Most active faults generate earthquakes by sudden slip along slip planes (at a rate of 10^-2 to 10² ms^-1). Between seismic slip events, the faults are essentially stationary. In contrast, some faults move continuously (or creep) at very low slip rates (10^-12 to 10^-10 ms^-1) without inducing earthquakes. The ability to distinguish between seismic and aseismic faults is important for mitigating the seismic hazard of earthquake-prone areas. Previously, aseismic faults have been identified using analyses of both seismic and geodetic data. We will show that analysis of geodetic data alone (particularly, kinematic vorticity number of velocity gradient) can discriminate seismic from aseismic faults. As an aseismic fault can transform into a seismic fault without any precursor, possibly inducing huge earthquakes and related infrastructural damage, precise monitoring of kinematic vorticity number may substantially reduce hazard potential. The question then arises as to what controls the different kinematics between the two types of fault. We propose that fault mirrors (shiny mirror-like fault surfaces) showing extremely low frictional healing rates are likely to induce faults to creep aseismically. Densely-packed sintered nanograins of the fault mirrors hinders the chemical and physical processes that cause frictional healing of fault surfaces.
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