Section of an active fault that has not experienced a significant earthquake for a long time is called

Seismic Gap

Explanation:

Seismic Gap is a kind of an active fault that has not suffered with any of the significant earth-quaked from a long duration of time.

And Seismic Gap is the lack of large as well as big earthquakes in a particular place with a tectonic front. It is also an active fault which generally becomes the reason for other earthquakes.