asperity: one construct, many systems

In contact mechanics, an asperity is one of the microscopic peaks on a rough surface; because no real surface is perfectly flat, two solids pressed together actually touch only at the tips of their tallest asperities, so the true area of contact is a small fraction of the apparent area and the local pressure at each contact is high. Greenwood and Williamson's 1966 Contact of nominally flat surfaces modelled a surface as a population of asperities of varying height and showed how the real contact area and the number of load-bearing contacts grow with the applied load, which in turn sets friction and wear. The referent is a load-bearing surface peak, measured by its radius, height distribution, and the real contact area it carries.[1]

Unit

The real area of contact carried by surface peaks, and their height distribution

Recovery means

Not applicable

Measured by

Characterised by surface-roughness statistics, the real-to-apparent contact-area ratio, and the number of load-bearing contacts versus normal load

Seismologists borrowed the word for a strong patch on a fault surface that stays locked while the surrounding fault creeps, so that stress concentrates on it until it finally breaks and radiates most of the earthquake's seismic energy. Aki's 1984 Asperities, barriers, characteristic earthquakes and strong motion prediction used asperities (and their counterpart barriers) to explain why slip and high-frequency radiation are patchy across a rupture rather than uniform. It is the same idea as the contact asperity of tribology (a small, strong, load-bearing region that governs how the whole interface behaves) scaled up from micrometres between two blocks to kilometres on a fault, and tracked by the size and stress state of the locked patch.[2][3]

Unit

The size and accumulated stress of a locked, strong patch on a fault

Recovery means

Not applicable

Measured by

Inferred from slip-deficit and coupling maps, patches of high moment release in rupture inversions, and zones of high-frequency radiation