Using an atomic force microscope (AFM) to zoom down to nanometre dimensions, researchers in the US have improved our knowledge of the friction between surfaces at geological fault lines. The experiments show that chemical processes can act to bond the surfaces together – a discovery that may lead to a better understanding of how earthquakes are triggered.

Understanding how these surfaces interact is crucial because earthquakes result from frictional instabilities along active fault lines. Stress builds up along the fault and a slip occurs when this stress overwhelms the friction, releasing the stored energy and triggering an earthquake. Geophysicists know that friction between opposing rocks increases the longer the surfaces remain in contact. This is known as frictional strengthening or "ageing" and has been observed in both natural and laboratory settings.

Quantity or quality?

Two competing theories have been put forward to explain frictional strengthening. First, the points of contact at the rock interface may grow and increase in area over time ("plastic creep") – this is known as the "quantity" argument. Second, chemical bonding along the fault can increase the contact strength – this is the "quality" argument. However, these mechanisms are difficult to study because they usually occur deep within thick layers of rock.

Now, a team of physicists and geologists has shed light on frictional strengthening by considering the problem from a nanoscale perspective. Robert Carpick and colleagues at the University of Pennsylvania carried out experiments using an AFM, dragging a tiny silica tip across a silica surface in order to mimic rock-on-rock friction. Silica was used because it is a major component of rock.

The frictional force between the tip and the surface was found to increase logarithmically with time on a timescale of about 100 s, as is observed for macroscopic rock interactions. The researchers then investigated the source of this strengthening by sliding the silica tip across surfaces of diamond and graphite. Because these two materials are chemically inert and do not easily form bonds with silica, any observed frictional strengthening would be caused by a change in contact area, not chemical bonding.

Quantity ruled out at the nanoscale

The results were conclusive: the silica–diamond and silica–graphite interfaces showed almost no frictional strengthening, thus ruling out the contact-area mechanism. The researchers conclude that the silica–silica strengthening must be caused by chemical bonding at the interface, possibly via the formation of siloxane (Si–O–Si) bonds.

"Right now, most of the models of earthquakes are empirical and not predictive," says Carpick. "This adds a piece to the puzzle, and we think it's an important one because it proposes a specific mechanism to include in model friction: chemical bonding at the interface. With nanoscale experiments, new insights can be gained that have previously been elusive."

Although chemical bonding accounts for frictional ageing on the nanoscale level, it remains to be seen whether this mechanism can fully explain real-world, macroscopic rock processes.

Which process dominates?

"These results suggest that chemical bonding is dominant in chemically compatible surfaces, whereas other results distinctly indicate that the quantity of contacting surfaces plays a key role," Jay Fineberg of the Racah Institute of Physics in Jerusalem told physicsworld.com. "I would suspect that both effects are at play, and which of these dominates may well depend on how reactive any two contacting surfaces are. In any case, these are important issues that still need to be understood," he adds.

Carpick agrees that chemical bonding might not be the only contributor. "At the macroscopic level, interfaces between materials are extremely complex. We are not ruling out other mechanisms (such as growth of the contact area), but we are showing that chemical bonding is one mechanism that should be included in any comprehensive analysis," he says.

So how could we better understand the relative contribution of these two mechanisms? "We would need to look at the higher stresses, and also higher temperatures, that span the conditions found in geological systems," says Carpick. "We may see plastic creep at these higher pressures and temperatures, but the chemical bonding may also increase."

The research is described in Nature.