The hidden mechanisms behind earthquakes – the world of physics

It is adhesion tolerances, not wear, that dominate fault friction and are reshaping our understanding of slip behavior.

Earthquake boundaries (Courtesy: Shutterstock/Karen Grigoryan)

Earthquakes occur when tectonic plates rub against each other, become temporarily stuck, and then suddenly release the pressure built up as they slide. Although earthquakes have been studied for decades, the microscopic mechanics that cause faults to stick, slip, and generate friction are still not fully understood.

In this research, scientists use a granite-on-granite system to investigate these processes. Granite is common in continental crust and is mechanically similar to many fault rocks, making it a strong laboratory counterpart. The researchers used three complementary methods. First, they conducted controlled experiments to measure friction, wear, and surface roughness when two granite surfaces slide over each other, including tests with water, different temperatures, and different sliding speeds. Second, they performed molecular dynamics simulations of a silica tip (amorphous SiO₂) sliding on quartz (crystalline SiO₂), the dominant mineral in granite, to observe how atomic bonds break, phases transform, heat builds up, and friction emerges. Third, they applied theoretical models of contact mechanics (how surfaces actually come into contact through small disparities) and flash heating (how much local heating occurs and whether it weakens the material).

Traditionally, earthquake models assume that friction comes from mechanical processes such as roughness meshing (bonding of high points together), plowing (pitting hard grains into the opposing surface), and gouge grinding (crushed particles resisting movement). However, this study shows the opposite of what those models predict: more wear leads to less friction, and less wear leads to more friction. Instead of friction from drilling or grinding grains, it arises from small irregularities that flatten plastically and are cold-welded together, resisting slippage because their welded atomic bonds should break. This represents a major shift in how false friction is understood.

The study also found that friction is largely insensitive to temperature, sliding speed, and holding time, suggesting that classical rate-state friction laws may not fit real faults. The simulations identify three main energy dissipation mechanisms: bond breaking, plastic deformation, and stress-induced phase changes. This shows that flash heating at laboratory velocities is too small to weaken quartz, whereas earthquake-level sliding velocities would generate much stronger thermal weakening. They also reveal that some polymorphs of quartz can only form from stress, meaning their presence in natural faults does not necessarily indicate high temperatures.

Together, these results suggest that fault friction is dominated by adhesive bonding at varying grades, rather than mechanical grinding, and that tectonic motion may be governed by creep-slip more than classical stick-slip behavior.

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Earthquake Physics by Hiro Kanamori and Emily E. Brodsky (2004)

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