Wiktionary
n. A geologic fault which has exhibited seismic activity within a geologically recent time period.
Wikipedia
An active fault is a fault that is likely to become the source of another earthquake sometime in the future. Geologists commonly consider faults to be active if there has been movement observed or evidence of seismic activity during the last 10,000 years.
Active faulting is considered to be a geologic hazard - one related to earthquakes as a cause. Effects of movement on an active fault include strong ground motion, surface faulting, tectonic deformation, landslides and rockfalls, liquefaction, tsunamis, and seiches.
Quaternary faults are those active faults that have been recognized at the surface and which have evidence of movement in the past 1.6 million years - the duration of the Quaternary Period.
Related geological disciplines for active-fault studies include geomorphology, seismology, reflection seismology, plate tectonics, geodetics and remote sensing, risk analysis, and others.
Usage examples of "active fault".
It was formed by one of the most active fault lines on the planet.
I would never have noticed the peculiar courses of the rivers, which had right-angle displacements to their courses along the more active fault lines, if Jack had not pointed them out to me as we flew over them in his private starship, XSS Scurra II.