be determined. This is commonly described by the coefficient of friction of the rock, µ, defined as the ratio of shear stress, τ, (the stress in the direction of sliding) to normal stress, σn, (the stress pushing the blocks together) and is an important physical parameter used in the modeling of earthquake behavior.

In a triaxial experiment in which forces are applied to the sample in all directions, a cylindrical sample of rock is sealed in a leak-proof jacket and then placed inside a pressure vessel. The vessel is pressurized to simulate the forces acting on all sides of the rock at depth in the earth. The rock can then be further squeezed along its vertical axis with a piston, simulating the additional tectonic forces in the earth. In a triaxial apparatus, the failure strength of intact rock (force required to break the specimen) can be determined, as well as the frictional strength (force required to slide on preexisting fractures).

The total amount of slip between rock surfaces in both biaxial and triaxial experiments is limited by the geometry of the apparatus. When greater fault displacements are desired, a third type of apparatus called a rotary shear machine is used. In this type of experiment, two rings of rock rotate past one another so that the relative displacement on the simulated fault surface can be very large. Also, this apparatus can be operated at high slip rates, simulating the actual rupture speed during an earthquake.

Other important parameters that affect the behavior of faults in the earth are pore fluid pressure and temperature. These conditions are both reproduced in laboratory triaxial experiments by injecting water into the rock and heating the sample during testing. In this way, all of the important conditions controlling earthquakes on natural faults are reproduced on test specimens in the laboratory.