An asperity is an area on an active fault where there is increased friction, such that the fault may become locked, rather than continuously slipping as in aseismic creep. Earthquake rupture generally begins with the failure of an asperity, allowing the fault to move.
See also Asperity (materials science) Asperity (geotechnical engineering) Earthquake Fault friction Fault mechanics
References
External links IRIS page on fault asperities with simple cartoon video of an asperity on an active fault IRIS page on "Modeling Asperities on a Strike-Slip Fault with Spaghetti"