A team of researchers at the Brown University and University of Pennsylvania have demonstrated earthquake friction impact at the nanoscale, paving the way for better understanding of the calamities.
A photo illustration of an atomic force microscope probing the San Andreas fault. (Credit: D.K. Lynch)
For the study, the research team used an atomic force microscope, which is suitable for studying bonding strength at the meeting point of two surfaces on the nanoscale. The team imitated the rock-on-rock contact with silica. A silica tip was pressed against the surface of silica for various time durations and then pulled out to gauge the amount of friction experienced by it.
The team repeated the same tests with surfaces made of graphite and diamond, which are chemically inert materials. Hence, it was difficult for them to create chemical bonds with silica and thus if frictional aging takes place with them, then it is not because of augmented bonding strength but caused by the modifying contact area. The findings demonstrated that frictional aging significantly varied with the different type of materials.
However, the research team had to relate the intensified ageing at the nanoscale with the weaker aging occurring during the actual earthquakes. The answer is based on the fact that only some contact points inside an area are opposing the sliding movement at their full strength due to elastic coupling. Hence, some will move in advance, and others will move later. Thus, the overall resistance not only depends on the optimal resistance provided by some contact points but also on the small amount of resistance offered by the other contact points.
Robert Carpick, who led the research team, stated that a further study will deal with higher stress levels wherein contact quantity will play a major role. The team plans to conduct experiments at various temperatures, which are vital in terms of the geological context, to observe the contact in real time utilizing an electron microscope, Carpick added.