An active fault is a fault that has displayed recent seismic activity while an inactive fault hasn't displayed recent seismic activity. Don't be fooled by the word "recent" however, as we are talking about "recent" from a geological perspective, which is much different from non-geological perspective because of the fickle nature of plate tectonics, an active fault could have earthquakes as often as once every few years or once every one thousand years. Conversely, it's very hard to call a fault inactive if we don't know it's quake history, and for some faults, geologists will wait ten thousand years in between quakes to call them inactive....... Hope I helped you..
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